Are all Speedometers off by 10% or so ?

A few people have made this claim to me over the years, that for saftey reasons all speedometers overestimate your speed by around 10% (so if your speedo says 66 MPH your actually doing 60). I was always a bit skeptical (couldn’t really see the point myself), but on a recent road trip, I tried testing with the aid of a cruise control and very long straight desert roads (with mile-by-mile markers), and it appeared to be the case, at least, for my (pretty new, in working order) car.

So whats the SD ?

I’ve checked my spedometer (2004 VW Passat) on a straight and level road against a very accurate portable GPS device (which shows current speed). It was off by only a few mph.

YSMV.

Sorry, and by “a few mph”, I acutally mean “1 or 2 mph”

Like you, on a boring drive, I once used police mile markings to test the speedometer of the car I was in. It too was spot on.

I’ve calibrated every single car I had against radar devices of some sort while maintaining 55 MPH.
Out of 6 or so cars, I’ve NEVER had 10% off.
The most I’ve ever seen was 2 MPH off. I saw 2 MPH high, I saw 2 MPH low, and my Chevy Caprice was SPOT ON, but I’ve never seen 5 MPH off, which would approach the 10% mark.
Check out BMW. Supposedly they do what you heard about… intentionally making the car optimistic.

I’ve got a Magellan GPS navigation system/copilot in my car. It always reads a few mph lower than the speedometer. The variance increases with speed. I never know which to believe, the GPS, which isn’t deadly accurate via position, or the speedometer, which is physicall connected to something. After reading this thread, I guess I’ll trust the GPS.

I have a Garmin StreetPilot III GPS and a 1999 Toyota Sienna. At a speedometer reading of 75 MPH the GPS consistently shows about 74.2, which is a -1.067% error.

Anecdotal, but 10 percent is a huge error for a speedometer.

Over the years I’ve had a whole mess of cars, and frequently checked the speedomenter. Often they were 2-3 mph off. The last few cars I have had (in last 5 years or so) seem to be right on at all speeds.

Our town puts out a portable radar detector trailer with large readout to warn speeders at various places. My current two cars have always been dead on, which I’ll admit surpises me, especizlly the '97 Mercury Villager (which is really a Nissan Quest). Not that surprised by the '01 Jag, though.

When they are in a 35 mph zone, I’ve pumped my bicycle as hard as I could, but could never get it over 30 mph going by the radar detector. Oh, well…

Actually,the little computer speedometer on the bike was surprisingly accurate too. Maybe i’ve just been lucky.

I rode with someone once who’s speedometer was off by quite a bit - but then, he had just had his tires changed, and gotten the wrong size.

Many people with Honda Shadow motorcycles in the USA say the speedo is 10% high. Probably a safety/liability issue?

The older mechanical speedos tended to be inaccurate at the extremes and fairly accurate at mid range. 10% is a lot and I think you would only find that kind of error at the extreme high end of the reading. I don’t know if this applies to electronic speedometers, or not. As has been mentioned, tire size can dramatically affect speedo accuracy.

BMW motorcycles seem to be optomistic by a similar amount. In many cases the odometer will be more accurate, or even off in the opposite direction.

Many cars with electronic speedometers have a calibration procedure available.

Bicycle computers are just counting wheel revolutions and comparing against their clock. If the clock is accurate, they’ll be accurate to the degree you were accurate in entering the wheel circumference when you set up the computer.

How do you go about calibrating a speedometer? I didn’t know that was possible.

Do you have those radar unit automated roadside speed warning signs anywhere around? They show the speed of each car as it passes by. That is a cheap way. A police friend could also help.

My speedometer does read low by quite a bit. I believe it is about 5mph at normal driving speeds. I derived that from the signs. It also helps me explain why I seem to be speeding everywhere I drive and I have never gotten that type of ticket as an adult.

I calibrate my speedomoter against a cop’s radar periodically and it is usually spot-on. Mind you, this is a very expensive way of calibrating.

Just take it on faith that the reading at upper part of the band is probably correct.

The Automobile Association of New Zealand did a study on this about 8 years ago, amidst a rash of people insisting they’d been clocked by speed cameras for speeding, despite being positive their speedo said they were obeying the speed limit.

End result: Most speedos were only off 1-2km/h, with the worst offender being the Mistubishi Chariot, which was OVER by 10km/h at 100km/h… so, if your speedo said you were doing 100km/h in a Mitsubishi Chariot, chances are you were actually doing 110km/h!

You don’t see many of those roadside speed displays here, unfortunately- I think they’re a great idea, though.

So you don’t actually recalibrate the equipment, you just make a mental note that it’s off by 3 mph or whatever it is. Right?

Oh, when I say that, I mean I set the cruise at 55 MPH and drive past the “YOU ARE GOING THIS FAST” machine the cops set up to make you aware of your speed headed into a construction zone, etc.
I then note any difference, and make a mental adjustment when choosing my speeds.