How accurate is your speedometer
 

After noticing a lot of cars tailgating me, I thought I would check my speed with a GPS. I found the speed to be out minus 10 kilometers/6.2 mph when traveling at 60k/37mph. Do you think this is excessive and Is there any way to have this adjusted?

You can have your speedo calibrated. That seems pretty excessive, I wouldn't expect to see more than a +/- 2 mph difference.

Speedometer error
 

Do you have stock tires?

Wayne

I've got the standard 18's. It's a bit of pain having to calculate your actual speed to compensate for the error all the time.

I would take it to a dealer and say that its a brand new car w/ stock wheels and tires and have them fix it. That is very excessive in my opinion

Do u really think a signal from a satellite is going to give u a better reading than ur car? Go to the track n use a laser. That should be a lot more accurate.

Yeah they are spot on! I've got two GPS units that both measure my speed identically. They are also spot on when approaching one of the "Your speed is" signs. ;-)

Its usually a percentage, not an actual number that is off.

On my mini it was roughly 9% off.

the Z is a little better, at 7.7% .

I checked mine with my GPS and it is off about 1 MPH.

^ at what speed?

70 - 85 MPH (or 71-86 MPH, however you look at it :p)

My Z is right on, perhaps just a small fraction off at highway speeds. No more than 1 MPH at 60 though.

Again, you need to study up....:shakes head:

"Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems, perhaps 0.5mph accuracy in typical automobile applications would be what you can count on."

trust the gps

According to all the local radar stations, my speedo is dead on.

Hmm GPS is not necessarily accurate. Depends on how many satellites, etc you are receiving. Likely less than normal unless you have a roof antenna. The greater the distance traveled at a fixed speed will also increase the accuracy.

I think the standard 18'' rear tyres is a bit smaller than the 19'' ones..

How would you go about calibrating your speedo? For OCD types like me, I would like to have it recalibrated if I were to get different tires/wheels.

Mine seems pretty accurate.....its my cruise control I'm worried about.

It seems that it can fluctuate a lot. I've been like 6 mph over going down a hill.

mine simes to be about 2 mph off from my GPS, i dont lose sleep over it.

A possible reason for the error is Nissan my have calibrated the speedo to suit the 19" rims and not the 18's. Does anyone know if the rolling diameter is different for both wheels?

the GPS speedometers are usually pretty accurate unless you are accelerating hard or slowing down in which case, there is some lag in the calculation.

I had a portable Garmin and used it in 5 different cars (sometimes I drive my parent's cars) and it always seems like the car speedometer is about 1.5 - 2 mph faster than the car.