Speedometer Error. Need some help
 

My speedo is running about 3-5 mph high at city speeds and then is about 5-7 at highway speeds.

It was like this when I purchased the car.

It also makes a ticking noise (only audible at low speeds for obvious reasons) once every 50 feet or so.

I originally thought it may be a skip in the speedo gear but, that would make it read low.

Any thoughts on what this could be or how to remedy it?

What size tires are ya using? If they are much different from the original equipment....that is your error.

Palo Alto or North Hollywood Speedometer can do a recalibration based on the current tire size.

Good thought......I should have mentioned upfront that they are stock sizes: 205/55-16 and 225/50-16 so no error accountable to that.

Sounds like you have a Carrera. Many of the speedometers on Porsches and other cars are notoriously optomistic. You can send it off to get recalibrated, or, I think you can adjust the potentiometer yourself. Do a search on this board, I know someone posted a how to on this. Of course it will be hit and miss to get it right, but you save the $100 to get it recalibrated.

1983 SC

Chris,

Your 'ticking' may be the OXS counter ... it can be disabled by removing the brown ground wire.

The speedo is electronic, and has an internal trimpot that can be adjusted. It faces the front of the car in normal mounting position ... when you remove the speedo ... a hole will have to be drilled to access the trimpot for adjustment. Do a search, as suggested above, there have been several threads discussing the procedure.

Chris,

I believe Warren is dead on. I live with my '83 SC being slightly off, though the difference is less than yours. The speedo is rather easy to fix and it is fully electronic.

Thanks for the key words. I'll look again.

Hey Ron,

Those look like my knees. My daughter calls them "fuzzy"

Scott;)

I have read all of the posts I can find regarding the adjustment of the speedometer by drilling a hole to get at the trimpot.

My question is this. My Carrera's speedo is off by a steady 13 per cent across the range. Does the trimpot adjust the initial position of the needle, ie. a constant 5 mph reduction across the range, or does it adjust a "scaling factor" in the speedo, ie. I can adjust out 13 per cent everywhere in the speedo's range? Probably not very clear. Just curious if an adjustment at 60 mph will correlate to better accuracy at all speeds?

Maybe I should just let North Hollywood Speedomet do it.

Any one know or remember which way to turn the pot? My speedo reads higher than actual and if I know which way does what it might save me a step.

Gentlemen,
I'm not the last word on this.. so if you got any news afterwards please give it up.. so this is what I did/may not be the most professional, but it worked for me..

1-remove the screws at the rear of the speedo for the plastic wire connector.. push connector aside and eyeball where the trim pot is.. you'll need itty bitty eyeglass screwdriver

2- align screwdriver on outside of case to get a straight shot at the pot after you drill hole

3- set up the speedo so it's outside the dash and you can still drive the car

4- I followed a new Honda on a straight flat road.. the Honda had the cruse control set for 60mph.. then pulled of road to adjust an itty bitty amount.. like the difference in 5mph is about a move from the 1 O'clock to the 2 O'clock position.. A turn CW increases the speedo reading, I believe..

5- did the same clocking at 30mph.. no adjustment necessary

then I borrowed a portable GPS to confirm.. but couldn't get the GPS to work.. so after driving on big roads, knowing that the old folk set their cruse to the 70 mph speet limit..[it's obvious] I just happened to be dead on at 70mph also.. did the same at low speeds..
not saying that it's a linear adjustment/don't know.. if you set at 60mph your lows may be off/don't know
the millage clock stayed inaccurate.. but a measured mile and a stop watch may help.
and N. hollywood is a great outfit.. you do what you gotta do.. but my old RE-71 tires are a different tire dia. than the new S-03s for the same tire size.. no big deal..