Speedometer calibration, well now driving on the road, 42 miles to work 30 on interstate, well I cruise at 80mph and the Georgia Troopers (GSP) will usually leave you alone. So had to make an Atlanta run and was moving along at speed, pulled up WAZE on my iPad to check traffic and when my speedo was reading exactly 80 mph the WAZE said I was going 87 mph! Now I had been pushing 90 on the I-285 loop (also known as the ATL 500!) so I am guessing I was close to the century mark, I am sure the GSP would have wanted to talk to me about that! So my question is can the speedometer be calibrated for my car? My 924S when I went to new wheels and tires last year I did have the speedo calibrated and it is spot on at 80 mph with my hand held GPS, but that car has a cable drive and I am guessing the 87 928 is electric.

 

you need to know what size tires in the rear you have as compared to the stock size,
from what I understand there are 3 different speedometer gears .
Once you get the same size rear tires as stock on your car then verify what the speedo is doing,
then you can pick a gear,
Call Roger, he has the them
To swap a gear ,it requires a pod removal,
and the speedometer dismantle,
then gear removal to count the teeth you have .

 

Your speedometer is reading 10% lower than your actual speed via GPS. Check the spec for the OE tires on your car, noting the rolling radius, or the circumference. Than measure yours. IF the numbers for your tires is 10% higher, then your speedo is correct for the car - the tires are the problem. Either get the correct size tires rolling radius or circumference, not bead size (which is 15, 16, 17, etc and has nothing to do with the distance you travel in one tire revolution.). Or, keep your current tires, and use your instantaneous 'mental math' skills to add 10% to the speedo reading!!
Stan is correct if you want to change your speedo gear - not a 10 minute job!

Hope you keep avoiding donations to the GSP fund.

Gary