Mine has a 5% speedo error with stock wheels and tires. so, 2 mph at 40, 4 at 80, etc. I documented this by one of those radar "Your speed is ___" signs near my house. Now, my track tires are 18s, so that introduces another 2% error for a total of 7%, so when my speedo says 153 at Road Atlanta my gps tells me its really only 143.
2008 M3 Coupe / DCT
// StopTech BBK 380/355 / Pagid RS29 / Motul / Apex Arc-8 / BFG R1S / Ground Control / MS filter / Performance spoiler / BPM tune // Road Atlanta 1:39.70 / VIR Full 2:10.87 / Barber 1:42.20