sg530

NOW, a new question regarding speedometer accuracy.
Had a friend check his handheld GPS against Nav in car. His GPS showed vehicle speed was roughly 2 mph less than my spedometer in the 30-60 mph range. I have used GPS for speedo calibration with excellent results. I will verify with my own GPS soon. But is my speedometer as much as 4% off? Has anyone else calibrated their spedo?
The reason I ask is related to another problem I foresee that nobody in this forum has addressed... how to verify speedo accuracy when changing wheels. It seems half the people in this forum are changing wheels or tires and no mention is made of how the speedo is affected. Could it be that it is always off on everybody's car? How about computed mpg accuracy? How about getting tickets in places where they stop you for going less than 5 mph over?
I asked dealer regarding speedo calibration after tire substitution and they said they never had to do it yet. Manuals say nothing either. Thought I'd ask this group before going through BMW channels.

heezy545i

Many BMWs have speedometers that exaggerate mph, but this usually occurs at higher speeds.

For maintaining accuracy with upgrading tires or wheels, try to keep to the recommended sizes to keep the same radius and diameter.

This site may be helpful http://www.powerdog.com/tiresize.cgi

GarryF

I was finding 2-3 mph difference with hand held too, didn't seem to matter what speed you were going at.

buddy

Yeah they are set that way at the factory from what i am told. I got a ticket just because and i knew i was not going that fast and had the spedo checked, 2 too 3 miles off slower than the actual reading i was told that this is like on all bmws

sg530

As an engineer, I expected more precision from the car...especially in the speedometer. I am disapppoited to hear someone say all bmw's are like that. I have calibrated my friend's cars and my old ones using gps and even though there was a mix of manufacturers, I never had as big as a 2 mph error. My friend's toyota was dead-on. Another point that I have verfied is that the error is roughly constant (as noted by another here) from 35-65 mph.
I also showed the cruise control might fluctuate by 1 mph on the bmw speedometer but the gps showed considerable and rapid fluctuations in the 0.5 mph range and occasionally as much as 0.7 which caused the speedo to click up or down by 1 digit. I am not complaining at all about the cruise control. It is quite good and the best I have seen. I am merely showing the presision the gps provides.
It is the 3.5 to 4% error I am upset about and in my mind out of line with the german engineering precedent.

skidster

which, of course, means that your 0-60 numbers, well, aren't your 0-60 numbers