Odometer / speedometer accuracy

The odometer should be reasonably accurate, there's a lot riding on that, warranty expiration for example.

Speedometers invariably read higher than actual. How much higher can vary, country by country I think. The intent with that discrepancy is to get people driving a little slower, and ensure you're never being told you're driving slower than actual speed.

15" vs 17" outside diameter variation is small enough that it gets "lost in the noise". 17" is actually the smaller of the two.

 

Some fun with numbers: If a 17-inch wheel registers 60 kph, then a 15-inch wheel will register 53 in the same car.
Circumference (Pi x D) of the 17 is 53.38 inches, and circumference of the 15 is 47.1 inches. Multiply 60*47.1, then divide by 53.38 to get 53 kph. Tire pressure and remaining tread will also have an effect.

 

... except that's not exactly how that works. The 15-inch and 17-inch are the wheel dimensions. The tires go around that. So the actual circumference of the tire is what you need to use, which is wheel + 2x tire sidewall:

15-inch wheel uses the 195/65R15 tiles. So the total diameter is 15inches + 195mm x 0.65 x (1 inch/25.4mm) x2 = 24.98 inches
17-inch wheel uses the 215/45R17 tires. So the total diameter is 17inches + 215mm x 0.45 x (1 inch/25.4mm) x2 = 24.62 inches

So the difference is 1.5%

 

I have 15s, the dash shows 55 mph but obd2 says 53mph on flat road. Radar shows -2 mph also, no wonder other drivers hate prius, they always drive 2 mph slower on roads & highways

 

How would those morons notice that difference when they're going 95+?

 

Where I drive it’s 2 lane country side and trucks are they’re not able to do that here. I drive the speed limit at 57 mph to be a true 55 mph.

 

I drive 10 over and they pass me on the right shoulder, even with 2 or 3 lanes on my left.

 

Speedometer in miles the US has a very small error. Speedometer in km/h in Europe has about 10% error. Yes, that is true and it is hardwired in the display assembly. Toyota knows the exact speed (that is: the exact number of revolutions the tyre makes) but due to regulations in Europe they choose to add this error.
There are two rules: the indicated speed should be higher than the actual speed and the indicated speed should be lower than the actual speed times 1.1 plus 4 km/h.
And lots of car manufacuturers sit around that 10% error. And that irritates the hell out of me because it is not hard to show the exact speed. My 60 year old Peugeot displays the correct speed so why can a new Toyota not do that.