[200q20v] Speedo help.

[hah at srv.net]

>
> My last '91 Avant,(oem rims and 215/60 tires) always
>measured 72mph when the rev counter  was at 3000 rpm's in
>fifth gear. In addition, when I would reset the average
>speed button on the computer, the initial reading always
>corresponded to the reading on the analog speedo gauge.
>
>The newer '91TQA  (identical rims and tires) measures 77mph
>on the speedo gauge at 3000rpm's in 5th gear. But when I do
>the average speed reset on the computer, it reads73mph--4
>mph lower.
>
>Could it be that Audi put 2 different rear ends in these
>cars?? I remember reading an ongoing thread several months
>ago where Chris Miller and another lister were contending
>3.89 vs 4.11 ratios as OEM??
>
>Would you guys run the 300rpm 5th gear test and let me know
>what speedo and avg speed reset readings you get? In the
>mean time, I'll hit the Mass Pike with a stop watch over
>some measured miles.

Are the tires the identical brand on the new car as on the old car? 
Actually, that is only relevant to any real road speed difference, the 
relationship between speedo and tachometer is fully determined by the 
transmission and differential gearing. I think that it is far more likely 
that there is a 5% difference (call it manufacturing variance) in the 
speedometers of the two cars than two different ratios in the differentials 
(although 3.89 to 4.11 is suspiciously close to 5%).

I use the Bentley's overall 5th gear figure of 25mph/1000rpm for highway 
cruising in my car, because 3k rpm reads nearly 80mph on my speedo though 
the trip computer is quite close to 75mph at 3k rpm (also verified with GPS & 
stopwatch/mile marker). I also wonder about the accuracy of the highway 
mile markers, apart from the designated "speedometer check sections" which 
are presumably more precisely placed. Averaging over many markers is good 
but might be hard in traffic.

HTH
Henry Harper
http://www.srv.net/~hah
1991 200 quattro, 112k, not going to drive it until the AC is fixed
1988 GTI 16v, 218k, not as fun to "calibrate" speedo w/GPS :)