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Re: Tire Rolling Radius

To: Carmods@aol.com, tigers@autox.team.net
Subject: Re: Tire Rolling Radius
From: Bob Palmer <rpalmer@ames.ucsd.edu>
Date: Sat, 27 Mar 1999 08:14:50 -0800
John,

This is a slick way to calibrate your speedo/odo, but I think it needs a
bit of clarification. In the first iteration, it makes no difference how
fast you drive the ten miles; just note the odo readings at mile zero and
at mile ten to get the distance traveled according to the odometer. Then
use your formula to ratio the distances in terms of the number of gear
teeth. If you drove for ten minutes at exactly 60 mph according to your
speedo and used these odometer readings, then you would have a problem. For
example, let's say you just got your speedo back from calibration and you
want to see how accurate it is. You drive at a measured 60 miles per hour
for ten minutes and note that you have gone exactly 10 miles on your
odometer. Therefore, you conclude, everything is OK. WRONG!! This just
means the speedo/odo was calibrated correctly. You would get this same
result no matter what gears you were using to drive it.

Once you have your odometer consistent with the mile markers, then you
check the speed calibration; e.g., a mile per minute equals 60 mph.

Bob

At 09:14 AM 3/27/99 -0500, Carmods@aol.com wrote:
> Because of unknown rear end ratios, tire growth at speed, compounded errors
>in measuring, or wide allowable tolerances in tire manufactures data, the most
>direct way of determining the proper number of teeth in the speedometer cable
>is to actually test it. Assemble the car with the gear you now have and drive
>it on a highway that has mile markers. Drive at about 60 MPH for 10 miles and
>record the mileage as close as possible. Then use the following formula to
>determine the number of teeth needed on the driven gear.
>
>       Teeth on your present driven gear X Miles recorded/10 = number of teeth
>needed on new driven gear
>
> as an example: If you have a 16 tooth driven gear and you record 12 miles
>during the drive the calculation  is;
>
>       16 X 12/10 = 19.2 
> You would therefore use a 19 tooth gear or what ever is close. 
>
>After you check out the new gear and the odometer is measuring the miles
>correctly, drive at 60 MPH and you should pass a mile marker once a minute. If
>there is a speed error than the speedometer needs to be adjusted. If it's not
>too far off you may be able to adjust the needle return spring.
>
> As you have been discussing, there are many formulas that can be used to
>calculate the ratio but they are all subject to the accuracy the input data.
>
>                             John Logan

Robert L. Palmer
Dept. of AMES, Univ. of Calif., San Diego
rpalmer@ames.ucsd.edu
rpalmer@cts.com

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