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[oobflyer]

I finally got around to comparing the speedometer reading to the GPS reading on my iPhone.
I found that there is quite a discrepancy, but I don't necessarily know which one to believe!

When my speedometer reads:

60 MPH, the GPS says 57 MPH
65 MPH, the GPS says 62 MPH
70 MPH, the GPS says 66 MPH
75 MPH, the GPS says 70 MPH
80 MPH, the GPS says 75 MPH

So, if the continuous top speed is 75 MPH... I should see 80 MPH on my speedometer... right? 

Anyone know if the iPhone/GPS is considered accurate? For comparison I clocked the speedometer in our Prius against the iPhone/GPS and found discrepancies also, but not quite as large (only 1 or 2 MPH).

[protomech]

The iPhone GPS is probably pretty accurate over 10+ seconds. GPS can have significant error, but that tends to balance out over a longer stretch.

Most speedometers are off by 5-10% .. I wonder if the Zero speedometer can be calibrated.

[Richard230]

The speedometer on my 2012 Zero S is off by about 8%, when compared with those radar speed-check signs that the cops like to install all over the place.  That is about the same error as the speedometers on my two BMW motorcycles.  As I see it, calibrating a speedometer to read faster than you are actually going is a lot cheaper than adding HP.  I note that my 2002 Yamaha FZ1, which is the fastest and most powerful motorcycle that I have ever owned, has a very accurate speedometer.

The goofy main speedometer on my GPR-S, which was adapted from the made-in-Thailand Boxer-donor chassis, had a little button on the back that allowed you to calibrate the speedometer to read anything you wanted. It also worked to calibrate the trip and main odometer - which is likely a violation of federal law.