you know it's cold when.....
01-15-2004, 06:52 AM
#12
AudiWorld Expert
I think you're saying that your math shows that an 840 rev/mi tire is really close (appropriate)....
The Speedo and Tach are linked so long as the clutch isn't sliping. So I could put any size tire on the car and it wouldn't change the relationship between the speedo and tach. So rather, you've calculated that the right tire is apparently one that's close to 840 rev/mi 
If I had a GPS next to my speedo to see how fast the car was actually travelling, that would indicate how good of a tire size I have
If I had a GPS next to my speedo to see how fast the car was actually travelling, that would indicate how good of a tire size I have
01-15-2004, 07:18 AM
#17
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Yes, roughly :)
S4 seems to be one of the few cars that has an accurate speedo up top in relation to ground speed/rpm. Other cars/bikes apparently are commonly not accurate.
My own testing has shown it to be near dead on with a 24.8" tire, stock RE040s are 24.8" 840r/mi claimed. 245/40R17 kumho MXs are 24.8" with unspecced revs per mile.
Allowing for camera parallax, this was a dead on 120MPH speedo shot with cruise set from drivers point of view.
<img src="http://tychl.txf.com/dash-shot.jpg">
Horribly blurry laptop picture to back it up.
<img src="http://tychl.txf.com/craplapgps.jpg">
My own testing has shown it to be near dead on with a 24.8" tire, stock RE040s are 24.8" 840r/mi claimed. 245/40R17 kumho MXs are 24.8" with unspecced revs per mile.
Allowing for camera parallax, this was a dead on 120MPH speedo shot with cruise set from drivers point of view.
<img src="http://tychl.txf.com/dash-shot.jpg">
Horribly blurry laptop picture to back it up.
<img src="http://tychl.txf.com/craplapgps.jpg">
01-15-2004, 07:57 AM
#19
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The RPM <-> Speedo will always be correct
unless your clutch is slipping
The Speedo <-> Real Speed is what might be off, but yes, I've noticed the S4 seems to be right on.
-T
The Speedo <-> Real Speed is what might be off, but yes, I've noticed the S4 seems to be right on.
-T
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