More ado about the 996 speedo
 

If any of you are still mulling over the accuracy or lack of same of the 996 speedometer, here's a little tidbit to chew on.

At a steady state 80 mph on the digital readout, the GPS reads 77 mph. At 70 it's 67, and at 60 GPS say's 57. Plus or minus 0.2 mph. I refuse to do steady state 50 mph in a 60 mph zone. If I do 83 on the digital speedo, GPS will read 83, and so on. So, on my car and at these speeds, there seems to be a fixed difference of 3 mph between the digital speedometer and GPS.

There has to be a transition point somewhere. Otherwise, for the digital speedometer to read 0, I'd have to be going 3 mph BACKWARDS , heh-heh-heh.

Just thought I'd pass it on. :cheers:

I think my variance is a bit higher. My car came with 17" wheels, and I switched them to 18" with a 30mm side wall. The end result is a wheel with a smaller total diameter than stock, resulting in a slightly more inaccurate reading. I think I'm off by around 4 mph.

Well my speedo and my GPS read exactly the same.
The only thing is that it takes the GPS device I use about 2 seconds
to register a speed change. So if I am doing 60 and I go to 65 then
2 secs later my GPS will say 65.

mine is off by 4 according to gps. it didn't change when i went up to 19 inch wheels.

I heard from a police officer friend of mine that the speedos in the US are deliberately off in all cars. The speedo is supposed to show you going faster than you actually are. Evidently, for safety reasons, this is so that you are supposed think you are going 80 mph while in reality you are really going about 75 or 77 mph. Thus, your speedo is always wrong, while the GPS is right on. My GPS consistently shows me going slower than the speedo too...on all my cars. I guess some politicians didn't think we would be smart enough to drive at a sane speed.

my speedo is dead on in my work ride...but then again my work vehicle is a police car :)

My Lincoln, my Explorer, and my Harley are dead on by GPS. Only the Porsche lies.

It's not just the wheel size that you need to take into consideration, but the whole diameter of wheel and tire. If your speedometer wasn't affected after going up to 19", then the combination of wheel and low profile tire diameter must have been the same as stock.

don't they calibrate those specifically so that you can use it to legally guage speed and ticket people by following them?

Just dredged up this old post on the issue of speedomoter accuracy .

Any more recent thoughts? My new (to me) 02 Carrera is reading 3-4 mph faster than my gps at 80 mph (i.e. if the speedometer says 81 the gps says 77).

Hard to imagine that Porsche would not compensate for the difference between stock 17" wheels and factory-installed optional 18" wheels.

My VWs have always registered a bit high. I once read that this was as a result of German TUV requirements for accuracy. VW supposedly set the speedometers a bit fast on purpose to avoid the penalty for reading too slowly.

In the end this becomes almost a safety option as you are always going a bit slower than you think, which can be handy when Mr. Law makes a sudden appearance.

Common characteristic is my observation.

My 02 Boxster's speedo is off by 5mph at 70mph indicated, Actual speed is 65mph. My 03 Turbo's speedo is off by just 2mph, reading 2mph higher than actual speed. I can't say I'm 100% sure about my memory of my 08 CS but I seem to remember it's speedo was pretty accurate, with no real error in speed indicated vs. actual speed.

Couple of things going on. Word is that cars sold in Germany can't display a speed lower than actual speed. Since these cars are made in one factory they all come off the line with an optimistic speedo.

Since some models have several wheel/tire size options this can affect the speedo which is why some examples have a 3 to 5mph error and others have a smaller error.

As for odometers: I have looked into this and the odometer has very little error. I tested my 02 Boxster's odometer over 200 miles against a GPS device and the two disagreed on distance traveled by just 0.2 of a mile. However, this Boxster's speedo is at 70mph indicated 5mph optimistic. It is 3mph to 5mph optimistic at speeds from 30mph to well highway speeds. Above highway speeds I suspect the error increases a bit (which is why I generally tend to distrust top speed claims based on speedo displays).

Also, the few cars I had acess to I've found the OBD speed value to be accurate and in agreement with my GPS device.

Next automakers are on their honor to deliver speedos that are within +/-4% of actual speed, and distance. At 70mph the speedo could read 72.8mph or 67.2mph. Although if that info about speedo readings in Germany accurate the speedo would be set to be optimistic always. Thus, to avoid a 2.8mph under reported speed, the speedo could be set to have a 5.6mph error.

What I do is characterise my cars' speedos and then just use that to note my speed. I trust my speedos enough that I have passed CHP patrol cars and other LEO cars in other areas that were going a few MPH under the limit though of course only when there was no other reason to observe a speed under the posted limit.

Sincerely,

Macster.

I have to laugh and feel sick of the speedometer for speed limits game at the same time. Speed limits for safety? Yah to make it safe for bad drivers going down the road. The old 25-50 multiple guess question test for a license to kill on the road for some of the clowns out there.

My 2002 Targa reads 105 kph on the speedo for 100 kph on the GPS. Standard 18 inch whhels and tyres. Have just go used to it and adjust accordingly.

My 964 had the same issue -- and back them it was fixed by installing a different resister value in the speedo head unit -- did a search (google I recall) and found an audi repair of the "same" VDO head by fitting a different value resister -- and there was a calculation in that search result as to the required value to correct the error.

The car does not know or compensate for wheel size. On Porsche approved sizes for wheels/tires, the overall outer diameter of the tire (the only thing that matters for speedo accuracy) will be the same between 17s and 18. If you go away from that overall diameter, your speedo will be off by a percentage from factory, not a fixed number...

I don't have it at hand but there is a formula to calc. tire dia.

It has been awhile since I delved into this but not all Porsche recognized/approved 17" and 18" tires will have the same outside diameter. There will be differences.

My info is that Porsche arranges the speedo to be most accurate with one particular wheel/tires size, and all other wheel/tires packages come in worse, that is all other combos result in the speedo being optimistic by some amount.

It may be possible to have the dealer -- for a fee/labor charge -- to have the speedo corrected to read more accurately. But I never explored this and do not know for sure if this is possible. I have characterised the speedos in my cars and just adjust the speed accordingly. Currently my collection of cars includes a Boxster: 5mph optimistic so I drive 5mph over; Turbo: 2mph optimistic so I drive 2mph over and know I'm at the posted limit.

Sincerely,

Macster.