'84 911: Inaccurate Speedometer (faster you go, worse it gets)

[sneader]

Hi all!

This problem is with my '84 911 Cab.

While the odometer does not work (I realize a common problem), the speedometer works, but is not accurate at higher speeds. The faster I go, the more inaccurate it becomes. While it's pretty close to right at, say, 25 mph... by the time the speedometer says I'm going 70 mph, the real speed is actually closer to 60mph (based on GPS)

Could this be caused by having the wrong tire/wheel size? I have 16" Fuchs wheels.

Thanks for any advice.

- Scott

P.S. I would like to fix the odometer, too, but I see plenty of info on that fix.

[universeman]

Send it to North Hollywood Speedo. $100 and it comes back working. Odometer as well as calibration. Plus it's warrantied to keep working.

I know you can buy the gear and take it apart yourself, but why risk ruining it? The needle removal is (I hear) a real PITA.

[Lapkritis]

If your tires are too small then that can be the cause. What size do you have on there?

[sneader]

@universeman: Thanks for the North Hollywood Speedo recommendation. Sounds like a good plan!

@Lapkritis: I have Bridgestone Potenza 225/50R16 tires on it.

[gliding_serpent]

Not to hijac, but for this era of 911, will a shorter ring and pinion change things for the spedometer? Or is is measured in a way that will not be effected?

[Lapkritis]

Same size on front and rear? I see the tirerack sells a different size rubber for the front and a tire rotation might find those on the rear by accident. The 2255016 appears to be correct for the rear wheels where the speedo pickup is.

[sneader]

@Lapkritis: Rears: 225/50R16 Fronts: 205/55ZR16