Why BMW speedo overshoots but the other speedos don't

[besiktas]

I was driving the glk and i was surprised how the speedo was showing the exact speed of a gps speed monitor tool. Unfortunately, in my car, the speedo is off a bit. Then gps shows 77, the car shows 80. Today, I was watching the c&d video of x1 and the video mentioned how this is an issue for BMW and not for other cars. Anyone knows why?

Link to c&d review:
http://youtu.be/eoqinOfudzY

[Rich_Jenkins]

First BMW?

http://en.wikipedia.org/wiki/Speedometer

"The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.

The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h."

[Parkanzky]

Volkswagen have historically had very optimistic speedometers, although our most recent '09 and '10 TDIs actually read about 1 MPH slow.

[Rshane]

My Audi's do the same thing...3km less than what's indicated on the speedo.

[bdaddylo]

BMW programmed a factor to display speed higher than actual due to Euro laws what requires speedometers to never read below actual speed. This factor can be coded out.

[1MOREMOD]

know issue. quite a shock at the track once i started getting gps data that showed i was slower in the straight than i had thought.

[besiktas]

Interesting article from C&D - Speedometer Scandal!

http://www.caranddriver.com/features...ometer-scandal

[jphughan]

OP do you have 18" or 19" wheels? I'm guessing 19 by the pic in your sig, but in addition to the fact that BMW does indeed calibrate their speedos to display slightly faster than reality (though this can be disabled for the digital speedo via coding), wheel size makes a difference too. I don't know whether the factory adjusts the speedo calibration based on the wheel diameter you ordered, but if they only calibrate for one size, it would be for 19" to avoid risk of the speedo displaying too low when the car has 18s -- and if that's true then people who have 18" wheels would have their speedos showing faster by an even greater margin than those with 19". Of course even if the factory does calibrate differently for the different wheel sizes, if you switch sizes aftermarket all bets are off. Relative to the stock calibration setup, larger wheels will make you go faster than indicated, smaller wheels (like people who switch to 18s on the track) will make you go slower than indicated.

[1MOREMOD]

Originally Posted by jphughan

OP do you have 18" or 19" wheels? I'm guessing 19 by the pic in your sig, but in addition to the fact that BMW does indeed calibrate their speedos to display slightly faster than reality (though this can be disabled for the digital speedo via coding), wheel size makes a difference too. I don't know whether the factory adjusts the speedo calibration based on the wheel diameter you ordered, but if they only calibrate for one size, it would be for 19" to avoid risk of the speedo displaying too low when the car has 18s -- and if that's true then people who have 18" wheels would have their speedos showing faster by an even greater margin than those with 19". Of course even if the factory does calibrate differently for the different wheel sizes, if you switch sizes aftermarket all bets are off. Relative to the stock calibration setup, larger wheels will make you go faster than indicated, smaller wheels (like people who switch to 18s on the track) will make you go slower than indicated.

most people are sticking to similar outside diameter tires when switching. my 18 set up is .1 inch different than my 19 inch set.

[mdosu]

Originally Posted by besiktas

Interesting article from C&D - Speedometer Scandal!

http://www.caranddriver.com/features...ometer-scandal

rofl, did you see the comment written on this article? lol

"My BMW 335 is off by 5 mph at 80. That is almost 6%. So when the vehicle gets to 50,000 miles, the odometer, and the NADA value, will think the vehicle has 3,000 more miles on it than I actually drove. When speedo error is so far off that it effects your resale value, that seems a bit excessive."

[1MOREMOD]

Originally Posted by mdosu

rofl, did you see the comment written on this article? lol

"My BMW 335 is off by 5 mph at 80. That is almost 6%. So when the vehicle gets to 50,000 miles, the odometer, and the NADA value, will think the vehicle has 3,000 more miles on it than I actually drove. When speedo error is so far off that it effects your resale value, that seems a bit excessive."

i had this the opposite way with my last truck. had 35s on it and so registered lower miles than i drove by a big amount.

[Car54]

That's the craziness...BMW knows exactly how fast you're going and all the computer's know it as well. With coding you can have it be actual.

They just choose to change the speedometer for the safety reasons. Mostly all motorcycles are like this too.