irish44j UltimaDork
5/15/18 10:44 p.m.

My '88 924S speedo is reading the opposite of what it should.

Stock tire size is 195/55/15 and my current fronts (where the speedometer cable attaches to) are 205/55/16

So it *should* be reading 6.1% lower than actual speed due to larger tire diameter. i.e. when reading 70mph the car should actually be going about 75mph.

However, I'm having the opposite issue, with the speedo reading (roughly) 8-10% *high*, when compared to my GPS speedometer (which i verified as accurate against my WRX and CX-9 speedos).. So when it's reading 70mph I'm actually only going like 62mph.

I've installed a new speedometer cable (same issue with new and old), and tried both the car's original speedometer gauge and a spare one I have from my parts car. Again, same problem with both. Otherwise, everything seems to be working fine. Speedo action is smooth, odometer works, etc.

Now, while this will probably save me from speeding tickets in a bright red Porsche, it is still pretty annoying. Any thoughts? Not sure - can the magnet or hair spring inside the gauge go bad?

Side note: this weekend on a road trip I plan to test if the odometer (which is fully functional) is reading actual or if it is also off. 



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