If you look at the bottom of your speedometer, you will see a 3 digit
number. Probably 805. This is the number of revolutions needed to
register 1 mile. The odometer is directly driven. The 185R-14 tire turns
approximately 830 revolutions per mile. This is the first source of
error. As the tire wears, this error will increase slightly. The
speedometer error is further exaggerated by its calibration. Especially
in the 80-85 models. Many cars have the Speedos calibrated this way to
make us think we are driving faster than we actually are. I think VW
uses this to give us improved performance perception and better fuel
mileage! All of their models seem to have this design feature.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Richard A. Jones
Sent: Wednesday, October 24, 2001 6:48 PM
To: vanagon@GERRY.VANAGON.COM
Subject: speedo/odo error confusion

I just returned from a 3100 miles trip in El Jefe.  Well,
3100 according to its speedo, or rather its "odo"....

I've know the speedo reads high from some time.  Then
I watched the odometer between some mile posts and
decided it was high, too.  I thought it was gear driven
and should be OK even if the speedo was off because of
the calibration of the spring.  What do I know?

I have the stock size tires--185 R 14--Michelin Agilis 61s.

Then I decided to take advantage of I-70 which starts
from I-15 in Utah, with mileposts to the east.

Here's what I recorded
    my trip odo             I-70 milepost
       77.8                      0
      186.2                    100
      292.0                    200
      323.0                    230 (Colorado state line)

Converting to percentages:
       8.4%
       7.1%
       6.6%

So if I drove an arbitrarily large number of miles my
odo error would be arbitrarily small?  What's going on?
Why should the odo error not be roughly constant.

The only variable I can think of is the day started
cool and warmed up as I drove, but only cool/warm,
not cold/hot.

Can anyone explain this?

[Trip report and photos to come later....]

                                Richard A. Jones
                                Boulder, Colorado
                                '81 Vanagon   Mr Bus
                                '87 Syncro Westy   El Jefe