speedometer runs fast

[mattsnow]

My speedometer always indicates that car is traveling 2 mph faster than that indicated by the speed signs on the highways around here. If it was only one sign, I'd blame the sign, but it is all of them, and it is a consistent 2 mph. Is it possible that the car thinks I have larger wheels than i actually do? What else might cause this problem?

[hyde]

Originally Posted by mattsnow

My speedometer always indicates that car is traveling 2 mph faster than that indicated by the speed signs on the highways around here. If it was only one sign, I'd blame the sign, but it is all of them, and it is a consistent 2 mph. Is it possible that the car thinks I have larger wheels than i actually do? What else might cause this problem?

This appears to be normal based on previous reports that you are going slightly slower than it is displayed on your dash.

I like it, and I don't like to be reminded of this actually. It can save me from tickets because around here with all these new speed cameras, they send you a ticket if you go 10 mph over the speed limit.

[mattsnow]

Thanks for response.

If the speedometer is running fast, wouldn't the odometer also run fast? Wouldn't this mean that our odometers indicate that we have more miles on them than we actually have?

[NVRfollo]

did you change wheel size? the speedometer is calibrated for one size wheel. when you go up or down wheel size, it affects your speedo accuracy

[twelve535i]

I was told BMW does it purposely. Maybe to make sure you're staying under the speed limit?
If you get your car coded, and choose to turn on the digital speedometer, you can choose for it to display your actual speed. At least that's how it works on my F10.

[MattBianco]

Mazda Miata did this, and designed their speedometer to bounce when it fell to zero. Mazda believed it would have a psychosomatic effect on performance. it "feels" faster. There is a maximum legal tolerance for speedo error, and I think it is 5mph at 55mph....automakers create the illusion of more performance. As to odometer being actual, I don't know, since the OBD2 port can tell you the real speed, I suspect the odometer is accurate.

[MysticBlue]

This is normal. It's not really 2mph so much as it is some percentage... probably around 3%. There are government regulations (mostly European, but some variation exists in every country) that say speedometers have to have a certain accuracy and describe it as being a certain percentage on the high side and zero percent on the low side. So if a manufacturer makes a speedometer that is 4 percent accurate (+ or - 2%) they are going to make it so it is always around 2% over so that none of them will ever read under. German cars generally can read anywhere from 0 to 5 % over. I use an Escort radar detector with GPS speed indication and my last 4 BMW's speedo's have all been 2-4% over. (Speed indication derived from GPS is highly accurate). And, no, you don't have to worry about the odometer. That has other regulations dictating how accurate they have to be, and comes from different measuring methods.

[Ashot12]

If you change wheels and overall diameter is smaller then the speedometer will show higher speed than you actually traveling, and the other way around. Do not confuse larger rim size with overall diameter of the wheel including tire.