The speedometer on my 2013 Persona shows high by 2 mph. Is this caused by the 17 inch wheels? I wonder if the dealer can reprogram for this?
No. My car is also 2 mph high with standard 15" tires. This is a standard for all manufacturers to not be on the low side. The car actually knows the correct speed for other purposes but displays high.
This is about normal. You will find many speedometer error threads here on PC. The systematic bias in the high direction is an industry norm, and is (indirectly) required in some areas. An error spread is a fact of life, so the typical displays must be biased high so that the low end of the distribution does not violate local laws forbidding any car to read lower than actual speed. European market cars typically have a larger bias than North American market cars. Odometers do not have the same systematic error, at least these days, because it leads to class action lawsuits for warranty fraud. If you install an aftermarket engine monitor such as ScanGauge, you should find that the internal speed known to the engine's computer is much more accurate. Only the dashboard display contains this systematic bias.
There are quite a few speed readouts set up by law enforcement these days. My experience after many "tests" is that the Prius reads about 3-4% high at 60mph, while both my Audi and Volvo are basically spot on. Perhaps some manufactures make their speedometers read high because they know that's what US drivers expect, while other manufacturers are obsessed with accuracy.
The bias also helps with maximizing MPG, as people are driving slower than they think. This with the optimistic MPG readout makes a lot of people happy.
