Speedometer accuracy?

[Reach]

Driving back from the Florida purchase of my '05 M3 Competition w/19"ers I found it hard to believe the I 95 traffic was running 80-85 as the speedo said we were doing. Sure enought, per my son's GPS the car's speedo is at least 5 mph too fast. What I haven't determined yet is if the reading varies with speed. This is an issue since we live in Alexandria, and often drive thru Falls Church, both of which have main streets with 25 mph limits and police with radar hiding in driveways and behind hedges in hopes of nailing "speeders," assuming one can apply that term to someone doing 31 in a 25 zone.

Yes, on our cars it does. There were some metrics done on this a few years ago, and here were the results:

Indicated----Actual

40-----------38.2---1.8
45-----------42.9---2.1
50-----------47.6---2.4
55-----------52.3---2.7
60-----------56.8---3.2
65-----------62.2---2.8
70-----------67.0---3.0
75-----------71.9---3.1
80-----------76.5---3.5

I found mine to have perhaps a slightly higher delta, about 4mph at 72+ and 5mph at 81+. The point is, the ratio can be taken to be slightly off and therefor the difference is not constant. Below 50 MPH I take my speedo's speed to be accurate (and of course, its necessary to be conservative on northern VA roads due to enforcement) and at 'Highway speed' I shave 4 mph off to estimate actual speed.

Of course all this varies by model.