Bigger tires, changing gearing.

[Syncronoid]

I've been discussing this offline with knowledgeable members and have gotten some good advice.

Here's some interesting data. I did a drive test today to check GPS speed vs. RPM's to compare to the plots that Gears presented. With my stock gearing, 1st - 4th (3.778/2.063/1.225/0.854), 4.86 R&P and 27.9" (215/70R16) tires, I collected data that was more exemplary of stock gearing as above with 29" (235/70R16) and 5.43 R&P below. I'm at a loss to explain.

1st
MPH RPM
10 2450
15 3800
2nd
MPH RPM
15 1900
20 2600
25 3200
3rd
MPH RPM
30 2300
35 2700
40 3050
45 3500
50 3800
4th
MPH RPM
50 2650
55 2900
60 3200
65 3500
70 3700
75 4000

_________________
'90 Syncro Westy w/GW2.5L

[E1]

[jackbombay]

[E1]

You park a jet in your creek? 😳
_________________
‘84 Westy, 2.1L with Digijet, 5.43 R+P, GT Gears
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere."
— Colin Chapman

[levi]

All this bantering about and nobody brought up the most fuel efficient rpm to be driving at?

Anyone want to guess what that is?

If a change in tire diameter brought a driver into that perfect point, where otherwise he was way off, I could see it helping if other factors were not also introduced that negated that effect.
_________________
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

[E1]

👍🏼 Precisely, my good man! 👍🏼

Hence the banter. Thread leapt off cliff, beer far more interesting.
_________________
‘84 Westy, 2.1L with Digijet, 5.43 R+P, GT Gears
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere."
— Colin Chapman

[hdenter]

OK, I'll chime in a little. When if went from my stock 4spd to my ASS 5speed I went from a .85 X 4.86 4th to a .82 X 4.83 5th. No other changes-same stock wheels, same cruising speed on the highway. I found tank after tank that I went from an average of about 17.5 mpg to about 19.2 mpg. It wasn't a major change, but it was noticeable. The funny thing is, when I recently pulled that trans out of my weekender and put it in my work van, I saw no significant change in MPG performance on that van. Why? I suspect because between the weaker 1.9 and the much heavier load in that van I can't pull the 7% grade I drive every day in 4th gear like my weekender could. I have to drop to 3rd and I'm still having to crank up the RPMs to climb that hill at 50mph. It hasn't been in the work van long enough for me to do any full highway tanks to see how the straight highway MPGs will compare to the 4spd in that van. I seldom do road trips in that van, but eventually I'll have to drive it up to the Bay Area or L.A. and I'll let you know how it does.

Hans
_________________
'79 triple white convertible bug
'84 sunroof vanagon
'85 weekender

[E1]

My opinion straight away is you gained mpg in adding another gear -- which I wish we'd done. This goes well beyond just the open highway benefit or loss of a changed top gear alone, meaning it would show little change in a highway/top-gear-only drive but a significant one when using all gears -- like on a back road.

Imagine a ten-speed transmission, and the easier revving in all gears. Serious power gain while all else is the same as before. That converts to better mpg and is a likely factor of manufacturers going to five-speeds from the start.

Performance gains are also gained when shortening the time between shifts, as in a race car.
_________________
‘84 Westy, 2.1L with Digijet, 5.43 R+P, GT Gears
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere."
— Colin Chapman

[Syncronoid]

Yes, strange indeed. But, for whatever reason, there seems to be spare RPM's for larger tires, without a gear change. Investigation to continue.

[Waldi]

[crazyvwvanman]

What are you using to precisely measure the engine rpms? The dash tach needle isn't so accurate for this purpose.

Looking at the 4th gear numbers, about half of your discrepancy is due to error in the program GT is using. This error is common to many gearing calculator programs. The programs consistently get the tire size slightly wrong and this makes the results slightly wrong but consistent. The error isn't very important for comparisons between gearing and tire options within the program but you shouldn't mix it's numbers with real world numbers. If you wish to do that you need to correct for the error it makes.

Mark

[Pcforno]

[Waldi]

Here something to google translate:

http://www.spiegel.de/auto/aktuell/rad-und-reifen-...04825.html

http://www.automobilclubs.org/magazin/reifen-der-z...verbrauch/

[gears]

[Jake de Villiers]

[Syncronoid]

From the BFG KO2 page.....

215/70R16:
Overall Diameter
27.9
Revs/MI
746

235/70R16:
Overall Diameter
29.0
Revs/MI
718

Seems the Revs/MI are a tad larger than tires with similar diameters. Can tires with the same diameters have different Revs/MI values? What would cause such discrepancies? Tire pressure?

Based on the numbers presented by BFG, I think I could reasonably move to their 235/70R16 without considerable impact my existing driveability; only a 3.9% diff in Diameter, 3.75% diff in Revs/MI. Would I even notice?
_________________
'90 Syncro Westy w/GW2.5L

[E1]

... or sidewall deflection.

And D-rated tires generally roll further than C-rated tires.
_________________
‘84 Westy, 2.1L with Digijet, 5.43 R+P, GT Gears
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere."
— Colin Chapman

[Waldi]

[E1]

Are you saying you don't like KO2s, Waldi?

I ask as ours are absolutely fabulous.
_________________
‘84 Westy, 2.1L with Digijet, 5.43 R+P, GT Gears
"Adding power makes you faster on the straights.
Subtracting weight makes you faster everywhere."
— Colin Chapman

[Syncronoid]

To each his own. I have KO2's now. Love 'em.