Several months ago I had my GPS in my '84 911 and was comparing the speed on the GPS to the speedometer. Way off the Porsche was 10 mph faster than the GPS. 0-40 was same after 40 I was gaining 1-2+ mph per mile, by the time I was up to 90 the GPS was at 80. I have checked the GPS against other cars and it is dead on. I have 255/40 ZR17 wheels on the back. But was assuming the speedometer was at fault and not the gearing, because I thought if it was the gearing on the speedometer it would be incorrect at all speeds? Any thoughts. Also the speedometer does not start working until you get to about 22-25 mph, otherwise it just sits there. Thanks for the help.

 

Porsche is known to build in an average of a 5% error, although they allow for up to a 10% error within their standards. For example a car doing 70 will exhibit 73+ on its speedo. Add to that the fact that you have 17" wheels and tires on a car designed for 15" tires and you are adding an additional factor of about 6%. So that would explain 90 being 80 on the GPS. I have the same issues on my car and find it frustrating that the speedo is so much faster than actual speed.

 

this old calculator thing is pretty cool http://www.miata.net/garage/tirecalcold.html
255/40/17 is still very close to stock diameter.
there's a tire wheel expert floating around here somewhere.

 

I thought I read in Rennlist before...I did a search, but came up empty, that there was a plastic gear that could be changed in the speedometer to correct the tire situation. Does anyone remember that and does it work? Or does anyone have the link to that thread? Thanks

 

Mine is off by about 10%...I have stock wheels and tire sizes. The speedo can be recalibrated by a vdo speedo shop

 

The odometer is driven by a plastic gear. Thats the fix when it stops working but I have never heard of that for the speedometer.

 

Yep, there is a fix available. Supposedly the guys at www.odometergear.com can determine the required # of teeth if you provide them with several other factors, i.e. tire size, rear gear ratios, current # of teeth on the gear in question, etc.

Personally, I'd rather not take my speedometer apart, I'd prefer to send it in to a pro, regardless of who or where.
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