Date:         Wed, 5 Mar 1997 20:01:05 -0500 (EST)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Re: RESULTS - Tire Survey

In a message dated 97-03-05 13:56:34 EST, mdstockr@neo.lrun.com (mdstockr)
writes:

> I am just going by tire diameter, not odometer set-up.
>  
>  I calculate tire outside diameter to be approximately the 
>  following:
>  
>  	185R14   25.9"    0%
>       195/70R14   24.7"   -5%
>       195/75R14   25.5"   -2%
>       205/70R14   25.3"   -2%
>      27X8.50 14   27.0"   +4%
>          225R14   28.5"  +10%
>  
>  Now, all mechanical speedometers are typically 
>  conservative by approx. 2%.
>  
>  So, with 27X8.50 14 tires, I would expect the speedometer 
>  is reading 2% slower than with 185R14.  At an indicated 65 
>  MPH, that's 1 MPH or an actual speed of 66 MPH.

This assumes that a 27" tire is really 27" and that the virtual rolling
radius is the same as the static unloaded radius of the tire.  In actuality,
a 27" tire is about 25.7" to 26.3" unloaded diameter depending on the
manufacturer.  Then, load the tire with the weight of the van and start
driving and the tire's shape can deviate a lot.  Other variables are
inflation pressures and the width of the rim used.  Tires are measured by
manufactures using a particular rim width.  Wider rims will diminish the
overall diameter of the tire.

About the only way to know for sure is to compare the actual odometer
readings with a known distance driven.  Using your math above the "right"
tire diameter for a Vanagon would be about 25.9".  But even then, the "right"
diameter would depend on rim width and tire pressures.  

Speedometer error is not directly related to odometer error.  The odometer
error can be eliminated and you may still have speedometer error.  The
odometer has a direct mechanical link to the road and can be adjusted by
changes in tire size, inflation pressures, and the gear ratios in the
odometer drive.  Speed is more or less "inferred" from the speed of the cable
which throws more variability into the speed measurement than the distance
measurement.  Speedometer error is also not necessarily linear.  Many
speedometers will read less accurate at low speede and actually be more
accurate at higher speeds.  One of the car magazines (Car & Driver I think)
includes a table of indicated speed vs actual speed in their road tests.
 Most new cars are really very close to the actual speeds and usually the
indicated speed equals the actual speed.
 
Regards,
Jim Davis
87 GL Syncro
88 GL Wolfsburg