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.:(

I hardly look at the speedo...get a calculator for rpm speed that uses your gearing and tire size...I just use the tach.
Mines about 6 mph off and everyone with an SC that I know says about the same.

I had mine rebuilt, the gear broke inside. They also 'calibrated' it.
I replaced the funky reed switch in the tranny at the same time.
It works but it's off by about 5 mph. Who knows why.

If you drive with the traffic your odds of getting a ticket are low.
If you get something like a 9500ix and radar is in the area, it will
show you your speed. The 9500ix is an expensive investment
but with its built in memory, update capabilities, accessory"live" mapping on
your droid or iphone... we'll its a pisser!
Can't beat it and less false alarms than the Valentine.
One of the best investments I've made besides wide band AFR display.

an optimistic speedo is the cheapest way to get performance out of the car. :D

When I fixed my odometer gear, I used a gps to calibrate the needle, making small adjustments until they agreed.

Smart idea!
What did you adjust on the board?

Not sure what your question is.

I read an article (not Porsche specific) that claimed this was because of variations in actual tire dimensions from different manufacturers and because of variations due to under/over-inflating the tires.

Mine reads way fast and gets near accurate at high speed.........Like someone said...they are all off........maybe just live with it......

If I did this math correctly, then this example of using a 16-in tire on a speedo calibrated for a 15-in illustrates how significant the difference can be.

A 15-in tire has a theoretical circumference of 47.1 inches.
A 16-in tire has a theoretical circumference of 50.24 inches.

If the speedo is calibrated to a certain number of revolutions, then:

800 revolutions
15 in: 47.1 * 800 = 37,680 in (total distance traveled)
16 in: 50.24 * 800 = 40,192 in
delta = 2512 in (6.7%)

1200 revolutions
15 in: 47.1 * 1200 = 56,520 in (total distance traveled)
16 in: 50.24 * 1200 = 60,288 in
delta = 3768 in (6.7%)

1600 revolutions
15 in: 47.1 * 1600 = 75,360 in (total distance traveled)
16 in: 50.24 * 1600 = 80,384 in
delta = 5024 in (6.7%)

Oh and a 6.7% calibration difference at 20mph would register a difference of about 1.34mph and at 60mph would be a difference of about 4mph. I think this is consistent with what most people see as the difference between real and actual.

15 inch tire? 16 inch tire? Those are rim diameters, not tire diameters The rim diameter has nothing to do with the outside diameter of the tire, and the circumference thereof. The only thing that matters is the rolling diameter of the tire, which is 25.0 " for the most popular tire sizes for SC or Carrera, 205/55/16 and 225/50/16.

From all my experience and what I've seen reported on this board for the last 9 years, the typical difference is twice what you suggested.

How did you "adjust the speedo".
What procedure?

My SC is off by almost 9%. At 80MPH indicated my GPS will read 73MPH. Must by a German thing. I also have a BMW R1200RT and an R60/6. Both of those speedos are also optimistic.

I believe the German government has VERY BIG FINES on a manufacturer if any of his cars are found to UNDER report speed. Thus they design the stock speedo to OVER report speed by 5% or so. Not sure when this practice started.

You are correct. However, since I didn't have the actual outside measurement handy and I was too lazy to look it up, I just used those simple numbers. The actual numbers don't matter for the sake of illustrating the point that a larger tire circumference makes a difference in distance traveled for the same number of revolutions of two different tire diameters and mechanical speedos are calibrated to a number of revolutions/unit time.

Seems like I saw a post on here once (or maybe I read it somewhere else, I don't recall) on how to calibrate your speedo using an electric drill and a 12v power source.

Thinking out loud here, but what happens if someone changes the gear ratios in the transmission. Does the final gear ratio change at all?

I thought about the wheel size as well. Although my 81SC came from the factory with 16-inch wheels, the PO or dealer changed my speedo - I think my car came with the idiotic 85 MPH speedo. What I have often thought about was when the speedo was replaced, were there two speedos available? A 15-inch version and a 16-inch version, each calibrated for the appropriate wheel size? It just seems like my speedo is calibrated to 15-inch wheels, not the 16's I have.

I wonder how NHS "calibrate" their speedos? Is it done on a bench? If it is, how do they know if they are calibrating for a 15-inch wheel or a 16-inch wheel, because when they "calibrated" mine, they did not ask, although they had my car at their shop. I simply assumed they used the dyno rollers because I saw them there. That is why I was quite surprised that after my speedo was calibrated, it still registered higher speeds than I know I am going. And for me, I just didn't like the fact that my mileage is probably higher as a result of the incorrect speeds.

I looked for it a little while ago and couldn't find it. I think it was a guy sitting at his kitchen table with a cordless drill and a car battery. He had the drill hooked up to the speedo cable somehow and I can't remember what he used to measure the RPMs.

According to one source I found says the US requires something like no more than 1.5% variation. About 5% seems pretty common so it is kinda interesting that so many people are seeing greater variation than that.

Not too much you can do about it though. Send it off to a shop that knows what they're doing or live with it. If I ever send off my gauges I would have it done, until then I'll live with it. I keep the Escort app running on my smartphone when it counts the most anyway. :)