Speedo reads too optimistically, too fast
 

Any reason why this would happen? When it says 35, I'm really doing about 27. When it says 90, I'm really doing about 75. What gives? It's a 77 911S, if that matters.

Installed non-standard sized tires?

My 87 is off by 5 mph as well: Google Navigation and my PASSPORT 9500ix are both within 1 mph amazingly

Decent thread here on the subject
http://forums.pelicanparts.com/porsche-911-technical-forum/723314-need-help-w-88-carrera-speedo-problems.html

And I'd echo what jlex commented on, tire size.

same thing in my 83SC. speedo always reads higher than i am going with 16" fuchs and 16" msw wheels.

I have the same problem in my 1987. Speed reads higher and seems to be more "off" at higher speeds than lower speeds.

Mine is off too. by about 10%

A couple questions come to mind:

1) If my Speedo is off 10%, is my Odo also off 10%? i.e. the car has 85k miles, does it really only have 77k?

2) Can it be recalibrated to remove the 10% so that it reads true speed?

1. probably not 77k but definitely less than 85k. the speedo was right at some point :) fwiw last year my on my herhsey run my odo read 886 miles and my gps said 855.

2. i think so but i am no expert

Standard tire sizes here.

My Audi S4 does this as well. At 80 MPH on the speedo, nav device says 75. Worries me that it depreciates faster because of inflated miles. I have complained to Audi, and they care not at all.

This is a very common issue in all of the classic 911s. The fix is to have your speedo reconditioned and recalibrated by North Hollywood Speedometer or Palo Alto Speedometer. Mine was 5-15 mph higher than actual, and I finally sent it off to NHS for rebuilding which cost $150. It came back looking brand new, and the readings were absolutely SPOT ON at every place on the dial.

i've thought about doing that myself but worried that the recon speedo would make the other gauges looks worse. not sure i want to spend quite a few hundred to have them all match.

Most of the speedo error is due to different size tires. Older 911's stock tires usually rolled at 800 revolutions/mile. So if you put on a tire that has less rev/mile, then the speedo will read too slow. If the tire has more rev/mile then the speedo will read too high. Go here. Put in your stock and actual tire size. It should approximate the percentage you are off. Of course, if you tell NHS what size tires you have they can calibrate your speedo to match your tires so the speedo will not be off.

Tire size calculator

I think that it is not possible to have always an exact speed reading.
There are many variables that affect the speedometer.
Tire size, correct air pressure, tire temperature, road roughness, if there are many curves, if it is a straight road. If you have new tires the reading will be different if you have worn tires. When worn the reading will be higher than when new. Very few milimeters in tire diameter affect a lot the speed reading.
I think speed reading is an average.
It is better to have a higher reading than a lower reading.

I didn't realize the effect such a small amount of diameter could have...

However, I'm not sure I agree with the last sentence. I think it'd only be better to have a Speedo reading higher if you didn't know about it.... When I think about my speedometer reading 10% too high, my biggest concern is racking up 1.1 miles for every mile I drive, thus expediting depreciation. Granted, I didn't buy my car for the resale value, but I certainly don't take comfort in this knowledge.

Oddly, my speedo was calibrated by NH Speedo after some repairs were made by them, and it still appears to read faster than what I am actually going. It has always read faster, and frankly, I was a bit surprised that it still read faster after calibrating. NH has a dyno-like roller setup, but maybe they calibrated it with something else? I didn't bother calling them back afterwards to ask them, but my car has always read about 10 MPH faster (at highway speeds) than the speedo reads, and it is the same AFTER NHS calibrated my speedo.

I run original 16-inch Fuchs and original tire sizes, and it was reading high even when I had the original Pirelli P7 tires.

+1 on th North Hollywood Speedometer treatment. I had my 79SC speedometer repaired there recently (infamous broken odoneter worm gear). I also had the face plate replaced from 150 MPH to 160 MPH/140 KPH. NHS also recalibrated the speedometer and it is SPOT-ON! I have factory 16" Fuchses with factory specified tires.

I never could figure out why this mis-calibration issue is so damned prevalent. (And that's with proper tires on the car). I've known only a handful of 911 owners who had accurate speedos,..and they were ALWAYS reading higher than expected...mmm....weird.

A proper calibration will "set you free".

BEST!

Doyle

I am told there is a calibration knob on the back side...you might want to look

I've not heard of that but may exist for specific models. There certainly is a potentiometer on an '89's circuit board that allows for proper calibration.

BEST!

Doyle

i need to send mine off to the speedo people in california too. my speed sounds exactly like yours, which doesn't bother me now, since my odometer isn't working because of the famous gear issue. there are so many things i'd rather buy with that $150.:(