I have seen it stated that the odometer/speedometer accuracy can be checked by comparing to the GPS. can someone explain how I would do this with my Prius' nav system?
Use the GPS in a Smart Phone or a dedicated GPS (Garmin, Tom Tom, ...). You need to have a system that you know is independent of the circumference of your tires. JeffD
Avoid curvy roads when you are making the comparison. A GPS can read little low on a real curvy road.
Note that the displayed odometer and speedometer figures will have different errors, so measure each separately. Don't measure just one and assume the other is the same.
Just a prediction: speedometer will read about 4% high (52mph when speed is actually 50, for example), and odometer should be quite accurate (assuming you have stock tire size).
My experience in a 2012 Prius Three was 1 mile high from about 30mph up. In my 2012 PIP its about 1.5 miles high from 30mph up. Checked with a Garmin nuvi 1450 which, by the way, is what I use for navigation. The Nav in the Three and the basic PIP isn't to my liking.
My speedo generally reads 1mph higher than my Garmin. Not sure about the odo. You'll need to have one of the trip odos displayed to see the tenths. On the same route I believe the nav & the nuvi show the same mileage. I will have to make a point of checking it on the way home.
International in the sense that the Euro regs apply to more than one country, not that it is close to universal international law. There is a Euro reg that the Europeans go by, you could call that international law because there is more than one nation involved. The US regulations are different and the Euro regs don't apply in the US. IIRC, in Europe the speedometers are required to exaggerate more than they are in the US
Here is a copy of the U.N. regulation that governs speedometer accuracy, it originally came out in the late 1950s. This is the 2003 version and there may be a later one but they are pretty much the same.
Do note the list of ratifying countries at the bottom of page 6. The 40 countries are heavily weighted towards Europe. No countries in North / Central / South America have joined this particular Agreement.
I use a mixture of My Tracks, GPS Test, GPS Essentials and GPS Odometer on my Android phone. The odometer is very accurate with OE tires and air pressure at 38psi on flat roads. The Speedo error varies by speed but average is about 1.5mph too fast +- .5mph. Since the speedo doesn't read in decimals it is hard to really say.
