Functionality of speedo measurements?

[M3Racer]

Does the speedometer get its data from the rotations of the front axle, the rear axle, or some other way?

The reason I ask is that I'm looking at tires for when my RAC Monolite 20's arrive. I'd like to run Michelin PS2s.

Available sizes:

front:
255/30/19 Overall Height: 26.1"

255/35/19 Overall Height: 27.1"

rear:
295/25/19 Overall Height: 25.8"


Stock is about 27.0".

I'd prefer not to have any more speedo error than we already have and running around 26" will give an additional 3.7% or so error.

So, here's what I'm thinking...if the speedo works off the front axle somehow, maybe I'd get the 255/35/19 fronts to keep the speedo in line and get the 295/25/19 rears since they'll lower my gearing a bit and not mess up the speedo accuracy further.

If its on the rear axle, then I might have to choose some other brand. I wish they had a 305/25/19...still some error but not as much...or 305/30/20 which would give 0.7% the other way.

[M3Racer]

anyone??

[Second Childhood]

I can't say with any assurance where the M5 takes its speedo measurement, but most vehicles take a feed off transmission near the end to monitor actual revolutions going to the rear differential. If this is, in fact, the case with the M5, the rear tire circumference would directly effect the accuracy of the speedo reading. My experience with my M5 is that the speedo reads consistently 10% higher than actual speed with standard tires. I am planning on looking into how to adjust the speedo reading to obtain a more accurate reading. If the current setup is entirely an digital vs. analog setup, this may be more involved.
This may be more simplistic than what you were looking for, but it seemed you were looking for some response.