It is all big brother... the government doesn't want speeders, so they make
the car manufacturers install speedometers that read high on purpose.  Hence
you can do 70 in a 65 zone and "get away with it" as you are really doing 65
or 66 MPH.

The best way to measure speed is with a GPS - very accurate, both in the
speedometer and odometer departments.  The speedometer part can easily be
corrected by adjusting the needle - pull it off and reposition it the amount
the error is at 65mph.  The odometer will require a different cog at the
pickup.

My Syncro with 205R14s installed was spot on speed wise and a little low
odometer wise (2%).

David Marshall

Fast Forward Automotive Inc.
4356 Quesnel Hixon Road
Quesnel BC Canada  V2J 6Z3
mailto:info@fastforward.ca <mailto:info@fastforward.ca>
http://www.fastforward.ca <http://www.fastforward.ca>
Phone: (250) 992 7775           FAX: (250) 992 1160

. Vanagon Accessories and Engine Conversions
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-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com]On Behalf
Of Richard A. Jones
Sent: October 24, 2001 3:48 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