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      05-22-2010, 02:42 AM   #34
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by gr8000 View Post
The figures posted are an approximation of the true speed i.e. the 302km/h is my "desktop" approximation of the true top speed of a 6MT M3 (assuming no electronic speed limiter). Here is my methodology:

First I calculated the theoretical top speed following this:

The theoretical top speed for the M3 I came up with based on the above (assuming 19" 265/35 tyre) is 315.4km/h for the 6MT and 335.4 for the DCT.

Then what I have done is to apply a 15% "tyre distorition factor" (which basically is a manual factor adjusting for tyre compression at higher speeds) in order to come to an approximated true top speed. The 15% figure came by comparing my theoretical speeds at redline for each gear of the DCT versus the actually measured true speeds for a stock DCT as these were measured by GPS equipment from a reliable Greek auto magazine. I was happy to see that my 15% correction factor was uniform accross all DCT gears and yielded the same true speeds as those measured by the magazine.

Then I applied the empirical 15% distortion factor to all gears of the 6MT.

The true top speeds I came up with are 302.3km/h for the 6MT and $321.4 for the DCT (assuming no electronic limitation).
Hmmm.. Not so sure about your method. The 15% drivetrain and 10% tire losses sound just about right. The 10% tire loss is speed dependent but at about 180-190 mph 10% is probably very close. The only other possible corrections are tire growth and indicated speed vs. true speed. I'm not so sure about the 15% empirical correction factor. Of course you must know tires expand at speed, not shrink.

Coincidentally our calculations (mine based solely on hp (with losses) and drag alone) agree quite closely. Your 302 km/hr is 187.6 mph, I get 186 mph.