I have an '86 951.

I have this issue where my speedometer needle reads 110% higher than it should. In other words when I go 1 mile I'm really going to 1.1 miles according to the odometer.

I pulled my gauge and counted the teeth of the gears and I have a 20×22 gear. I thought maybe I had a 20 x 21 gear and that would explain why my speed is off, assuming more teeth would mean the speed would be reduced if I went with a numerically higher teeth count. Now that this is not the case since I have the 20 x 22 gear already, what else could I look at what else could be wrong?

My tire diameter is correct for the car so I know that is impacting it

I forgot to mark the needle before began twisting it off now I'm not exactly sure where it goes when I reinstalled it is there some reference out there I can use to relocate the need?

Final thing is there a way to test the lights on the dash before for reinstalling it? I am putting LED replacement bulbs in and it indicated they could be 180° out and I would have to take them out and rotate them. I would like to avoid that.

 

Mine also reads higher ('84 NA), I'm getting 120 km/h at 100. About 4 km/h comes from a slightly smaller diameter tyre but no idea about the rest of the error.

It is pretty annoying, though.

 

This is interesting as my '91 S2 speedo reads 10% over - says 110kph but GPS says 100. I`m on stock wheel/tire size

 

It is pretty common for manufacturers to make their speedos give a high reading so that no one can come back and try to sue for speeding tickets. My car is odd in that it reads about 6mph high through the whole range, so I suspect that the needle is just offset a bit.

 

My speedo also reads faster than the GPS.

 

I thought all Porsche speedo's read high, and at a non-linear rate.

 

I've yet to encounter a 944, early or late, that didn't consistently read high, but by an inconsistent amount. If you're really concerned, just use a GPS speedometer app, because you're sure as hell not going to get the gauge to line up with reality.

 

I'm not so concerned about the speed reading higher as I am about it putting on miles to the odometer at 110% of its actual mileage

 

Not 100% sure re: the late dash, but for my early dash, I was able to do a partial install and check the polarity on the bulbs without having to completely pull and reinstall the cluster. Will depend mostly on the amount of slack in the connecting cables, though.

 

So you can't use a multimeter and just touch the leads and if there is continuity? You actually have to power it up from the battery?

 

Not sure- I was able to get enough slack in the lines that I just did the whole thing sitting in the driver's seat and flipped around the bulbs that didn't work. I never really needed anything more involved than that in my case.

The LEDs care about the polarity of the power coming in, so it's really not a continuity question, it's a question of 'which supply line is positive' and 'which side of the bulb expects positive', so I'd imagine it's just easier to plug 'em all in and see which ones work.

 

Shameless plug:

http://members.rennlist.com/tom86951...r%20Page1.html

 

For the record, Tom, that's freaking awesome. I keep seeing people do random crap with Arduinos that's interesting, but less than actually useful. This is perhaps the first application that doesn't seem forced!

 

While this is completely awesome it is also mount Everest over my head

 

Sorry, missed your responses. It's really not that hard to put together. The hardest thing was figuring out how to condition the speedometer signal, but with that sorted out, and with the software in hand, it's not so bad. If you search for "speedobooster" on google you can find an further iteration of the printed circuit board, which you can order off OSH Park cheap. I ended up combining the speedometer calibrator with a boost gauge controller, which lets you remap the factory boost gauge so that it reads however you want (e.g., 1 = 10psi or 1 bar boost, 2 = 2 bar or 20psi, etc.). Here's a picture of the latest, which does away with the arduino and just uses it's processor chip. The little blue pots on the end are to calibrate the speedometer and boost gauge.