[net.auto] speedo blues

bentonh@tekid.UUCP (Benton Holzwarth) (08/19/83)

    I've noticed a problem with my speedometer. It reads about
15% high. That's more then can be accounted for by switching
from the stock 185/70 tires to 205/60's which seem to be about
a 0.2 inches shorter in radius. Any ideas? Are speedos normally
adjustable, or do you just buy a new one? And how much do you
normally have to replace? Just the guts, or the entire dash
assembly? When they go, do they often stay linear in their error
or will it skew off at a certain range?

    And locally (Portland/Beaverton Oregon)...  does any one
know of a place I could get it calibrated, once I've got it
fixed, or retweeked?

    The car is a '79 BMW 320i. And yes I stand behind my
statements that BMW's are reliable cars. It's fair for one
or two parts to go sour at 90k miles.

    Benton Holzwarth
 ...tektronix!tekid!bentonh

    Tektronix, Oregon

4563eab@whuxk.UUCP (08/20/83)

The article about the speedometer reading 15% high sounds interesting.
I too have a '79 BMW 320i, and I too recently
switched to 205/60R13 tires. But my speedo now
reads about 10% too low.  Of course, most of that
error is due to the smaller radius of the tire as
compared to the stock 185/70R13 tires

                       -Chip Baines
                       whuxk!4563eab

wookie@alice.UUCP (08/22/83)

Speedometers are typically driven off the output shaft of the
transmission (some are taken from one of the wheels).  The only
thing that will affect thaese the transmission takeoff type is tire
size, rear axle ratio, or the speedometer driceve gear.  Since
the tires do not account for the cghange and I assume you haven't
changed the rear axle to a different ratio then maybe the
gears in the transmission are wrong.  Have you had the transmission
changed lately?  These gears are usulyyally easily chengesd without
removing the transmission from the car.

However, assuming that none of these things have occurred, then
the speedometer itself must be going bad.  The thing works by
having the speedometer drive either a fluid or friction clutch
in the gauge itself.  If the clutch should increase in friction
for some reason then the raeading will increase.  I have seen
them wear out the other way where the reading drops.

a Tesa testA test for this would be to compare the speedometer with the odometer.  
If the clutch is at fault then the odometer should
still read accurately so find a source of known mileage
and see if the odometer agrees.  If it too reads high
then I think you had better take another look at the
tires, rear axle ratio and transmission speedo gears.

Let us know.......things like this are intrigueingfascinating

				Keith Bauer
				White Tiger Racing