If you take a look at the Bently manual, you will note on page 90.30 ,

"...Starting May 1983, the only requirement [of the Department of
Transportation] is that the vehicles' speedometer indicate a higher
speed than is actually being driven.
...starting with December 1984 production.  These speedometer heads have
a tolerance of (+3) miles per hour."

When the DOT bureaucrats dropped the 85mph speedometer rule, they
instituted a new rule that required all speedometers read faster than
actual speed.  All in the effort to keep people from speeding.

Todd
'88 Westy

"Richard A. Jones" wrote:

> 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