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.