Speedometer/Odometer Accuracy
 

Is tire size not directly related to speed and distance readings in the speedometer/odometer? If it is, how do you calibrate the speedometer when you change tire size? I am concerned because I believe my speedometer is reading about 10 mph high at 65 and that means I am showing more miles on the odometer than I should be. That means I get to the 2500 mile insurance limit before I should and I have to stop driving it. That would really be a bummer. Or am I off on a goose chase without a reason?

Its the tires height that effects speedometer calibration. The stock height for C-1s approx 26.75 inches.

That is my point exactly. Tires come in all kinds of sizes. My 63 SWC has 215/65 SR 15's on it. It's "stock" size was 6.70/15 I believe. Since the first number is the width and the second is the ratio of height to width and since I do not know that ratio for the "stock" tire, I have no idea how the heights of the two compare. And (back to the original question) if they are different, then the speedometer/odometer readings will be different, so how do you make sure you are getting the right readings if the height changes?

You can change the speedometer driven gear down at the tranny. I put in a new driven gear with one more tooth when I went to 215/65/15 tires, since these tires' diameter was less than the stock tires. If you tell us your current tire size, someone here will likely chime in with which driven gear you should swap in to get the speedo reading approx. right.

The tire size R205/75/15 is closest to the original in a C-1. One way you could calibrate your speedometer to to follow another vehicle whose speedometer is accurate at a certain speed and compare differences.
Depending on whether your speedo is fast or slow ypu could change the plastic gear in the trans to compansate.

As I mentioned in the posting above, I've got a C-2, a 63 SWC. I won it in a raffle last year and it came with fairly new 215/65/15's on it, so I am not looking to get new tires. But I have already run it against my wife's car and it reads high, so I know it is out of calibration. I guess what I need to know, according the posts so far, is what driven gear do I need to install in the trans to make it read right? What kind of a job is it? And given the two size tires, 215/65/15 and the old 6.70 x15, is there any way to tell from my current odometer readings how far I have really gone? There must be some mathmaticians out there who know the height comparisons and can figure this one out.
Actually if someone can give me the two diameters I might be able to make a stab at it myself. Thanks for any help you can give on any of these questions.

You need to know the tire revs per mile specifications. Maybe someone knows them. Maybe they are in the archives. Hope this helps.

Duke

I have the reverse problem. My 65 has the repro bias goldline tire (7.75) with a 3.70 rear and the speedometer is reading about 12 mph to slow. I have the "green" gear in the tranny which I think has 21 teeth. Should I replace it with less teeth and then how many?
Thanks

WWW.tirerack.com
Click on the specs to get revs/mile. Typical 205/75/15 is 769 revs/mile.

(1/revs/mile) x (engine rpm/axel ratio) x (60 min/hr) = MPH

For 'Old Blue':
(1/769) x (3000/3.36) x 60 = 69.66 mph

Havn't had a working speedometer in years.

There isn't a division sign on this d--n keyboard! :cuss

Each tooth in the drive gear affects the speedo about 5 MPH.If the speedo reads high you must add the appropriate higher number of teeth gear . ( 10 MPH high = 2 more teeth to what you have) The converse is true if your speedo reads low.

No, your statement is incorrect. Each tooth affects speedometer readings by about five PERCENT, which is 5 MPH at 100 but only 2.5 MPH at 50. You have to do these calculations in a proportion format like percent, not an absolute format like MPH.

The typcial driven gear tooth range is 18-22 (1/20 = .05 or five percent), so you can make changes in about five percent increments to compensate for tire revs/per mile variation from the OE tires.

The OE 6.70-15 are speced at 760 revs/mile, and the 7.75-15 are speced at 775. GM did not change the speedometer gearing when they transistioned to the 7.75-15 size in 1965. The just accepted the additional two percent error on the high side.

The revs per mile of any tire can be found at The Tire Rack or the tire manufacturers web site. Then you compute the PERCENT difference in revs per mile. If the revs per mile are about five percent HIGHER for the new tire and there is an available driven gear that has one more tooth, then you can correct the speedometer.

You would be unlikely to install a tire with LOWER revs per mile than OE. Most of the popular replacement sizes have higher revs per mile than OE, so the speedo will read high, and you need to increase the driven gear tooth count to slow it down. This assumes that you have the correct OE speedometer drive and driven gear setup installed for the installed axle ratio, which is not always the case.

If your speedometer is off more than TEN PERCENT, and you don't have unusually "short" tires installed, then I would suspect that the speedo gear setup is not correct for the installed axle ratio.

Duke

Duke

Duke,
Thanks for the explaination and correction. I must have misinterrupt the
info I was reading.

Donald #31176

FWIW my 67 had a driven gear with 20 teeth when I bought the car with Brand X 205/75 redline radials. When I put on 215/65/15 Yokohama AVS tires I went to a driven gear with 21 teeth and the speedo now reads correctly (or so close to correct as not matter). Keep in mind that the exact same nominal size tire will have a slightly different diameter from one manufacturer to the next, so your mileage may vary.

The Tire and Rim Association promulgates tire dimension ranges for each nominal size group. Though there is some latitude, most manufacturer's specs are very close to nominal, and you will generally find well under one percent total range variation.

It is always best to use the rev/mile spec for your specific tire. It it's not available the next best bet would be to use a spec from the same manufacturer for a similar model tire.

If this won't work, just check out the specs for a handful of tires of the same range and use an average.

Also keep in mind that between new and the wear bars, tire revs/per mile will increase by about 2 percent, which will increase speedo readings by 2 percent, so if your speedo reads two percent low with new tires the error will decrease as the tires wear, but the speedo error will increase by an additional two percent at the wear bars if it starts out high.

Duke

Thank you! That is exactly the information I was looking for.

[QUOTE=SWCDuke]The Tire and Rim Association promulgates tire dimension ranges for each nominal size group. [QUOTE]

Duke, I recall this same conversation about four years ago when I put radials on my '63. My speedo is near correct as per that old thread. BTW, I like the word "promulgates". :D Keep posting. :cheers:

I thought green speedo gear had 22 teeth. So if I have a slow speedometer (40 mph but should be 50 mph) should I get a 24 yellow tooth or a 20 blue tooth?

From what I am getting out of the info above, you are 10 mph slow at 50 mph so you are 20% slow. Given that you need 1 tooth for every 5% difference, you would need 4 fewer teeth or 18. If that is not right someone please correct me.