83 SC --- Speedometer reading higher?
 

Hello All,

I've noticed that my SC speedometer is running/showing a bit faster.

Could not believe that I was running at 90 mph the other day. I would have estimated it to be closer to ~ 75 mph, based on speed limit and awareness of local traffic.

Few Q's:

1) Has anyone also experienced this --- with SCs, Carrerras?
2) If speedometer does show faster velocity than actual, does that mean the odometer also logs miles faster (e.g. more miles than actually driven)?
3) What's the diagnosis/fix?

Your thoughts/experiences are appreciated.

Much Thanks!

My SC indicates about 5 mph faster than my gps says I'm going, so I think the speedo is off. At 65 mph on the gps, I indicate about 70 mph. Fifteen mph off at 90 seems to be a bit excessive.

Mike

Mine reads about 10% fast. I just use the knowledge to impress chicks, man.

Chicks, man.

Mine does too.......they come like that from the factory.
You can have them adjusted (NoHol) but it probably will only get you a ticket....:D

Are you running stock size wheels/tires?

My question exactly... I think, but not sure, that SC's were delivered with either 15's or 16's depending on what was ordered or delivered....

It can be affected by the overall diameter of the tire/wheel combo. If you have a shorter sidewall than stock then technically your wheel is turning faster, causing the speedo to read high.

Remember, the sidewall number on a tire is a proportion to the width, not its actual height.

as long the OD on the tires is the same as the stock size the 15 or 16 can be the same as long as the profile is adjusted to compensate.

From what i have heard the speedo's do read a bit on the high side from the factory ...

Mine on my 80SC at a shown 80 is a real world 73mph per my gps others seem to experience about the same ... I am running 17"cup2 and the OD per the tire shop is very close to the stock (can't remember what it is)

Same deal here - the speedo in my car measures 5 mph fast from 70 on up to 85 or so, as measured by GPS, gradually getting a little faster still as speeds rise. Have no idea if the odo also is fast, but certainly would like to know.

Yes, of course the odo is fast.

Thanks to all who chimed in with your own experiences, opinions, diagnoses, and suggested fixes.

I am running on:
205 55 16 up front
225 50 16 at rear

These are Dunlops. Seems pretty standard to me.

Here is a thread on adjusting the speedo.

http://forums.pelicanparts.com/porsche-911-technical-forum/97658-speedometer-error-need-some-help.html

I just moved the needle to agree with my GPS at 75 mph. It's probably a little off at 65 and 85, but it's probably not that far off.

Much thanks for the link.
Extremely helpful!
I am going to look into NoHo option.

German law of the time allowed a faster-than-actual read......but "zero" tolerance in the other direction. To accommodate all variables.....most Porsches therefore were set up to run showing 10% high.

Doing this had 2 other advantages.

1.) made you think the car performed better than it really did, and
2.) If matched to the odo.....all routine services were done sooner generating service repair revenue sooner. Might mean you piled up 10% more-than-actual mileage, however.

That all said, and knowing this.....I still checked my 85 Carrera against known gearing and tire diameters, and compared them against factory spec books, ...and it was surprisingly "spot-on".

This pic shows the trimpot, but it is not on all of the speedometers, others have a resistor that would need to be changed to alter speed. These adjustments don't affect the odometer reading. I adjusted this one for my 3.2 conversion in my 73.5.
http://forums.pelicanparts.com/uploa...1289521785.jpg

Note - from calling North Hollywood speedo a few months back they say the odometer is gear driven not potentiometer driven. You can "trim" one but probably not the other as E Sully says.

Mine is dead accurate but I can't say whether the original set up was so. It's a Euro with a "Miles"(vs kms) speedo that was probably calibrated when installed upon arrival in the US. Sounds a little weird that it's SO wacky ....almost as though someone has changed the calibration in the past, perhaps to compensate for larger diameter tires. The sizes that you're currently running though don't sound too out of whack, size wise. Cheers

my 83SC is way off. i just happen to know that at 2950rpm in 5th i am doing 70, so i roll with that.