Speedometer Error

[huck387]

My 2005 Echo has huge speedometer error. I didn't know this until I got a GPS. The error is greater in more speed. So I was going 65 MPH on the GPS's Speedo, while the Car's Speedo was 70 ish. Yes the error is THAT big. The tires are the same size as the ones that came in stock.
What can cause this?

I also threw the GPS in my Prius and the error was 1 or 2 MPHs off. So the GPS is accurate.

Thanks

[Radioactive0110]

Do you have the right size tires? :P I know that is the easiest way to mess up a speedometer. If not it might be an electonic problem.

I know I srewed up my spedometer by taking the needles out to change the backlight color. I reset it by seting the cruse control using my gps, then sticking the needle back onto the unit at the right spot while I was driving. Better if you have a friend to help you on that one tho.

[ericlp]

it's normal for a car to be off 3-5 MPH matter of fact they always over clock the speedo 3-5 MPH faster so that it makes us all a safer driver.


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.

But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage.

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.

So there you have it: the raw, unvarnished truth about speedometers, laid bare without the underhanded aid of secret pyrotechnics. Readjust your comfortable indicated cruising speeds accordingly.