deandavis1

Ok now I have GPS I know my speedo is out by 10km/h. I have tested it in many other cars and know the GPS is correct. Question is how do you adjust the speedo setting on an '89 944? is there a trimming pot I can adjust?

Thanks

FRporscheman

Is it always off by 10kph, no matter your speed? If so, you need to remove your speedo gauge and have the needle repositioned.

75ohm

Is the thing zeroed out correctly or did someone pull the needle off to replace the odo gear? How about rim size and tire size? Are they stock? Something up with the sending unit on the transmission?

deandavis1

Thanks for the ideas. Yes it is zeroing. The tires are stock. They are due for replacement but I would not expect this to add 10kph. I think that it is always 10kph no matter what the speed (but will double check). I dont believe the odo gear has been replace (actually I am just about to do that). Dont know about the sending unit. How do I check this?

So is there no pot adjustment I can tweak?

Thanks again all.

bearone

stock as in what's on it now or as it was equipped when delivered?

87951

aprotogerellis

Hi there,

I've also got very similar problems... The speedo shows more than I'm actually doing. The amount of difference seems to increase as the speed increases. So there'll be a 10kmh difference when doing 100 and 20kmh when doing 140.

Any ideas on what the likely cause of this could be?

Thanks,
Alex

El Commandante

Yours would probably be from a non stock wheel size, that usually adds or subtracts a certain % to your speed. I have the same problem as Dean, mine is always off by 4mph no matter what the speed.

aprotogerellis

Hmmm... any way to adjust it so that it does show the right speed in the end? or do I just need to up my mental maths?

El Commandante

check your tire and rim size and compare it to the stock ones behind the gas cap cover. I dont know if you can adjust it besides putting it back to stock. YOu might just have to figure out the percentage, and do math. My audi 5000 has larger wheels and tires than stock, and now it is 10% off, so i know when the speedo says 50, i'm going 55, 60- 66, etc. You could verify all of this with a GPS

bearone

bogus info.

my speedo reads dead on what the garmin nuvi200 shows.
fronts 205/50/17, rears 255/40/17

the mercedes is off by 1-3mph with the garmin.

87951
95sl320 dd

aprotogerellis

Strangely, I do have stock tyre and wheel size. Just changed the lot of them and put the standard config on.

Anything else that may be causing this?

bearone

it's difficult to reply.

this thread's been running since may and you haven't mentioned the year.
early had a speedo cable, later, electronic

did anything change when you went back to stock tire size, are they 15's or 16's?

87951
95sl320 dd

aprotogerellis

OK...

So, the car's a 1986 turbo. The wheels are stock 16's. And nothing changed when I changed the tyres (they were always stock...)

Thanks a lot!
Alex

PennyWise

Most older cars and some new ones have optimisic speedo's and the higher the speed the bigger the gap. Live with it.. At least when you get caught speeding the fine is less than you thought because you were just under the go-to-jail threshold and not on it.