MPH display - Scanguage vs dashboard

It could be the speedometer is calibrated slightly higher than actual speed.

For example Germany have a law that states the speedo must show 7% more than actual... so 100mph actual = 107mph indicated.

I have the same problem here in the UK.. GPS (3 different types and a radar gun - used on a local racetrack) show the actual speed at 3.5mph lower than speedo indicated speed.

I'd prefer it to be spot on to be honest.. especially with stock wheels and tyres.

Anyone know how to calibrate the speedo ?

 

I doubt it's a calibration issue. The display shows what the OBDII data shows. My Scangauge II reads exactly what the speedo does. It could be a time delay if your speed is changing, or it could be roundoff errors.
I -think- you can change how the Scanguage II calculates speed, ie, enter corrections. Perhaps a correction has been entered into it (for mileage correction)?

 

I have a GPS which also reads 1.5 to 2 MPH less than the dash. Im pretty sure all Prius' are calibrated high.

 

My GPS (a Garmin GPS V) reads about 1 mph lower than the dashboard display on my Prius at 66 mph indicated on the dash, about 65 on the GPS. (original tires at about 15,000 miles)

As to German cars reading low by law: if true in Germany it seems not to be borne out on my US models. My 1987 BMW 325i and my 2002 Audi A4 both read very, very close to my GPS. 7% high is many times the margin of any error they have. My 1975 2002 predated my GPS, but until it had a failure it did not read anywhere near that much too high either (calibrated on freeway mile markers over many mile stretches).

 

This kind of questions were raised many times.
http://priuschat.com/forums/care-maintenance-troubleshooting/52888-speedometer-calibration.html

The speedometer reading is intentionally biased to meet the UN ECE Regulation.
http://www.unece.org/trans/main/wp29/wp29regs/39rv1e.pdf

Japanese NHW11 meets old Japanese regulation.
Japanese NHW20 meets the UN ECE regulation.

Ken@Japan

 

The odometer is another story.
My observation is the odometer is very accurate, within 0.5% error.

Ken@Japan

 

Not that it's much more than echoing the opinions already posted, but I'll weigh in. My Jetta was 3-4 MPH over at 70 with a stock tire size, until I changed the 225/45-R17 tires to 225/50-R17 tires. As I expected, now it's right on, according to the little radar stations that the troopers set up near construction zones. Oh, and because of the size change, my odometer now reads 100 miles for every 105 traveled, so that was right on before.

When bringing home the Prius and talking to my wife (following in the Jetta) the Prius was reading 1-2 MPH over the Jetta, i.e. 1-2 MPH over actual.

Scientific? No. Groundbreaking? No. Just another data point.

~ J

 

According to the specs Ken provided...

 

FWIW, this is seemingly a pretty consistent phenomenon. The speedo in the TCH does the same thing. When the gauge is indicating 80 mph, the GPS (Magellan) says 68. My SG will be coming soon, and I expect the same from it.