Mike in CA

Exactly. According to that quoted regulation and spec, the accuracy range is plus or minus 5mph @ 50, meaning it would also be okay per the standard if the speedo read 45mph when you're really doing 50. It may be perfectly reasonable that manufacturers err on the optomistic side with their speedometers so that customers don't end up with tickets due to them going faster than their instruments tell them they are going. But as to this being mandated by government regulation, I doubt it. At least the regulation posted above doesn't prove the case.

Fred R. C4S

There are other regulatory bodies in the world besides the US DOT. There's several others in Europe (EU, Switzerland, UK) and Japan. A manufacturer only wants to build one version of a product that will meet all regulators. So they usually choose the toughest and meet it. The only exceptions is for regulations for a very small market. For those, they might build a special version if it makes economic sense.

Edgy01

I'll look up the real reference that used to refer to an intentional error. Just been a little busy lately to go look it up.

MichaelL

After going through a multitide of local, low speed road radar sensors, the digital speedo seems to read 3 mph high consistently. No observations for speeds over 50 mph.

SBPORSCHE

ditto on the 3 over

acao

Another marginally interesting note:

3 mph seems like a rather arbitrary figure. But, 3 mph is roughly 5 kph. 5 kph is slightly less arbitrary. It would be interesting to see if, on metric digital speedometers, the difference is 3 or 5 kph. The needle of course, would be 3 mph & 5 kph.

Minok

I've gone through several low-speed radar signs and they don't read consistently. They are not calibrated either.

As I pass one sign with an indicated speed of 40, its telling me I'm doing 43.
Two miles up the road I can pass one with a speed of 35 and it tells me I'm doing 32.

In the end, all of the speedometer solutions need to account for variances in wheel diameter due to inflation and temperatures and a host of other variables. Having the cars speedo that is using wheel rotation, transmission rotation, or even GPS try to estimate speed results in some variance in that estimate.

hpowders

I would rather have a speedometer that "fools" me into thinking I'm driving faster than I really am. Better than the other way around.

Fin Fever

No, it is not as if they build a speedometer and plug it in, and that is it. Speedometers are individually calibrated, and they can even be recalibrated. I have one on my truck which I change the setting on depending on which tire set I mount on it. on the p-car it makes perfect sense to indicate a couple of miles higher speed than is actually being driven:

1) The driver thinks he is going really fast so he is happy
2) The police officer sees the happy driver driving the speed limit, so he is happy
3) Porsche does not get sued by the happy driver for the ticket or the accident which was due to a misleading speedometer, so Porsche AG is happy

Everyone wins!

cuttysark

Indicated speed on our 997.2 is 3 MPH higher than the GPS on the radar detector.

Minok

GPS is great for average speed over a distance, but not very accurate for instantaneous speed determination over a very short distance traveled because of the geometry of the calculations and precision of the system.

HiWind

My aim solo dl shows speed below 100km/h as about 4% slower than instrument cluster, and above 100 it goes to about 5-6%. My scooby is much closer like 2-3% slower. But would tend to agree w minor ^^

Alan C.

I see around 3 mph high on the digital speedo of my 11 GTS compared to 2 different GPS units.

Macster

No link to post to the info but I take it on faith it is mandated (at least in the EU and obviously since the EU countries represent a large market for Porsche it is going to adhere to the mandate) that the car's speedo must not ever indicate less speed than the vehicle is going.

IOWs, the speedo in the car can be optimistic -- indicating 70mph when in fact the car's only traveling 65mph.

OTOH, the speedo can not be pessimistic -- indicating 65mph when the vehicle is actually traveling at 66mph.

Also, years ago when I first encountered this difference in indicated speed vs. speed from the OBD2 link and GPS speed, I have tested my Boxster's odometer over 200 miles of driving and found it disagreed with a GPS's distance traveled by just 0.2 mile. This works out to 50 miles in 50K miles.

I have a very good sense of both of my cars' speedo optimism -- the Boxster's I had a speed shop calibrate it -- and adjust my driving to allow for speed optimism.

In the case of my Boxster the speedo reads 5mph fast and in the case of my Turbo 2 mph fast.

Relax. The speedo in your car reads a might higher than actual speed. The odometer is not affected, so no biggie.

Sincerely,

Macster.

sdawsonracing

We just purchased a 2006 911S and while the wife was following me on the ride home she asked me why I was driving so slow. After a little testing we found our Speedo display reads 3mph fast compared to our newer Chevy Suburban. Will test with a local state trooper soon and report findings to the group. I think I will need a calibration. I would prefer it be accurate.

Sincerely,

Steve