as i was driving past one of the speed markers ( the ones that are battery operated and can run for a few good weeks and had the large digital speed reading as you go by them) left by chp or local police station, i noticed that my speed on my speedometer and the electronic did not corespond together? of course i was making sure, like everyone passing it, was going at the correct speed levels. well i noticed that i was showing 45 mph but the marker was showing that i was going at 40 mph . of course that looks good in front of the copper but isnt my speedometer supposed to show closer to the markers number? i then did it again by doing a u turn. again i tried a different speed and again it was off by about 5mph. i then looked for another loction for a different marker. i still have not seen another one yet but has anyone noticed this or is it just my car that is doing this? cause if its my car then i just racked up alot of miles that is not correct then.

When i usually go by them they usually have my speed down give or take 1mph. Do you have 19" wheels that can mess with your speedo.

A while ago, someone used a GPS system to check the speedometer and odometer calibration. The speedo was optimisic by 3-4 mph, but the odometer was accurate to the tenth of a mile over a pretty long distance(can't remember specifically). It is just a needle calibration issue, not anything to do with the measurement of the mileage.

On the contrary,

A good way to check the accuracy of a speedometer is to use the odometer. Usually this is done over a distance of 5 miles.

On another note, almost all car manufactures have speedometer error. There was an article in Car and Driver I believe on this subject, suprisingly GM had the best accuracy.

Personally I would trust my speedo before I would trust the speed marker. Depending on how much deflection there is between the radar signal and your vehicle, there can be a good bit of error.

I have checked my forester XT by comparing actual vs. theoretical, and it is within .5 mph
Later

P-ROD

Originally posted by kosoku
When i usually go by them they usually have my speed down give or take 1mph. Do you have 19" wheels that can mess with your speedo.


i have stock wheels and stock tires. nothing else has changed. i would raher have it read slower then im am actually going. i try to stay within the speed limit but i notice that everyone is literaly speeding by me. i also thought that the speedometer is in relations with the miles that is racking up on my car? or is there a different meter attached to the tranny?

For what its worth, my wife bought me a new handlheld GPS for Christmas (Magellan Meridian Platinum) and I use it in my 04 WRX wagon whenever I drive. The speedometer on the car and the GPS speed indicator are close to identical at all times. I've yet to check the odomter and the elapsed miles on the GPS, but I'm sure they will be close.

KK

I too have used GPS to check the Speedometer. It was accurate up to 125.

im 3 mph fast.

with snow tires 205 55 16 and even with my summer tires 205 50 16. its still 3 mph fast.

The only certified calibrated speedometers will be found on law enforcement vehicles :)

Jay