Indicated vs actual speed - observation
#16
Race Director
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.
Last edited by Mike in CA; 12-15-2011 at 02:53 AM. Reason: sp
#17
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Don't forget........
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.
#21
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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.
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.
#22
Drifting
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.
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.
#24
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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.
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!
#26
Drifting
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.
#27
Race Car
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 ^^
Last edited by HiWind; 02-12-2012 at 12:46 PM. Reason: Addition
#28
Rennlist Member
I see around 3 mph high on the digital speedo of my 11 GTS compared to 2 different GPS units.
#29
Race Director
In case anybody is wondering, I had the opportunity to observe the indicated speed as shown by three sources: speedometer, OBD, and GPS. The OBD and GPS signals came from my Aim Solo DL.
OBD and GPS show the same speed value. The speedometer displays a higher value. The delta varies, but at 65 mph on the GPS and OBD, the speedometer shows 68.
The bottom line? The car has a more faithful measure of speed, but Porsche has chosen to not display it on the dash.
OBD and GPS show the same speed value. The speedometer displays a higher value. The delta varies, but at 65 mph on the GPS and OBD, the speedometer shows 68.
The bottom line? The car has a more faithful measure of speed, but Porsche has chosen to not display it on the dash.
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.
#30
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
Sincerely,
Steve