jox

Had a GPS unit in the car during a road trip this weekend, and noticed that while going 50 km/h (~31 mph) on the speedo, I was infact doing 60 km/h actual speed. Whoa! Good thing there weren't any speed traps!

When it reaches 100 km/h - it is spot on with the sat nav, and as it climbs up higher it becomes inaccurate the other way.

Is this common? Can I even adjust this somehow, seeing as it's speed-dependant?

CraigyB

everry speedo I've checked vs a sat nav, the car speedo reads high 5-10%

kurquhart

Mine is ~10% fast at high speeds (135mph actual vs. 150mph indicated). I never checked it at low speeds, however.

Tom W

Your speedometer uses a certain tire circumference to compute speed, if your tires are different (and they are always changing due to wear) your speedometer will be wrong.

jimq

My 964 isnt off by much but my 996TT reads about 3-4 mph faster then actual speed.

Akerlie

Dont know what setup Porsche's have but i know that Toyota speedos show approx 6% to much while the odo counts correct distance.

IIRC EU directives say that the speedometers are allowed to show 10% to much to 2% to little.

The percentage should not vary from 50 to 100 km/h, i would think that your gauge is failing.
Try to contact a speedo shop, they should be able to point you in the right direction.

Don't know where in Sweden you are but there is in good shop in Kongsvinger, Norway, just across the border.
No affiliaton.

mpeters951

Ohhh..........speedometer; I'd thought maybe you were having a coverage issue with your beachwear

seg1959

On my 964, today, I got:
GPS........Speedo
72..........78
61..........66
I wouldn't know anything about higher speeds, of course :-)
So speedo was about 9% over real speed.

seg1959

Indycam

Jox
Your speedo has a linearity problem .

Colin 90 C2

My speedo is within 1 mph of my gps, at all the speeds I checked.

springer3

Road and Track used to check speedometer error with every car report. Every Mercedes is spot on. My 964 and most cars are about 5% too high on both speed and odometer. I suspect this is deliberate because warranties expire sooner, you are more pleased with you fuel economy, and you think your car is faster than it really is.

jneteler

Mine too...

But weirder yet nothing agrees.

I timed the Mercedes over a 10km course as measured by the roadsde mile markers and distance vs. time and speedo and GPS all disagree by significant amounts. All readings vary by more than any possible stop watch timing errors. If the time delta is converted to distance then the error is several hundred meters. The car was running on cruise control and never varied by more than 1kph from the set point according to the digital speed readout.

The Mercedes is about 4kph higher than the GPS while the 964 indicates about 6-7kph high.

I finally decided that this was the automotive equivalent of a woman asking, "Do I look fat in this outfit?"

JNeteler

Fred's 964

Maybe that's how the manufacturers met the Mile per gallon mandates forced upon them by Congress. If we think we are getting better mileage, everyone wins. Let's have a group tree hug.....