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markf1
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Joined: September 21, 2019
Posts: 286
Location: Telluride, Colorado
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 Posted: Thu May 27, 2021 5:40 am Post subject: Do larger wheels affect speedometer/odometer? |
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We put 215 R15 BFG Ko2 on our syncro. Previously had the stock 14” wheels.
Now I’m wondering if that had an effect on the speedometer and odometer? If yes, is there something that can be done to adjust for that?
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1986 Westfalia Syncro
2.1 L Waterboxer
Some Westy supported adventure photos:
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dobryan
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Joined: March 24, 2006
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Location: Brookeville, MD
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mikemtnbike
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Joined: March 26, 2015
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Location: North Carolina
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| Posted: Thu May 27, 2021 5:48 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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In short, yes.
My "adjustment" was mental. I used a speedo app on my phone to determine that my speedo is reading about 7% slow. I make the mental adjustment to the speedo and odometer readings in my head accordingly.
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1991 Vanagon GL 2.1 AT Westfauxlia. "Frankie" Totaled  https://www.thesamba.com/vw/forum/viewtopic.php?t=764510&highlight=carnage
1995 Eurovan Camper "Marzivan"
2020 GTI SE manual
Last edited by mikemtnbike on Thu May 27, 2021 5:56 am; edited 2 times in total |
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danfromsyr
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Joined: March 01, 2004
Posts: 15143
Location: Syracuse, NY
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| Posted: Thu May 27, 2021 5:50 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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a larger tire rolls slower at a given speed than a smaller tire.
it's only the outer diameter that matters.
pure and simple math.
215/75R15 tires have a diameter of 27.7", a section width of 8.5",
The circumference is 87.0" and they have 729 revolutions per mile
185R14 is a Metric tire with a section or tread width of 7.3 inches,
overall diameter of 25.7 inches. 185R14 tire has a circumference of 80.7 inches, making 785 revolutions per mile
while they may be only 2inches taller.
they have a 7inch greater circumference when rolled out on the road.
it WILL also effect your overall gearing.. and you will spend more time in 3rd gear in the Mtns than before.
as well as putting more strain on the transmission internals. though that largely depends on your driving habits.. DO NOT LUG THE TRANSMISSION.
down shift and keep the RPMs up when loaded up and it's slowing down.
soo next question is what is to lug?
https://www.motorauthority.com/news/1105621_heres-why-you-should-never-lug-your-engine
| Quote: |
To start, let's talk about what it means to lug your engine. This is the term used to describe when you're puttering along in a higher gear than you need to be in and the engine is turning low RPMs. When you're cruising along in 6th but you should've downshifted into 4th about a half mile back, you are lugging your engine. Why is this bad? Engineering Explained is here to lay it out for you.
When you lug your engine, you are forcing it to work far harder than it should. Low RPMs are fine if you're just cruising or slowing down, but when you accelerate, low revs leave your engine with a gearing disadvantage.
To get your car up to speed, the engine has to work a lot harder. Engine temperature will rise, and cylinder temperature will rise. This is where things can start to go screwy.
Picture yourself on your old 10-speed road bike. You're cruising along, having shifted up into the higher gears. Suddenly you need to accelerate and when you try to turn those pedals, you find it takes a lot more effort than it would if you were in a lower gear. You are working far harder than necessary just to get that bike moving again.
In this example, you've lugged your body because you're the engine of the bike. It's not fun to do that, so why do it to your car? |
those bike pedals are your transmission gears. and that extra power is pushing too hard forcing the oil out of the gears and applying too much side load to the gears forcing unnecessary wear into the bearings and bushings that hold the gears in place. this is more of a problem with the big tire and big motor crowd. they end up learning the hard and $$$$$ expensive way..
I only mention these because I know you have had recent (last year?) transmission issues that cost a fair amount already.
say after me. it's OK to downshift on hills.. 
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Last edited by danfromsyr on Thu May 27, 2021 6:00 am; edited 2 times in total |
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dobryan
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Joined: March 24, 2006
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Location: Brookeville, MD
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MarkWard
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Joined: February 09, 2005
Posts: 17110
Location: Retired South Florida
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| Posted: Thu May 27, 2021 6:12 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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The Speedo face has the value for number of revolutions per mile that it should have been calibrated for.
Even calibrated for the correct rotation, larger tires can impact mpg.
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bobbyblack 
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Joined: May 21, 2015
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Location: United States, Iowa
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| Posted: Thu May 27, 2021 6:23 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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I don't know about Syncro front ends, but changing the geometry of 2WD front ends by changing tire diameter and width puts stresses on the suspension that wasn't engineered into the system. How do you feel about your balls? Do you like them? In the 2WD world, they're the first thing to suffer from taller/wider tire mania.
I also don't know about the 4WD world in general. I know that in snow or slush, you want to cut through and bite on the hard surface under that to get your gription. Wider tires float up on top. It might be the same with muck and mud for the 4WD folks who do that stuff, I don't know.
As for the speedo, again, in my 2WD world, I know the speedo's were reporting incorrectly right off the factory floor. So, will yours be closer or even further from the actual speed? I can't decide. But, I went from 195R14 at 25.5" to a 16" rim with the lowest profile I could find at he time, and got to 26.1" and my speedo is now spot on between 40 and 84 mph. Lower is off a bit, and I have not taken the trouble to see if higher is 'on or off' as I'd be in for a serious ticket... (South Dakota Interstate has an 80MPH limit)
-bobby
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markf1
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Joined: September 21, 2019
Posts: 286
Location: Telluride, Colorado
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| Posted: Thu May 27, 2021 6:45 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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| danfromsyr wrote: |
I only mention these because I know you have had recent (last year?) transmission issues that cost a fair amount already. say after me. it's OK to downshift on hills.. |
Thanks to you all. that's what I thought but now I understand even better.
Dan- You are right about that! However - I think our transaxle blow up was due to 300K + miles versus lugging the engine. GF is the van owner and she does almost all the driving (while I mange the navigation, music and refreshments). She's pretty good at keeping it above 3K RPM when going up hills (which we do a fair bit of due to living in the mountains).
Mainly I'm thinking about the mileage interval for maintenance like tire rotation, oil change, tune up. We are getting ready to d o al that for the first time since putting on the new 15" wheels/tires.
729 revolutions per mile with the 15" wheels versus 785 revolutions per mile with the 14" is a difference of about 7.5%. So - I'm thinking if the odometer is showing 3K since last oil change/tire rotate that really = 2,775. Not that big of a diff but good to know. Also going to skew the gas mpg numbers I've been keeping track of as part of my co-pilot gig.
Thanks for all the helpful info, It would be nice to have the speedo and odo calibrated at a specialty shop but I doubt we will be able to find such a shop anywhere near us and I think we can get along OK doing the mental conversion.
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1986 Westfalia Syncro
2.1 L Waterboxer
Some Westy supported adventure photos:
https://www.openrangeimaging.com/adventure.html |
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markf1
Samba Member
Joined: September 21, 2019
Posts: 286
Location: Telluride, Colorado
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| Posted: Thu May 27, 2021 6:48 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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| bobbyblack wrote: |
| How do you feel about your balls? Do you like them?-bobby |
_________________
1986 Westfalia Syncro
2.1 L Waterboxer
Some Westy supported adventure photos:
https://www.openrangeimaging.com/adventure.html |
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MarkWard
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Joined: February 09, 2005
Posts: 17110
Location: Retired South Florida
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| Posted: Thu May 27, 2021 7:36 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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There is a company in Ft Lauderdale called Lauderdale Speedometer, that you could send the head to have it recalibrated. Probably not convenient. But I think you can specify the correction you'd like.
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syncrodoka
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Joined: December 27, 2005
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Location: Santa Cruz, CA
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| Posted: Thu May 27, 2021 7:59 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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Get a speedometer app for your smartphone and see if the speed is in fact off. The factory tire size on my son's syncro reads high on the speedo.
You can buy a geared adapter to fix the speedo error but you need to know what % it is off. Getting the speedo tweaked by a pro is also an option. |
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markf1
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Joined: September 21, 2019
Posts: 286
Location: Telluride, Colorado
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| Posted: Thu May 27, 2021 8:02 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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Thanks Mark. I just called that shop. Spoke with a super nice and knowledgeable person named Bob.
Cost $180- $230 = shipping. He says the odometer could be made somewhat more accurate but cannot really be calibrated. He could move/tweak the needle position on the speedometer and that would be more accurate but I believe he said +/- 3mph.
So... I think we just keep doing the mental math as occasionally needed. + we now know that 60 mph on the speedo is probably more like 55. I will set a gps on the dash and check speed next time we are driving the van to see how that matches up with the speedometer.
_________________
1986 Westfalia Syncro
2.1 L Waterboxer
Some Westy supported adventure photos:
https://www.openrangeimaging.com/adventure.html |
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brickster
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markf1
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Joined: September 21, 2019
Posts: 286
Location: Telluride, Colorado
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| Posted: Thu May 27, 2021 8:45 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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very cool brickster!
We are assuming the stock 14" that the speedo/odo are set for = 785 revs/mile (not sure how to verify that).
so 751 is @ 5% difference from 785.
Will still compare speedometer to phone app and/or gps on next drive.
_________________
1986 Westfalia Syncro
2.1 L Waterboxer
Some Westy supported adventure photos:
https://www.openrangeimaging.com/adventure.html |
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brickster
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Joined: January 05, 2004
Posts: 512
Location: CO, USA
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| Posted: Thu May 27, 2021 9:10 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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According to this handy dandy pdf (probably also here somewhere?), your original revs/mile was 820 or 798 depending on how it was equipped.
http://oldbluesblog.com/files/VanagonTireSpecs.pdf |
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4Gears4Tires
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Joined: October 08, 2018
Posts: 3018
Location: MD
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| Posted: Thu May 27, 2021 9:13 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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| syncrodoka wrote: |
Get a speedometer app for your smartphone and see if the speed is in fact off. The factory tire size on my son's syncro reads high on the speedo.
You can buy a geared adapter to fix the speedo error but you need to know what % it is off. Getting the speedo tweaked by a pro is also an option. |
With 27/8.5/14 tires my speedo reads about 2mph high at highway speeds. That's about perfect imo.
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'87 Syncro
Ferric Oxyhydroxide Superleggera Edition |
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DuncanS
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Joined: October 17, 2013
Posts: 4583
Location: New Hampshire
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| Posted: Thu May 27, 2021 9:39 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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| Photoshop a pic of your speedo dial and make it read correctly. Odo won't read correctly, but the speedo will. |
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Steve M.
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Joined: July 30, 2013
Posts: 6829
Location: Fort Lauderdale, Fl.
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| Posted: Thu May 27, 2021 9:43 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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Syncro correction gearing for speedometer are available.
https://www.brickwerks.co.uk/catalogsearch/result/?q=speedometer&x=0&y=0
The speedo cable comes from the front dif so it is gear driven and gears are changeable.
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Steve M.
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Location: Fort Lauderdale, Fl.
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| Posted: Thu May 27, 2021 9:47 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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There's an Excel spreadsheet for tires sizes and gear ratios showing rpm outputs.
http://www.syncro.org/Files/syncrotireandgearratiosdd.xls
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syncrodoka
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Location: Santa Cruz, CA
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| Posted: Thu May 27, 2021 10:19 am Post subject: Re: Do larger wheels affect speedometer/odometer? |
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Those plastic speedometer drive gears shouldn't just be swapped, they are matched to their drive gear. |
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