Originally Posted by clived' post='331260' date='Sep 5 2006, 03:38 PM

There is no "exact" difference - every time your wheels make a revolution and lose a molecule of rubber, the delta will change ever so slightly.... so unless you're going to have a wheel-rotation based speedo that calibrates itself constantly, forget having it accurate.

Originally Posted by monacobmw' post='331136' date='Sep 5 2006, 02:08 PM

Totally agreed here.
I understand that the 5-Series is about 3-4 mph off actual speed at 60mph. You can check it with GPS.

Has anyone figured out the exact differences?

Originally Posted by Hopco' post='331271' date='Sep 5 2006, 07:03 PM

My '06 530xiT has 18" OEM wheels and 4 x 245/40-18 tires with approximately 7k miles on them.

I recently ran a check vs. a GPS: at 70 MPH - the GPS read 68 MPH - or about 2 mph optimistic.

However, when I check my auto's speed against those 25 . . . MPH speed check radars located by local cops about our towns, my speedometer checks out as a bout 4 MPH fast. Perhaps the local cops tune their radars to read 2 MPH slow???

When we drive from Colorado to Connecticut next week I plan to do a series of speedo vs. GPS checks over a range of speeds (30-to-90 in 10 MPH intervals) and will post same.

Originally Posted by JDN' post='331274' date='Sep 5 2006, 04:07 PM

Mine checks out 2 mph fast at 60. This does not vary much through 75. Hence with a speed limit of 70, error of 2 mph, and a state law that allows 10% above the posted limit before getting points against the DL, I set the cruise at 78.