Speedometer accuracy...

I recently noticed that my BMW speedometer is not in line with my Garmin Speedometer, they seem to get further off the faster I go. I spoke with my local bmw mechanic about this and he told me it was because I have larger wheels than the stock car had; but didn't offer any suggestions on how to recalibrate the factory speedometer.

What can I do to fix this and get accuate speedometer readings? I was thinking I might get a diff. gps unit and have it slaved to wheel spin but that still doesn't fix my instrument panel readings. Any ideas?

The speedometers seem to be a little off anyway, even with factory tire sizes. My car (Z4 M Coupe) is +6 mph indicated at true 80 mph. I think yours is +3.

Pretty much all BMWs overstate their speed by a few MPH. If nothing else, you get pulled over for doing 85 instead of 90!

GR

Pretty much all cars are like that USDOT regs state they can be off -0% (cannot read low) but + almost anything (at least they used to.) Most manufacturers try for -0+10%

No laws against it being accurate, but the law prohibits them reading low from the factory.

Actually larger diameter tires will understate your speed, not over state it.

GR

Good point. He did not say which direction it was off. You should be running an aftermarket computer for your purposes anyway!

Your right! I was unconsciously reading between the lines and assuming he was referring to overstating the speed.

The funny thing is, on my 1988 M3 if you set the max speed alarm on the OBC, it is not in sync with the speedometer. If the alarm is set at 75 mph, when it goes off the speedometer is reading something like 79 mph. So I am guessing the alarm is more accurate. But I have not done any tests or anything to verify and I have not tried the max MPH function on my E36 to compare.

GR

I have found the OBC to be closer to the true speed on every car I have timed in the measured mile. Like I said, the speedo generally reads low because it was programmed that way. That way it could cover small differences in the rolling radius of the tires and the change in rolling radius due to speed without going against any DOT regs.

I got this from John Rosen, Washington State Department of Transportation:

"I researched our RCW's and WAC's and found very little on speedometers. The reference I did find had to do with commercial vehicles having a speedometer that is reasonably accurate, but no definition of reasonably accurate. Sorry could not help more."


I got this from Frank Corrado, Federal Lands Highway:

"I don't know what may pertain to private vehicles, if spedometer accuracy is tied to odemeter accuracy, or if the performance specification is limited to the manufacturer's specifications, but here's FMCSA's reg concerning spedometers for buses and trucks, dated Aug 15, 2005 from their website: 393.82 Speedometer. Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)."

And finally this: "Sounds like the question should be sent to staff at NHTSA", whom I have asked but received no answer as of yet.

Any word from US DOT? They are the ones that dictate what can be imported.

They referred me to NHTSA!

I did some checking and the -0% + whatever is actually a German law. The manufacturers would be held accountable if they read low so that you were unknowingly speeding.
The FHLA is part of the DOT, but does not necessarily write or know the rules. The NHTSA is the part of the DOT that does, that is why they sent you there. I haven't got time to look right now, but:
http://www.access.gpo.gov/nara/cfr/waisidx_06/49cfr571_06.html

So you have been waiting over a year for a response?

There was an article in Car and Driver dealing exactly with this issue in detail. You can find it at http://www.caranddriver.com/reviews/hot_lists/high_performance/features_classic_cars/speedometer_scandal_feature

Thanks for this article.

Does anybody have any idea on what equipment one can buy to obtain a more accurate reading? I've head you can slave a GPS to wheel spin but I've called around and most people have no idea how to go about doing this.

Thanks you everbody for your input so far!

Why do you ask that? No, but I have been waiting awhile...

if you want actual speed from your BMW, hit the the OBC button for MPH, and reset it to see what it is computing. you can do it on the fly...

I've actually done this, but it is still at odds with the speed that my GPS unit gives. I feel like the GPS has to be accurate although it sucks at giving directions so who knows.