using the Derek Drew tireandgearingcalculator

[Jon_slider]

A theory about tires and gearing, using percentages derived from miles per hour at 4000 rpm for a stock waterboxer, using the syncrotireandgeargratios Excel spreadsheet, written by Derek Drew available free from:
http://users.rcn.com/derekdrew/syncrotireandgearratios.xls

1. Assume as true that a stock 2.1 waterboxer with a 4.86 ring and Pinion, and .85 fourth, using a 205/70/14 tire with 819 rotations per mile, will produce a speed of 71mph at 4000 rpm.

2. Assume as true, that if the tire is changed to a 215x75x15 BFG TA KO with 748 rotations per mile, the resulting speed at 4000 rpm will be 77.66 mph.

For sake of discussion, lets use percentages. Then the 215x75x15 tire results in a speed that is 10% faster than the stock setup, as in item 1.

By using percentages, I have found that I can make comparisons between vehicles with varying ring and pinion, gears, and tires more easily, than by memorizing mph@4000rpm.. both are accurate, the use of percentages is convenient for me, but optional.. in any case, to continue the percentage based comparison...

3. Assume as true, that a van with 245x75x16 BFG tire, with 680 rotations, and a 5.83 Ring and pinion, still keeping the .85 4th, would also travel at 71mph @ 4000rpm, what I refer to as 0% variance from stock gearing speed.

People who want to run tall tires, like to have shorter ring and pinions to offset the gearing change.

Over the years I have polled a number of Syncro De Mayo drivers, and others on the subaru vanagon and syncro vanagon groups, on their tires and gearing, and I have found that the majority fall within -3% to +10% of stock gearing, after accounting for variations in tires, gears, and ring and pinions..

The +10% configuration uses the BFG 215x75x15, with stock 4.86 ring and pinion, and stock .85 4th gear. Many syncros with 2.2 and 2.5 subaru conversions use this 10% taller than stock tire and gearing combination.

4. Assume as true that if a 30x9.5x15 tire with 705 rotations, keeping stock .85 4th, and stock 4.86 ring and pinion, would produce a speed of 82.42 mph@4000 rpm, and this is 16% taller than stock. If the Ring and pinion is changed to a 5.43, the resulting gearing will be 4% taller than stock. And if a 5.83 R&P is used instead, the result will be -3% or 3% shorter than stock.

So from example 4 you can see that a 5.83 ring and pinion would represent the -3% from stock configuration..

Now for a controversial leap of faith. Talking about Diesels, instead of gasoline motors.

Lets look again at the impact of the 215x75x15 tire, with 4.86 R&P and .85 4th, on speed. We already know that at 4000rpm the speed will be 10% faster than stock. But lets consider the speed produced at 3000 rpm instead. Those tires and gears will produce a speed of 58.25 mph @ 3000rpm, and that is -18% or 18% shorter than stock. I propose that diesels lend themselves better to operating at 75% lower RPM than gas motors. If you grant me that assumption, you will find that the percentages produced are very much in the same range as the values discussed for Gasoline motors.. -3% to +10%

On a diesel, at 3000rpm, the configuration that produces +10% would use a 245x75x16 BFG, with a stock 4.86 R&P, and changed 4th to a .7

The configuration that produces 0% would use the 245x75x16 tire, with stock 4.86 R&P, and changing 4th to .77

Similarly, a 245x75x16 tire with 680 rotations per mile will produce a 3000 rpm speed that is 0% off stock gas motor speed @ 4000rpm, with a 4.57 R&P and a .82 4th

The reason I propose these ideas, is to help people predict the likelyhood of success when making various changes and combinations of tires, ring and pinion, and gears. I dont mean to say that using percentages the way I do, is the best, nor the only way to do tire and gearing projections, its just a way I find useful, and wanted to share and explain my reasons.

I hope it helps some people avoid tire and gearing mismatches. It is my opinion that gearing that comes closest to 0% variance is best, and that 10% is the outer limits of useful for the majority of people I have polled. Not to say there are not people who are happy at 16% or even -25%, but they are, in my opinion, the exception.

5. Specific to my experience with a 2.5 AEL TDi, I would like to run a 680 rotation tire, with a 1.88 2nd, 1.14 3rd, and .77 4th.. to me, that would be an ideal combination using a 4.86 ring and pinion.
granny and 1st would be -10%
2nd would be -1%
3rd would be -3%
and 4th would be 0%

furthermore if setup in item 5 was modified to use a 4.57 Ring and Pinion, then the results would be
granny and 1st would be -4%
2nd would be +5%%
3rd would be +3%
and 4th would be +6%

fwiw, Karl mullendore runs gearing that is +6% in 4th. I tried it, and felt 4th was too tall..

Even though a gasser is tolerable at 10% tall with a wider RPM band than a diesel, I dont recommend exceeding 6% tall with a diesel.

I have added some fields to the derek drew spreadsheet so I can see easily the percentage change caused by gearing changes both at 4000rpm, and at 3000rpm

Although it appears that a 1.08 3rd would fit logically between a 1.88 2nd and a .7 4th, in the real world Karl, and others, are finding the 1.14 is a more driveable setup than a 1.08 3rd, both if used with a .7 4th, and also if used with a .77

If anyone wants help figuring out the effect of tires and gear changes on their van, feel free to provide me your ring and pinion, tire rotations per mile, and transaxle gearing, and I will gladly run the calculations to determine the resulting speeds..

Happy New Year, and thanks to all the people that have helped me by providing their personal experience with numerous gearing and tire combinations..

see also Derek Drew's post on the syncro group:
http://tinyurl.com/2e7uoek
"Once you know what mph vs. rpm you are going in 4th gear at 4,000 rpm (3,000 rpm for diesel)"...

[MTV1960]

Thanks for all the awesome info on this complex subject. Im still confused as hell. So, all that being said I'm still hesitant and do not want to make the wrong decision on tire my purchase. I have a 87 Syncro resto project underway and will be converting to the 2.5L subaru gas motor, 175hp. I will be overhauling the trans at the same time. My conundrum: I want to run a taller tire, mostly for cosmetics. My concerns and wants are better MPG's, Cosmetics (gnarly as possible), and budget(re-gearing?).
Interested in the BFG 215/75/15, 215/70/16@743RPM and the Nokian WRC Cargo 195/75/16@761RPM, 205/75/16@744RPM and 215/75/16@729RPM the last is 28.7"tall. The word is the Nokians wear better and have better all season road capabilities and better MPGs and quieter than the BFG's. My drawn out question is what is the best fit for this van set up with stock gearing and suspension (no lift). 2nd. is it worth changing the gearing to improve mpg's, lesson engine wear and cruse at 70 to 75MPH at 4000 RMP. In other words what would you do?

[Jon_slider]

BFG 215/75/15, 215/70/16@743RPM and the Nokian WRC Cargo 195/75/16@761RPM, 205/75/16@744RPM and 215/75/16@729RPM the last is 28.7"tall.

I take it you live near sea level and dont have 10,000 foot mountain passes to deal with.. so the BFG 215x75x15 is the ideal syncro gnarly tire and you do NOT need to regear, and it would not increase gas mileage if you did.

the 215x75x15 will regear you by 9% already.. youre done.

nokians are really nice in snow, and if you are at high altitude you will appreciate the 195/75/16's 761rpm.. stock is 819 rpm so that is still 7% taller than stock, so again, NO regearing.

you need to verify if the 215x75x15 will fit with stock suspension. Most people I know who use that tire have lift springs.. Go Westy has the goods..

and absolutely NOT on the 215x75x16, the 729 rpm will be an 11% power penalty, and you most likely need spring upgrades

the nokian is more of a road tire than the BFG, so the nokian is less gnarly. Otoh, if you dont actually drive in dirt, nokians are a great road tire.

btw, I dont think any of the changes you are considering will have any gas mileage effect worth considering.. not the tire size, not the gearing, not the tread..

get the one that puts the biggest smile on your face, not the one that will leave you thinking and wondering what all the fuss is about with BFG's and Syncros..

I have a friend with Nokians.. he does fine on dirt too..
BFG's are just OK in snow.. Nokians excel there..

decisions decisions..
post a photo of your van

[MTV1960]

Nice info, im getting closer. I assume that you are taking into consideration the Subi power upgrade in relation to RPM at hyw speeds? What im gathering is that no need to mess with trans gearing unless you are going over 11% increase in RPM in size. All the below would be cool when running the more powerful motor.
BFG 215/75/15
BFG 215/70/16
Nok 195/75/16 WRC Cargo
Nok 205/75/16 WRC Cargo

I have not seen the WRC Cargo mounted on a van yet. I will see if Peter from Van-Cafe has a photo as he claims that is the way to go in a 195/75/16. Seems skinny to me @ 7.7" wide... I will be doing mostly hwy driving and sand driving as well so.... Planning a trip through Central Am. So noise and fuel are a consideration. What im hearing is not much difference with the above selections then. As far as know gowesty markets the 215/70/16 BFG and claims it to fit with no suspension change on the Syncro. the BFG 15" is about $200 less than the 16" for 4 on the tire rack. Thoughts? PS posted a photo on my gallery of the pre-resto.
_________________
86 Wolfsburg Weekender
87 Syncro Westy

[MarkWard]

This is all good information, but I should add a caution for others coming across this thread. While it is true that crank rpms, transmission gearing, and tire diameter are a solvable math equation, there are other considerations that should be taken into account when making a decision to change what you have in either direction.

Horsepower, torque, terrain, and load are also important to consider. Jon is correct that comparing gas engines to modern diesels is like comparing apples to oranges. The equation or formula does not take this information into consideration.

My TDI makes max torque at 2200 rpm. Max hp is at 3800 rpm. This is not a spec, but from our chassis dyno with an eddy current. I went from a Aircooled 4.86 r&p with a .85 4th gear to a 4.14 r&p and a .77 4th gear. Using 3000 rpm for reference and my tire size with the original aircooled transmission I was cruising about 62 mph at 3000 rpm. Well above the max torque, but below the max hp.

With the upgraded transmission, I cruise about 72 mph at 3000 rpm. At 2200 rpm my max torque, I am running around 50 mph. That is the math. Now I load up all our gear for a 3 week trip, drive out of Florida and when we get out west, I can not run 4th gear on long uphill interstate grinds. Mathematically I should be able to, but I don't have the horsepower and torque to do so. I can run 3rd gear to 65mph, but the engine is near redline.

So, you can gear too short or too tall if you only do the math. I have since installed larger nozzles which increased my hp and torque together and can run uphill in 4th gear fully loaded with the same tires and gearing I had before. Just another point of view. mark

[Jon_slider]

> no need to mess with trans gearing unless you are going over 11% increase in RPM in size

people think that more power means they can use taller gearing, but when they put on 11% taller tires their gearing is already suffering, which makes granny less useful, and makes the entire power band work harder, from being tall.

the gearing change to fix that is Ring and Pinion, big bucks. You should imvho Never add insult to injury by changing 4th gear to be taller, when you already used big tires, that made you too tall to begin with. This is all in reference to the 2.5 Subaru gas motor..

now on to diesels and rpm speed targets:
> With the upgraded transmission, I cruise about 72 mph at 3000 rpm. At 2200 rpm my max torque, I am running around 50 mph. That is the math. Now I load up all our gear for a 3 week trip, drive out of Florida and when we get out west, I can not run 4th gear on long uphill interstate grinds. Mathematically I should be able to, but I don't have the horsepower and torque to do so. I can run 3rd gear to 65mph, but the engine is near redline.

excellent points
my personal opinion is that gearing a diesel to 70mph @ 3000rpm in 4th is ideal, for me. I agree that adding power by chipping the motor and changing nozzles, helps make up for tall gearing, or extra heavy loads.

The gearing calculator alone does not take into account differences in the weight of the vehicle.

A tin top might weigh as little as 4000 lbs, while some syncro westy's weigh 50% more.. so they benefit from 50% more power.. or whatever gearing and power upgrade combinations get your 6000 lb van up a hill in 4th gear..

now some general assumptions I make:
a 2.5 Subaru has about 165 hp, and about 162 ft lbs of torque
my TDi has 140 hp and 212 ft lbs, weighs 5000 lbs, and is geared to 70mph @3000rpm in 4th, and 8mph@3000rpm in granny. I use a 1.88 2nd, 1.08 3rd and .7 4th, with 31" tires and a 5.43 Ring and Pinion.

its likely, if both vehicles weighed the same, that the subaru will merge onto the freeway faster. Otoh, it is likely that the Diesel will stay in 4th gear on steeper hills than the subaru.

this assumes the gas motor is geared to 70mph@4000 rpm, and the diesel is geared to 70mph@3000 rpm

gearing to 68mph@3000 on a diesel is very good for staying in 4th on hills
gearing for 76mph@3000 on a diesel is very bad for staying in 4th

factors impacting gearing include, tire size, ring and pinion size, 2nd, 3rd, and 4th gear choices, And the weight of the vehicle.. thanks for your input rsxsr

A 4000 lb 2wd tintop with a 2.2 Subaru and 26" tires can be fun, but a 5000 lb syncro westy with a 2.2 subaru, and 28" tires, is not so fun.. its the 25% more weight, And the 9% bigger tires, that make the syncro westy work harder a combined total of 34%...

the implication then is that the syncro would benefit from 34% more power, to match the performance of the lighter 2wd van with smaller tires, such as when going uphill on the freeway.. and the ability to stay in 4th gear..

at the granny end of gearing, I find
8mph @ 3000 rpm excellent for my diesel syncro. I did not like it when I was geared to 10mph@3000 in granny.
stock gas vanagons are set to 10mph@4000rpm in granny.

[levi]

About the 195 75 16, the wrc cargo, have them on my van several months now, and I posted my thoughts there.
Fwiw, on my van these are the worst yet as far as handling.
I've been to different alignment shops, and they agree it's dialed in to spec.
No tire pressure changes make it stable.
Yesterday it was windy here, and it was scary as hell even at 35mph.
Unless there's something funny about this particular set (unlikely), or a hidden suspension
problem, then these would be your worst choice.
_________________
One of these days I'm gonna settle down,
but till I do I won't be hangin round.
Going down that long lonesome highway,
gonna see life my way

https://youtu.be/cSrL0BXsO40

[MTV1960]

What is the best PSI to run in BFG all T/A's 215/75/15 on a Syncro full camper with 2.5 L subi? Tks Volt
_________________
86 Wolfsburg Weekender
87 Syncro Westy

[MTV1960]

So i finally have the restoration (mechanical) of the 87 syncro complete! My first observation is a problem with the gearing. I decided to run a bfg 215/75/15 with the stock gearing and the 2.5 subi 175hp motor conversion pkg. I calculated 3000 rpm @ 58 mph 3500 rpm @ 66 mph and 4000 rpm @ 75 mph. It feels like I need a 5th gear $%^#. This is bad news. So much $$ in this thing already. Im over it! Any one know of a solution and what gearing to change? I guess it's a 4 gear change in the front and rear differentials to lower the rpms where it feel like the engine is not so stressed? IDK. still have to paint and do the interior over to complete the project.
_________________
86 Wolfsburg Weekender
87 Syncro Westy

[derekdrew]

Easy: just do *exactly* what it says to do here:

http://users.rcn.com/derekdrew/syncrotireandgearratios.xls

Do not deviate from those exact instructions in any way.

This calculator is exactly tailored to your engine and vehicle configuration.

See also the text immediately below that I wrote for my syncro peeps:

==================================
How To Select Syncro Gearing
by Derek Drew
1/1/2011
==================================

When selecting gears for a vehicle, the FIRST choice to make is what
rpm you want to be going at a given mph in 4th gear. This
consideration stands tall above all others, and determines what you
will do next.

Once you know what mph vs. rpm you are going in 4th gear at 4,000 rpm
(3,000 rpm for diesel), almost all the other gears select themselves
for you, and you have only a few easy decisions to make.

The numbers you use should be based on a calculator (see xls below)
or failing that maybe a gps, since the speedometers are not accurate
enough for such critical calculations and mislead people frequently
into choosing the wrong gear among two choices that seem initially
equally reasonable. Other common mistakes: do not estimate your tire
diameter and then run calculations based on that. You should instead
calculate based on "revolutions per mile" (RPM) of your tire. Also,
do not estimate your revolutions per mile based on running the
numbers on your tires to calculate RPM. The diameter of your tire as
your gears see it is quite different from what YOU would calculate it
to be based on the published tire size. So, instead, get the
manufacturer's published "revolutions per mile" number for the tire
you plan to use and ignore a tire's published dimensions or published
height. Don't do *any* gear calculations (body clearance is maybe OK)
based on the observation that a given tire is, say, 28" tall, or 30.25" tall.

The material in this posting is simply a report on the best I can see
at the moment. We never stop learning.

PRE-REQUISITES, TIRE AND ENGINE

You must have already chosen between diesel vs. gasoline engine, and
hopefully, know exactly what engine you plan to use. You also must
know the exact tire size you plan to use. I won't address tires here,
but my advice is that if you are considering more than one possible
tire size, then choose the very biggest and go directly to the
ultimate end result rather than being timid about doing so. I don't
know many owners that were unhappy with having large tires, but I
have seen many owners who committed to a smaller size liking it so
much that they eventually wondered if they shouldn't have chosen a
tire larger. For people who like the sound of tire sizes with
published heights ranging from 29.5" to 30.5", I encourage them to
select these sizes and not drop back to something smaller.

4TH GEAR IN THE GASOLINE WORLD

In the gasoline world, the range for 4th gear should generally be
between 70mph to 76mph at 4,000 rpm, the smaller number being better
for climbing up hills faster, and the larger number being better for
gas mileage and ultimate top speed of rpm-limited motors like the WBX
2.1L or GoWesty WBX. Heavy campers with smaller engines should avoid
going much higher than about 72mph, whereas very powerful gasoline
motors have the power to handle a high number like 76mph at 4,000
rpm. Most very powerful gasoline motors these days tend to have a
wider band of happy revving, so the urgency to get more top speed
ability by bringing the speed up at 4,000 rpm is lessened for these
high revving motors (e.g., Bostig, subaru).

Gear selection for the engine I chose, the 3.0L H6 subaru
engines, tends to be quite non-critical because of the wide flat
torque range *and* revving range of that motor. Gear selection for
2.1L WBX motors tends to be quite sensitive simply because you have
limited power from that engine, and you want to get that power where
it is most useful.

From time to time, people get excited about installing "overdrive"
4th gears, and when I hear that term being used, it generally means
that the owner contemplating the 4th gear may not appreciate how
drastically the choice of an overdrive 4th gear may deprive him of
the ability to climb hills or accelerate in 4th, especially in
heavier vehicles like campers. Accordingly, when the term "overdrive"
is used in contemplation of a gear plan with 2.1L WBX, Tiico, 2.2L
subaru, and 2.5L Subaru, the owner is almost always unwittingly
contemplating a gear plan that is going to be less optimal than if he
had never heard the term "overdrive" at all. The gravity of the
mistake is less if a) the vehicle will be operated very light in
weight or in extreme flatlands, and b) the particular setup actually
delivers the promised benefit in gas mileage somehow, and the gain
given, however small, is a worthwhile gain to the owner given the
disadvantages. What concerns me most about the use of the term
"overdrive" is that it has very little meaning, since it is not clear
where in the range of 70mph to 76mph at 4,000 rpm the overdrive gear
would place the vehicle in operation. In other words, the term is
often a tip-off that the owner is not using a gear calculator and
more disciplined approach from which he would *know* the actual
effect on gearing, rather than a nice-sounding word that seems like
it would produce some benefits.

While the 2.1L WBX, Tiico, 2.2L subaru, and 2.5L Subaru and 2.5L GW
WBX people seem restricted in their choice of where to put 4th gear
in comparison to the 3.0L and 3.3L subaru people, the gear planning
for diesel people is more restricted still, and there is a massive
disadvantage in the diesel world to making the wrong choice, so it is
quite sensitive. The reason for this is simply due to the vary narrow
revving band of the diesel engines, outside of which the vehicle
simply won't operate well. You cannot, and will not, have the option
to operate your diesel engine at 4,400rpm on the highway. So you have
to get the gear selection right, because it is a bit of a straight
jacket into which you are locking yourself. The choice of 2nd and 3rd
gears for diesel is also very important, because poorly planned jumps
that wouldn't be noticed by a higher revving gasoline motor can turn
into quite an annoyance when you can't just rev your way happily out
of a sub-optimal choice.

4TH GEAR IN THE DIESEL WORLD

In the diesel world for 1.9L and 2.5L TDIs, I am still listening to
all the opinions (I never had a diesel) but it seems that many owners
are happy if 4th gear at is at 70mph to 71mph at 3,000rpm. Additional
input on this issue would be helpful to me to hear from owners, but I
cannot listen to any data on this matter if owners don't assert that
they have an accurate tac working and I know how they know their true
speed and I have some assurance that they are not reporting numbers
based on what their speedometers are saying but are instead deferring
to carefully calculated speeds (ref. xls below) or to gps speeds.

After you know what your choice is for 4th gear then the hard work
has been done, and you can then decide whether to set your low G gear
as low as possible to facilitate off-road driving, or just leave it alone.

I do not know whether a 1.9L or 1.6L diesel of lower power than a TDI
would select the same 4th gear as the TDI people would.

SELECT YOUR G GEAR (R&P)

It is unworkable for owners, especially with smaller engines, to
drive off-road with very large tires unless they change their G gear
to a gear of lower type. It is also unthinkable for some off-road
enthusiasts to select any other possible G low gear than the lowest
possible one, simply because, for off-road use, lower G is better,
always, no matter what the size of the engine, and no matter gasoline
vs. diesel. No matter what, slower is better, always because it
brings the torque up when driving over irregular obstacles, which
helps you keep from stalling and from destroying your clutch, and
allows you to simply drive slower over bad things rather than having
to keep your speed up when you really shouldn't, which results in
things getting crunched, body damage, etc..

The diesel people will sometimes say that they don't need a lower G
gear because they already have the torque down low. I would rephrase
this to say that the matter for them is less urgent, but the rule
still holds that, even for them, the ability to go lower will always
have advantages than not having that ability, as the terrain gets worse.

Since there is no method to actually choose a different ratio G gear,
the way we lower G gear is through the selection of ring and pinion
ratio, which ratio is 4.86 in North America by default, 5.43 in South
Africa by default, and can also be 5.83 or 6.17. As these numbers go
up, you can drive slower with more torque, and you can use
progressively larger tires, and your rig will become progressively
more off-road capable with less danger of you burning out your clutch.

There is one theoretical dis-advantage to running high number ring
and pinion ratio gears which is that as you go up in number, the
gears themselves become progressively weaker and more prone to
breakage. We have not heard reports of ring and pinions being broken
in the USA simply because they are too weak though. I have been
running 6.17s myself for many years, and as the list members who know
me can attest, I am extremely overloaded and quite abusive to these
gears. However, when changing the differential the other day to one
of upgraded type, we noticed that my 6.17 gears themselves looked to
be brand new, which you can see in the pictures in the thread on
"Vanapalooza", a sort of tech party where we are putting 3.0L H6
engines into my syncro campers and those of other list members, here:

http://www.thesamba.com/vw/forum/viewtopic.php?t=4...p;start=40


That said, we have heard reports of 6.17 gears in Germany failing,
especially with powerful engine conversions. Also, Gordon in South
Africa before he died had a failure of his front 5.42 final drive
pinion head (sheared off) after using a powerful gasoline engine there.

I have not heard of any high ratio gears failing in North America
unless the cause was something other than the strength of those gears
themselves.

If you are going to select an optimized G gear rather than keep using
what you are using now, then the procedure is very simple, which is
to select the highest number ring and pinion ratio that will support
you placing 4th gear where you want it using the mph vs. rpm choice
that you have already made.

For all practical purposes, this probably means that you can choose
6.17 if you have a gasoline motor, or 5.43 if you have a diesel motor.

The expense of changing your ring and pinion ratio is so high that it
is hard to justify making any changes for only one jump. So, it would
be hard to imagine somebody with a 4.86 justfiying changing to a 5.43
unless they were already rebuilding their transmission. However, if
you already had, say, 5.43 in the front and 4.86 in the back because
of a humorous set of life circumstances, in that case, if you had a
diesel, then I'd say it might be just as well to change the back to
5.43 rather than exchanging the front 5.43 ratio for a 4.86 ratio.
Still, it is a lot of work to change the back ratio, and hardly any
to change the front, so practicalities need also to be considered.
Moreover, the gear plan to run 5.43 and 4.86 for diesel would be
completely different, so 3rd and 4th gears would also have to be
changed, and the transmission completely rebuilt.

Owners of gasoline engines in the 2.0L to 2.5L range who plan to
install larger diameter tires and/or off-road their rigs very much
sensibly think about changing their ring and pinion ratios to higher
numbers since these engines tend to have uninspiring torque at very
low rpms, a matter that can be addressed well through a ring and pinion swap.

SELECT 2ND and 3RD GEARS

All the other gears write themselves after the selection of 4th gear
and the selection of the ring and pinion ratio since you just choose
them to space them out in a sensible manner (think, accordion) in
between. Generally, this means 1.88 2nd gear (if you can find it) for
sensitive cases (think, diesel), and then just eyeball 3rd gear and
space it in a reasonable manner in between 2nd and 4th and you are
done. The gear calculator at
http://users.rcn.com/derekdrew/syncrotireandgearratios.xls that I
wrote helps you eyeball 3rd gear (and 2nd) to make nice spacing.

For anyone using my calculator, note that the most reliable spot to
get it is probably from here since I have the most control over
updating that file at that location:

http://users.rcn.com/derekdrew/syncrotireandgearratios.xls

Most owners don't need to see me attempting to destroy my ring and
pinions here since they have already seen these pictures:
www.syncromadness.com