BMW speedometers

I recently read in a popular BMW monthly magazine (I think it was Bimmer but I'm not sure) that BMW speedometers commonly read 5-10 percent higher than the car's actual speed. Is this correct, and if it is, why would BMW intentionally calibrate their gauges to an incorrect value? I had noted a difference between the indicated speed for my 2007 335i coupe and my wife's Honda Accord, (with my car showing the higher value) but had assumed that it was the Honda speedometer that was at fault, in light of BMW's reputation for accuracy and precision. Also, if the BMW speedometer reading is incorrect, is the odometer reading incorrect also?

This is true, and on purpose.
If I remember correctly, (it's been posted several times before) there is a law in Germany where the manufacturer can be held legally and financially accountable for mechanical variations in MPH. (If someone is busted for speeding and can show that the car speedometer is at fault) As a result, BMW simply calibrated their speedometers to do exactly what you point out to eliminate responsiblility. I'm pretty sure VW does the same thing, a friend has a Passat. If you run a portable GPS in your BMW, you will see the difference clearly, the gap grows as you speed up. I believe they also may have added a larger gap in purpose than you might think necessary to also compensate for changed wheel & tire sizes by the owner.

I've also read that Miatas intentionally read 5 mph faster for the simplest of reasons - per Mazda: give the impression of going faster

The Miata's I have had were usually pretty spot on with the speedometer. The Z4 on the other hand is about 3-4 miles faster on the speedometer than the GPS. It's probably a decent safety factor, but I wonder about the "ghost" miles on the odometer.

The mileage on the odometer reads correctly. The speedometer is the only inaccurate measure.

+1 That is easy to verify. Just watch the mile posts on the highway. Checking over about ten miles will give a more accurate check, to cancel out the human error of pressing your trip odometer and "highly motivated" DOT workers who placed the mile markers.

Good to know!

Next time I'm on the road, I'll have to remember to compare the speedo number with the speed shown on the Nav.

Compare your odometer to the mile markers on the road. They are more likely to be correct, especially over a large distance. Compare over 10 or 20 miles if not longer.

Calibrate it as Grant told you. The most wildly inaccurate odometer I ever owned was in an old Audi Quattro. It was 5% off (registered 10.5 miles for every 10 miles traveled).

OTOH, you could have failing odometer gears. These are replaceable.

If it helps any, these are cheap (usually under $100) and you can do it yourself. The supplier is Odometer Gears (Google it - and tell Jeff I sent you.) and they are club supporters. If I am not mistaken, they will be at Oktoberfest, as they have been the past few years.

Speedo issues

I had the same question this past summer and here is what I got directly from the tech reps, this should clear any speedo/odometer concerns you have...

http://bmwcca.org/forum/attachment.php?attachmentid=1834&d=1252613253

Just paste this link or if it doesn't work let me know and I will send you the PDF file...

Have a good one!!!

Not to cast doubt on your findings, but how "scientific" is this?

I'm not doubting you, but I have to wonder why people are being so obsessive about this? I also see several instances where you want to be "scientific" but at the same time, you're choosing "fudge" words. Especially when I am seeing words like "about," "almost," "averaged," "i can account for 'a lot' of this mileage," etc.

First of all, not to discredit or cast doubt on any of your findings, but there's enough of a "fudge" factor for GPS to toss out the GPS numbers entirely. Do a search on "GPS errors" and you will find a surprising amount of "fudging" for GPS accuracy to make it a surprisingly questionable instrument for accurate numbers.

The average accuracy for GPS is within 65 feet of your true position, but if you can PROVE yours is significantly more accurate than that from a distance of 12,550 miles (the current altitude of GPS satellites) -- watching you prove it should be a very entertaining exercise. 65 feet out of 12,550 miles is still a remarkable amount of accuracy, but it's by no means a perfect system. If you disagree, I'd love to watch someone hit the exact geometric center of a 65 foot wide target from a distance of 12,500 miles with only a glass scope, a rifle, and 20/20 vision.

Accurate enough for 99.99 percent of consumers to find their location? OMG yes. Accurate enough to shave the hairs off a tick -- or in this case, overcome a 11.2 percent speedometer error? Not likely. 65 feet is still 65 feet, my friend.

http://search.yahoo.com/search;_ylt=AmT34Pv3CUmzmfwC6CrAKjybvZx4?p=GPS+error&toggle=1&cop=mss&ei=UTF-8&fr=yfp-t-947

I'm more inclined to take your word for the mile marker distances than I am for your GPS accuracy, without the need to buy a 10-mile long tape measure.

While we're shaving copper off pennies and splitting hairs, did you account for any mileage deducted while backing up, parking, stopping for gas, trips to restaurants, as well as compensating for the earth's rotational speed while slowing for a traffic light or a stop sign, etc?

Little stuff adds up. A fraction of a mile here, a couple more fractions of a mile here.

BMW never intended the vehicle odometer to be a scientific-precision instrument. If it were, we'd have aerospace quality sensors that would measure distance in INCHES, not MILES. That and every car sold with these diamond precision odometers would have to be sold for 10 times what they sell for now just to pay that sort of microscopic precision.

Wouldn't that mean our cars would have to be SWISS to be imported, and wouldn't the inside of an odometer look more like the guts of a Rolex watch than a circuit board?

You mention that you have to keep track of your mileage for business purposes, and that's understandable. But do you really think the IRS is going to come after you if your vehicle odometer is off by 11.2 miles out of every 100 traveled? I'm inclined to think that the IRS has far larger fish to fry than worrying about ONE odometer (yours) being off by 11.2 percent.

Either that, or you really are that obsessive-compulsive that this is keeping you up at night.

One more thing: mechanical speedometer gears are not likely to help you since the 2007 335i coupe doesn't use a mechanical odometer, and quite likely, your wife's Honda Accord doesn't either, unless it's an EXTREMELY old model (pre-1990).

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

I imagine that in the 70's when BMW was designing the E30 they had the immense foresight and vision to know that the E30 would be the most robust and thus, best, BMW chassis in its history ever, and, secretly engineered the speedometer AND odometer to be exactly as innaccurate as you have observed, with the express purpose of having those who would be maintaining these cars both spend more money, for maintenance, sooner, than would otherwise be the case AND substantially extend the road presence of the cars, in so doing.

Very difficult to prove, however, despite corporate espionage, company moles, and bribing secretaries to shuffle through files for incriminating memos when nobody's looking. At least you know you're changing your timing belt 11.2% sooner than need be, but really, isn't not risking an engine rebuild worth it?