Speedometer accuracy - reads faster than real mph?

It wouldn't surprise me. See Speedometer Error about 1 MPH?--Some evidence | PriusChat, Speedometer Error about 1 MPH?--Some evidence | PriusChat and a few posts that follow.

 

If anything the speedo is going to read high. A GPS unit is a better reference than an uncalibrated sign.

 

Maybe eco mode tells ya you are going faster than you are so ya let off the gas... Hehe jokes

 

I believe it is about 5% over-reading. GPS nanny on the screen verbal warnings is what I go on.

 

Can't speak for the US, but Australian regulations (ADR 18/03 specifically) implement the European Standard, UNECE Regulation No. 39/00.

The practical upshot of this standard is:

• Speedometers must never show a speed less than the vehicle's true speed. There is no acceptable tolerance for showing a speed lower than the true speed.
• Acceptable tolerance for displaying a speed greater than the vehicle's true speed is 10% + 4 km/h.

Two practical examples:

• If a vehicle's true speed is 80 km/h, the acceptable readings on a speedometer are from 80 km/h to 92 km/h.
• If a speedometer shows 100 km/h, this could mean the vehicle's true speed could be anywhere between ~87.3 km/h and 100 km/h.

I've yet to cross-check my speedometer against a GPS, but I did recently time a run past a series of odometer check mileposts with the cruise control holding 110 km/h. I missed the first one, but recorded 2 minutes 16.35 seconds to cover 4 km. That equates to an average speed of 105.6 km/h. You need to allow some tolerance for the accuracy of my use of the stopwatch, but this was the kind of result I expected.

 

My speedo typically reads 1-2 MPH high compared to GPS.

 

My C reads 1-ish mph faster than the Scangauge or traffic radar.

 

My prius C speedometer is very close to "truth". I have concluded that my various GPS and public RADAR displays are pretty accurate. My speedometer matches very closely.

I am thinking that the prius comes with a couple of different wheel and tire size combinations. Perhaps this would explain minor differences?

 

The final circumference of the tire/wheel combinations are the same.

 

Most speedometers read high. The obvious examples are German cars and motorcycles, which are known to read up to 10% high [e.g. 77 indicated, 70 actual]. American and Japanese speedometers are typically closer to accurate. I wouldn't be surprised in the slightest if the Prius C reads a little bit optimistic.

 

All Prii do this. The GenII was 2mph too fast and the GenIII is between 1mph and 2mph too fast depending on the wheels and tires that came on the car. The 17" wheels show a larger error than the 15" wheels due to the 17" tires being shorter.

 

mine reads 1 MPH higher then GPS on my C