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Posted by: matthepcat Apr 21 2016, 09:05 AM

Like the title of this thread states, what would cause my speedometer to read a higher speed than I am traveling?

Is this a mechanical issue with the factory gauge?

Posted by: Jeffs9146 Apr 21 2016, 09:08 AM

Smaller diameter than stock tires.

Posted by: tomrev Apr 21 2016, 09:34 AM

REALLY smaller diameter!

Posted by: era vulgaris Apr 21 2016, 10:55 AM

Wow, that is off by quite a bit. Mine reads about 8-10mph faster than I'm actually going. I'm running 205/55/15's.

Posted by: matthepcat Apr 21 2016, 12:16 PM

Duh! For some reason my mind was thinking that the speed was collected at the transmission and is somehow decoupled from the wheel/tire diameter.





Dumb question of the week complete.


Yes my new to me car is slammed and running low profile tires. Works well for the rotary motor.



^^ First project is to remove the BUBs ^^

Posted by: AZ914 Apr 21 2016, 01:37 PM

That is quite a bit of difference. You can go to Discounttire . com and use their tire size calculator to see what the speed variance would be between stock tires and your tire size.

Posted by: toolguy Apr 21 2016, 02:01 PM

That is a 50% error, You'd have to have something like go cart tires to read that far off.
Probably the speedometer magnetic assembly has oil / grease in it. .
May be able to take apart and clean but I'd bet someone has a real good deal on a used one. .

Posted by: porschetub Apr 21 2016, 02:09 PM

Only real solution is to have the speedo regeared then you are stuck that profile tyre.
The rims look like 16'' ? I'am guessing the profile is 45 series or thereabouts, from memory my 205/55/16's calculate out at somewhere close to 1% error.

Posted by: 914Sixer Apr 21 2016, 04:06 PM

Weight probably came off in the speedometer.

Posted by: matthepcat Apr 21 2016, 04:55 PM

They are 15" rims. I believe 195/50/15...maybe 195/55/15. Because the previous owner lowered the car as low as it would go, he had to run a small tire.

Posted by: struckn Apr 21 2016, 08:25 PM

Mine reads about 10MPH faster then actual speed. I decided to not mess with the Speed O Meter and just added a GPS that gives digitally actual speed. There's also an APP you can get for your smart Phone that will work as a GPS Speed-O-Meter..

The need thing about the incorrect SpeedoMeter reading is if you take some one for a ride tell them to watch the speedometer when you're fast into a tight turn or rolling down the Interstate.

Posted by: 914itis Apr 21 2016, 08:54 PM

Same tire size , reads 10 mph higher .

Posted by: PancakePorsche Apr 21 2016, 09:45 PM

Have Palo Alto or North Hollywood speedometer calibrate it for your car.