what is the stock tire diameter for accurate speedometer readings?

[chatas17]

My 84 911 has CS2 wheels (17").. The other day when I was driving 80 (according to my speedo) my friend had his cruise control set at 73 mph. So... I assume the tire diameter is creating the inaccurate reading. How do I correct this issue? Is there a plastic drive gear that "plugs" into the gear box? Any thoughts on what is the correct gear? Hey, at least I have fewer miles on the car than I thought! That's a plus!

Thanks!

[Oh Haha]

The standard tire diameter for 911s is about 25inches, give or take. From what I remember of your posting you have the correct sizes for tires to remain close to that. Different brands of tires will be closer, others not so much.

I think what most have found is that Porsche was a little optimistic in the speedo calibration.

Mine's in KPH so I use my GPS unit to know exactly how slow I am.

[Neel]

I've found that Porsche speedometers read about 5 mph faster than your actual speed.. When my odometer died, I sent it to North Hollywood, gave them my tire size and asked them to calibrate accordingly.. It 'seems' more accurate, but I have not verified it with my GPS..

[RoninLB]

ny 1977 has an internal adjustable pot

speedos changed over the years so a pot may not be in an SC or Carrera

[chatas17]

Thanks guys!