Speedo error
 

I have about a 10% speedo error, I'm reading a higher number than I'm actually going (verified by GPS). I know what you're all going to say-- wrong size tires, well, they're the right size, per the tag in the engine compartment. Why oh why would I get the wrong reading then? The tag is for the optional 16" wheels with 225/50 ZR tires, what is on the car (Pirelli P7s). I just bought the car and didn't realize the speed difference till just recently. Any ideas would be greatly appreciated. By the way its an 86 Carrera. It does have a chip in it, but isn't the speedo still cable actuated not computerized?

Thank you

Porsche was known for having the speedos be off by as much as 10%. If it bothers you it can be fixed. Do a search on speedo repair or, contact North Hollywood Speedo or Palo Alto Speedo. I've heard favorable reviews on both of the above.

Jeff:

Porsche speedos have been optomistic for 30+ years,...:)

As Joe said, it can be fixed by measuring how far off it is and sending the speedo to North Hollywood Speedo for correction. The speedo is not cable driven; its triggered by a magnet-sensor setup on the differential case of the transmission.

I drilled the case and tweaked the itty bitty pot screw. I posted a pic and the routine awhile ago.

Mine is dead on five miles per hour over in just about all cases. Always has been...

Tha big question though... How off is the mileage if the speedo is incorrect, or are they two different actions, not related to one another???

I believe German law requires plus 0% / minus 10% error tolerance...and Porsches have always been way-over on the safe side of error...meaning, high readings compared to actual speed....almost always spot-on 10% high reading. Done "on purpose".

Oh yeah.... provides a bit more maintenance income for those German shops too.....you tend to bring the car in more often then..:)

Wil

Geof- the speedo and odometer is one operation, at least on the older cars with cable driven speedo's. There's simply a gear (which often strips and leaves the odometer stopped) that goes off the speedo to turn the odometer. So if your speedo was off 10%, well so is your mileage.

My speedo is also off about 5mph on my 74 (cable driven), can I adjust this to make it spot on?

Can anyone confirm the final drive ratios for us so that we could guage speed by RPM?

Not for cable driven speedos

http://forums.pelicanparts.com/showthread.php?s=&threadid=83337&highlight=speedo+ drill