Speedo Error, Adjustments other than tire size? Options

[Van B]

So, compared to GPS, my Speedo is consistently 5mph too optimistic. My tires are 195/65-15. I’ve read in other threads that has shown to be a good size for correcting error.

So, not really wanting to go taller still, is there any other way to calibrate the Speedo?

Thanks,
Van

[Van B]

As an Air Force guy, I don't want to be any more reliant on GPS than what we already are ... tunnels, cloudy days, wrong area for sat coverage and your needle will develop ED real quick lol...

Clay,
I do have a couple questions for you though. Are you saying 195/65-15 is still too small compared to the mostly undefined 165HR15 OE tire size? Also, is there anymore info on adjusting this tensioner I can look at?

-Van

[ClayPerrine]

As an Air Force guy, I don't want to be any more reliant on GPS than what we already are ... tunnels, cloudy days, wrong area for sat coverage and your needle will develop ED real quick lol...

Clay,
I do have a couple questions for you though. Are you saying 195/65-15 is still too small compared to the mostly undefined 165HR15 OE tire size? Also, is there anymore info on adjusting this tensioner I can look at?

-Van

Van,

A 165/80R15 (165SR15) tire has a rolling diameter of 25.4 inches, and turns 796 revolutions per mile. So if you want an accurate speedometer, any tire you put on the car has to have a rolling diameter really close to 25.4 inches.


A 195/65/R15 has a rolling diameter of 25.0 inches and turns 809 revolutions per mile. It has the lesser circumference for 31 mm (1.22 in), which is 1.5% lesser than the 165/80R15. The actual speed will be 1.5% lesser than the speedometer reading.In the relation to the 165/80R15., the 195/65R16 has the lesser overall tire diameter for 10 mm (0.39 in).

A 205/50R15 has a rolling diameter of 23.07 inches and turns 877 revolutions per mile. In the relation to the 165/80R15, a 205/50R15 has the lesser circumference for 185 mm (7.28 in), which is 9.1% lesser than the 165/80R15. The actual speed will be 9.1% lesser than the speedometer reading. In the relation to the 165/80R15, the 205/50R15 has the lesser overall tire diameter for 59 mm (2.32 in).

I know for a fact that makes a 914 speedometer off by more than 5 miles an hour.


All of the tire size diameter comparisons came from here: https://www.tyresizecalculator.com/tyre-whe...tire-dimensions


I would make a suggestion for you. Instead of opening the speedo and attempting to re-calibrate it, just figure out the difference between what it is reading and the actual speed of the car. I use a GPS speedo app on my phone to verify the difference. Changing the speedometer spring tension only fixes the calibration of the speedometer, not the odometer. Unless you change out the speedo gears in the transmission, your odometer will be off no matter what you do. And I have never heard of different speedometer gears for the 914/early 911. Porsche built all their cars with the same rolling diameter tires for years. That's why the 944 turbo has the same diameter tire as the 914 and early 911.


I hope this helps.

Clay