Originalsly: 80kph/50mph is what it is ....but when it comes to speedometer readings why does it vary so much in trucks, cars made for the North American market and cars made for the Japanese market? eg..if a Toyota Corolla from the US and one from Japan are both clocking 100...the one from the US would actually be pretty much driving faster. Am I wrong? If not why is this so?

Siena: There are variations, not based upon the country they're sold in, speedo's tend to over-read. These are deliberate error margins built into the automobile.

So we have actual speed, and indicated speed. For example, When the speedo's indicating 70 mph, the actual speed is between 63 and 66 mph. The error margin increases the faster the automobile travels. So the error margin is smaller at 20 mph and greater at 155.

Fitting different size wheels will alter the error margin, either + or -, depending if the replacement wheels are smaller or larger than the factory offerings.

yungboss:
I dnt know how true the GPS is, i've actually checked my speed at 140km/h and 160km/h using the cruise control setting of my car. It was exactly the same as the GPS numbers measured...i think there's a level of accuracy with the speedo...

Originalsly: @Siena...I hear you on the margin of error and if the wheel size is changed. But my confusion is with the indicated speed....brand new car...Toyota Corolla... from the North American market and brand new Toyota Corolla from the Japanese market...if driven side by side at the same indicated speed the NA version would be travelling faster. I am now thinking the NA cars have larger wheels.