Wheel size setting

[BlackC2]

I have an 02 996 Cab w/Tiptronic that came with 18 inch wheels and 17 inch was an option. My speedometer reads several MPH fast and I'm wondering if there is a setting that designates 17 or 18 inch wheels. When I plug in an OBD2 scanner, the scanner gives the correct MPH which is different than the display in the dash.

Any thoughts?

Thanks

[Hilux2400]

Do you have the right size tyres with the correct profiles?

[BTERRY1]

I have the same issue with recommended tire size.

Subscribed

[RFM]

A deviation upwards, of appr. 5% to the GPS speed, is allowed to manufacturers for legal reasons. Is not obliged, it is admitted by the European regulations to avoid legal proceedings and claims against vehicle manufacturers. It is not prohibited to calibrate the speedometer annually at a company recognized by the administration.

[cdupont0734]

I have noticed the same problem. My speedometer reads high by about 6% vs GPS. 17" or 18" rims with the factory tire sizes doesn't change the wheel diameter on account of lower profiles on the larger rim. My dealer says that by law speedometers can vary up to 10% in Canada.

I find the whole matter annoying on an otherwise fabulous car. I had a hunch that the speedometer might read correctly for the GT3 model. So I did a quick calculation using rear tire sizes of 315/30R18 for a 2003 GT3 versus 255/40R17 for the base Carrera. The circumference is 5.2% longer on the GT3! That would slow the speedometer pretty close to the actual speed.

You can install an app on your smart phone. Ulysse Speedometer Android/Samsung app, works great.

P.S.: Some years ago Honda was sued because cars were reaching the end of their warranty too soon because of speedometer error. They extended the warranties on 6 millions of their cars. I don't think its a problem for Porsche as most 911 warranties end on time rather than distance.

[BlackC2]

The signal output to the OBDII is the correct MPH and is different than the signal received by the speedo in the dash.

[Loren]

The signal output to the OBDII is the correct MPH and is different than the signal received by the speedo in the dash.

 

Sorry, but that is not true - in 996 mark I the wheel speed sensors go to the instrument cluster - which is where the OBDII gets it.

In the 996 mark II the wheel speed sensors DME and then go to the instrument cluster via CAN bus.