Speedometer Accuracy?

[Stove]

Originally Posted by bkrownd

This is the way speedometers are designed. They typically read 3% or more faster than the true speed.

And there is an allowable tolerance of error during manufacture. Federal law is +/- 10% for Speedometers I believe. Most car makers keep it a lot closer to nominal. Some will read spot on and other will read a little high. It's really rare to find a stock car that has a speedo that reads slow. The requirements were set up around the old cable driven speedometers and have never been updated to line up with today's more accurate systems.

On a side note, there was some speculation a few years ago that car makers were manufacturing the speedometers and the related hardware so that it always read faster than what the car was going. This runs the warranty out quicker and before the car hits high enough actual miles to end the warranty. For example, if your speedo reads 5% higher than actual speed then your Odometer is also reading the miles 5% faster. On a car with a 36,000 mile warranty, at 5%, the car has only actually traveled 34,200 miles when the Odometer reads 36,000 (OP's post indicates around a 4.5% variance so he has approximately a 34,400 mile warranty). Obviously, the higher the mileage limit on a warranty, and the bigger the variation form true speed, the more beneficial it is to the manufacturer to have the speedo read higher than actual speeds and register miles faster than it should.

[bkrownd]

Originally Posted by Stove

For example, if your speedo reads 5% higher than actual speed then your Odometer is also reading the miles 5% faster.

That isn't necessarily true. The speedometer needle could easily have an offset that makes it different from both the odometer and reality. (It's possible the offset is smaller and/or more linear than in the old days, though.)