Are all Speedometers off by 10% or so ?

[LSLGuy]

A dealership can calibrate a sppedo to make it read corectly. Just expect to pay a bit for the service.

[R.P.McMurphy]

@CynicalGabe

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.

To some degree the GPS is reading point to point while the speedo is reading rotation of the drive train. Therefore, any curves in the road (or possibly changes in elevation) will be minimized by the GPS while the drive train is just reading rotations. That will explain some differential in the readings.

[CynicalGabe]

@Spartydog

To some degree the GPS is reading point to point while the speedo is reading rotation of the drive train. Therefore, any curves in the road (or possibly changes in elevation) will be minimized by the GPS while the drive train is just reading rotations. That will explain some differential in the readings.

Agreed, but usually when I compare them, I’m going down a straight highway. My speedo will say 80, and the GPS will say 76

[engineer_comp_geek]

@TJdude825

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

On some cars there is a hole where you can insert a small screwdriver and turn it to adjust the speedometer. I’ve only done it on one vehicle so I don’t know how common this is. One of our resident mechanics could probably give more details. After making an adjustment you can time your speed using mile markers and see if you adjusted it the wrong way.

The vehicle I did this on was a pickup truck, and had changed from 14" to 15" tires which threw the speedometer off by maybe about 10 percent. The mechanism clicked when I turned the screw. Two clicks adjusted it back to the right speed.

YMMV and all that.

[Pork_Rind]

This article explains why cars, espcially European models, often have some degree of speedometer error.

So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.

The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s.

There’s your explanation of high-reading European speedometers, with the highest readings on Porsches and BMWs that are most likely to lure owners inclined to fool with tire sizes. Of course, only the speedometer must conform. Trip computers are free to report average speed honestly. Try setting your BMW or Porsche cruise control and then resetting the average-speed function. Unless you’ve screwed up the tires, the trip computer should show a nearly accurate reading. Even General Motors, whose domestic speedometers are the best, must skew its readings slightly high on vehicles exported to Europe.

My car, for example, consistently reads about 7.5% higher than actual speed. Other owners report a very similar figure. Sadly, there’s no way to adjust the reading as it’s all software driven now. The odometer is spot on though.

Motorcycles are especially bad for some reason, to the point that most of the magazines report the amount of error at some given speed as part of the review data. One particular bike that I owned was off by more than 8%. No idea why, other than maybe to give guys something to brag about. 150mph sounds much more impressive than 137.

[Roderick_Femm]

@Pork Rind

My car, for example, consistently reads about 7.5% higher than actual speed. Other owners report a very similar figure. Sadly, there’s no way to adjust the reading as it’s all software driven now. The odometer is spot on though.

This is a question I’ve had - how can the odometer and speedometer be different? Don’t they run off the same information (number of revolutions of the wheels or some such)? (Naive non-mechanic here, if you hadn’t guessed).

My odometer measures about 10% higher than reality, based on roadside mile markers, and I’ve always assumed that the speedometer was following suit. I haven’t been pulled over for speeding yet while using this assumption. My car is a '97 Kia Sportage, and the odometer appears to be mechanical.

Roddy

[Pork_Rind]

@Roderick Femm

This is a question I’ve had - how can the odometer and speedometer be different? Don’t they run off the same information (number of revolutions of the wheels or some such)? (Naive non-mechanic here, if you hadn’t guessed).

In the case of most of the more recent cars, the the gauges are electronically operated. The engine computer counts the pulses from the rotation sensor and then commands the speedometer to display a calculated value. It’s easy to increment the odometer accurately and then, for example, indicate the current speed plus 7.5% with the needle. Guys trying to reverse engineer the ECU have figured out how to display whatever values they want with a computer plugged into the CANBUS connector.

Wouldn’t be too hard to do mechanically either. The spinning cable goes into the back of the gauge panel and turns the odo via a small gear. Use a slightly different gear for the speedo and there you go.

[Raygun99]

I got dinged twice for speeding tickets by speedometers that were off, in two separate cars. In both cases, I was actually speeding, but likely would not have been stopped if I’d been going the speed I thought I was.
Once I was clocked at 132kph when I thought I was going 120, another time at 115 when I though I was going 110. I was able to get the first ticket reduced by proving out that calibration, though with the cost of it, it amounted to the same. It was nice to know that it was off by 10%, though.

[Schuyler]

@CynicalGabe

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.

While your position estimate may bounce around a bit (depending on how much filtering is going on inside the GPS unit), the velocity estimate should be very accurate - as long as you are travelling straight-and-level, the Doppler measurement to the various GPS satellites gives a velocity estimate that is good on order to 0.1 kph (cite).

[Suburban_Plankton]

Our Land Rover Freelander consistently reads about 10% high, as measured against my GPS. From what I have read, this is typical of the model. I always assumed that it had something to do with the fact that it is a British made vehicle that was “adapted” for the US.

It still has the hood release under the right hand end of the dash, on the (for the US) passenger side.