speedo vs. trip computer - which is more accurate?

[Paul Meyers]

Calibrate the tachometer in top gear with a stop watch and interstate
highway mile markers. Easiest way is to set cruise and measure over several
miles then divide. Do this at 2 or 3 different speeds (say every 500 rpm).
Note the speedo reading at each speed. Plot these on a piece of graph paper.
You should get a straight line that intersects the x and y axes at some
point. The slope of the line is the gain, the y axis intercept is the
offset. On my cars, the speedos have correct gain, but are offset by some
number (from -5 to +4 mph). 

To calibrate the trip computer, set the cruise control to one of the
previously calibrated tach readings, engage the avg mph reading mode in the
trip computer, then hit reset. The first reading you get is the
instantaneous speed. You can leave it engaged to see if it changes. My trip
computers are within 1 mph of the computed speed (distance/time) at each of 
the three calibrated tach settings.

HTH, YMMV & LSMFT
Paul Meyers 
Gray 87 5KCSTQ 1.8bar eurolights fuchs 145 kM, replacing electronic ignition
loom 
Pearl 88 5KCSTQ 1.8bar eurolights fuchs bilsteins 128 kM 2pc-em k24 turbo 
Repainted Pearl 87 5KTQA 1.8bar eurolights TT wheels bilsteins 202 kM daily
driver
Blue 86 5KCSTQ parts car 


-----Original Message-----
From: Barton Oleksy [mailto:barto at nait.ab.ca]
Sent: Wednesday, June 20, 2001 1:18 PM
To: Quattro Mailing List
Subject: Q: speedo vs. trip computer - which is more accurate?


I've seen this now on all three Audis I've owned that have trip
computers ('86 5kt - 5spd, '89 200t, '91 200tq) - the speed listed on
the speedometer is *different* from the 'average speed' shown on the
trip computer.  It's been just on a road test where I've got the cruise
on, set to what the speedometer shows as a certain speed.  I then reset
the trip computer's average speed back to zero and it will quickly set
to between 2 and 7 km's HIGHER than the speedometer shows.

Which one is more accurate?  Neither?

Bart
Edmonton, AB Canada