[TMR013]

So in my 135, I quickly realized the the speedo read 3 MPH faster than I was actually going. After doing some research I found that seemed to be common on BMW's, and just assumed that it was a hidden safety feature they did on purpose.
Well in my M2 I assumed it was the same way, but recently realized that is not true. It tracks pretty close to actual speed, maybe 1 MPH faster. This was somewhat of a shock because I usually run about 10 MPH faster than the speed limit during normal driving to meet the unwritten 10 MPH leeway the cops typically give around here. And because I assumed the 3 MPH difference, I was actually exceeding it and lucky I didn't get a radar stop.
Wondering if this is an M thing, or if I was wrong in assuming the 3 MPH thing was common?

[DanG]

The M2 speedometer reads 1% high. If you go to the 255 275 tire combination it will read right on.

All car companies have their speedometers read high by a few percent. They don’t want to be sued for you getting a ticket while you thought you were going the posted speed.

[amancuso]

My non M BMWs read off about 3-5MPH, but both M's do not.

[champignon]

There are other threads on this and I'd suggest reading them if you are interested in this topic.

German cars (probably excluding VWs) are the worst at over-reading actual speed. I have had several VWs and 4 Subarus and all were spot on accurate.

As to BMWs with which I have recent experience:

My 2000 Z3M Coupe is consistently spot on, compared to GPS generated speed.

My (now gone) 135i coupe read about 2-5 mph fast, depending on how fast I was going; this corrected nicely (on the digital gauge only) using Carly for BMW to correct this. I also had a set of smaller snow tires and with those the error was different, but only by about 1 mph; I forget in which direction.

My 135is Coupe reads a 2-3 MPH fast but also was corrected on the digital speedometer only, with Carly for BMW coding;

My 1M is a couple of MPH fast, but the digital speedo also corrected nicely with Carly;

My M2 reads consistently about 2 MPH fast, but as far as I can tell cannot be corrected with coding.

Relatively recently produced BMWs, up until at least the 2013 model year, had a codable box that if checked would cancel the intentional speed over-reading by BMWs in the digital speedometers in North American cars; the analog gauge in these cars could not be "un-corrected." Supposedly, BMWs sold in other parts of the world could have their analog speedos "un-corrected," however I have no experience with that. If the F87 is any indication, either the coding software companies haven't figured out yet how to eliminate the "correction," or there is no such option in the chips in these cars, which may have been eliminated by the manufacturer.

There are standards for on speedometer accuracy for cars imported into the USA; all of the cars above and to my knowledge all others imported meet the standards, which are very lax as it applies to over-reading the speed, but do not permit the speed reported to be less than the actual speed the car is going at. You can look these regulations up with googling if you wish.