Does my speedometer know I have short ratio gears?
 

I put in some short ratio gears this summer. When my speedometer said 60, my smartphone gps said 55. When my car said 70, my gps said 63.

My old bmw reports higher speeds than reality with a shorter ratio diff... I am suspecting the same for my 911. Just wanted to confirm as someone told me that the car would not be fooled. I am very skeptical. I plan to jot some numbers down and find the real speeds using gear ratio calculators... Assuming someone does not have a quick answer.

I dontparticularily want a falsly high odometer reading in a few years. I know in the old bmw world you could get new gears for the odometer.

Tire size ?

Stock

You didn't change your diff ratio, did you?

Changed it from 7:31 back to 8:31

I believe the speedo reads off the differential carrier bearing. Since it is essentially reading the RPM of the driveshaft, the only driver is tire diameter. Regearing the transmission or final drive should not affect the speedometer reading. So yes, it cannot be fooled by gearing changes.

Thanks... Mine just likes to exaggerate then.

Most speedos do exaggerate. Or, is your GPS off ;)

I've found that speeds on my cell phone GPS seem unreliable. I speculate that it's because they are determining location from a variety of sources, but I don't know for sure. If you have a regular automotive GPS, it might be a better baseline.

A friend of mine says that German speedos are always 3mph hot due to some German law that holds the manufacturer responsible for speeding tickets if the speedo reads low. I don't know if that's true, but I will say that the speedos in all of my German cars (and my friend's German cars) read 3mph hot.

My tire pressures were low. Smaller effective rolling diameter