My '07 VW GTI reads 70mph when I'm really going 65...

 

It isn't exclusive to Minis either. My Ford F Series Super Duty is off by 3 as well. Checked by a ScanGauge II.

 

I never look at the speedo...

 

Here are some links for you to look at.

http://www.publications.parliament.u...t/10312w01.htm Scroll down to the part called Speedometer Accuracy.

http://en.wikipedia.org/wiki/Speedometer#cite_note-3 Scroll down to the parts about error. international agreements, Australia and England.

http://www.unece.org/trans/main/wp29...gs/r039r1e.pdf and in this one scroll down to section 5 Specifications. and SPecifically read Section 5.3 which gives the exact tolerances.

What every one here has stated about the speedometer reading high by law is very correct.

 

Apparently I am the oddball with the almost accurate speedo...

Stock 17" Crownspokes and Continental A/S

 

So, I accidentally stumbled upon one of those radar speed check thingies, and yes that is the technical term. You know, the ones that you pass by a sensor that you can't even see and your speed flashes on the screen to make you aware of your speed. Anyway, I had felt after 3500 miles with my Mini that it was probably dead on, maybe 1 mph too fast, but still accurate. Ummm... I was wrong, way wrong. My speed is off by 3 mph, which isn't huge, but I regularly go under the speed limit, which is 30, in my area because it is 100% residential and there are lots of kids around. I never realized that by going 28 or 27, I was actually going only 25 or 24. No wonder people would illegally drive around me when I was turning into my neighborhood. Well, I guess that I no longer need to talk to the local PD about my speed.


PS: I'm going to have my dad drive his MCS through it to see if his speed is as far off as he thinks it is.

 

It's actually the distance between your vehicle and the satellites that's being measured.

GPS satellites all send out a perfectly synchronized signal, but that signal gets to your receiver at different times from different satellites depending on how far they are from your vehicle. This time discrepency enables the GPS receive to calculate your position. Your position change over time determines your speed.

Interesting fact: The atomic clocks on the satellites get out of sync with the ground based master atomic clock, and they must be corrected on a regular basis. Why? Relativity! Thanks to A.E., we can correct for that.

 

My SA said that BMW purposely sets their speedometers to read 6-10% over actual speed. Apparently, all manufactures do this as they could get in major legal trouble if they made one read below actual speed. Makes sense. He also said the odometers were dead on for the same reason. BTW, if he's right, it will be the first time in the +1.5yrs I've had my MCS!

 

The one in my 5-series is way off. You'd be going 80 and it will actually be about 73. It gets more inaccurate the faster you drive. My VWs speedo is almost dead-on. Maybe 1mph off.

 

Tested it last night for first time, seems to run 4 mph faster at highway speeds as compared to my GPS, and with 3 others cars the GPS has always matched the speedo -- also seems to run about 2-3 mph faster than GPS at 25-30 mph.

 

It will generally be a consistant percentage of error at all speeds.

Yours looks like +10% which makes correction easy (30=27, 50=45, 70=63, etc.).

 

I used to think this as well, before being corrected in these forums by someone else.

GPS units don't measure your speed by comparing position readings, they measure speed by way of red/blue shift in the signal frequency. It makes for a much more accurate measurement than position sample differentials.

 

Reviving an old thread...

Our little family has now had 3 MINI Coopers -- an '06 Cooper (traded-in), and now 2, 2010 Coopers). Every one of them has a speedometer error that seems to be a combination of a fixed error and a percentage error. The fixed errors are between 2 and 3 mph, and the percentage error is about 2%. Only the 2% error shows up in the odometer.

I see the point of the fixed inaccuracy (European standards), and I have a theory about the percentage error. Our Coopers have the smallest tires available from MINI (175/65R15). They are almost exacly 2% smaller than the largest tires you can get through MINI (205/40R18). Rather than have different programming for each tire size, the manufacturer can just program the ECM for the largest stock tire they offer, and they'll still be within an acceptable error range for the rest.

Just a theory.

It sucks that my odometer is reading 2% too high. That means my warranty will run out at an actual mileage of 49,000.

 

my r56 with 205/45/17 (largest tire mini offers from factory) was off 3%+ always indicated 100 when gps indicated 96/97.

 

My MCS generally indicates about 4 mph faster than it's actually travelling, and that's funny about the faster the speed possibly the more inaccurate it is. last summer I was on an 80 mph stretch of I-15 south of Provo Utah, when I get radar-ed by a state trooper in Charger sitting in the shade under an overpass. The second my detector went off I checked the digital speedo, it said 89-- and the cop did nothing.

And Utah troopers usually dont cut one that much slack.

 

The error factor in the speedo is a percentage of the indicated speed, not a fixed mph. On my MCS, it's +10%. Almost all cars made in/for the EU have speedos set to read 7-10% fast. Big time legal trouble if they don't. If it said you were doing 89, you were probably going ~80. BTW, my odometer is dead on and most likely due to the same EU regs.

 

I've noticed the same thing in my car. Driving, say 65mph, my portable GPS reads that I'm going 61. Doesn't surprise me..

 

Yep. That's been my experience as well. The speedo indicates about 4 mph faster than my Garmin GPSMap478 indicates.

 

According to this explanation from an educational text created by the jet propulsion laboratory / nasa, it's the time shift that's measured:http://http://scign.jpl.nasa.gov/learn/gps2.htm

Also, TomTom, which should know a thing or two about GPS, has a similar explanation: http://http://www.tomtom.com/howdoes...D=2&Language=1

Of course, they may both be wrong. I would welcome your citation for the red-shift measurement explanation.

 

Is there no way of inserting some kind of rheostat into a circuit so you can tune your speedo to be accurate?

 

When I ran 205 45 r17's, the speedo error was about 2 percent. However, switching to 215 45 r17's dropped the error to 1 percent or less.

 

In a word NO. This si controlled digitally so a rheostat would not do the trick. As for the error it changes as the tires wear and the difference between new tires and fully used tires is between 1.6 and 1.8 percent for most tire sizes.

Also switching brands of tire or even different models of tires from the same manufacturer will change the error as much as two percent.

For example: for a 205-45/17

Dunlop SP Sport 01 DSST (OEM) runs 853 Revs per mile
Bridgestone Blizzak Snow Tire runs 859 revs per mile

so if we divide 853 by 859 we come up with the percentage of difference.

853/895= .993015 or almost 1 full percent change.

so now if we change from one tire size to another say 17" summer to 16" winter the change can be even more or in some cases less depending on make, model, and size.

Measuring off the ODBII port seems to give the closest reading but can still be off by some amount especially when changing tires.

BTW the ODBII and the Speedo in most cases are off by 2-3 MPH. This is built in to make the cars meet European laws as in the links in post #54 of this thread.

 

The same tire mounted on different width wheels will also cause a variation in the tire's diameter. The specifications of a particular tire will list the tire's diameter, and on what width rim that measurement was made.

My speedo has both kinds of errors -- the built-in 2-3 mph error, and a percentage error caused by tire size. Some 185/65R15s would eliminate the latter.

 

You might want to fixify you links - too many http://. Neither one seems to says anything about measuring speed. These are explanations of position fixes. GPS receivers really do use phase shift to measure speed, not position change over time. Position change over time would always over estimate speed with GPS position fixes because of the associated position errors, but the phase detection is VERY accurate. It has to be.

 

Citation? ... I'd be very interested in reading how the process you describe works.

Also, GPS position is accurate to within a few meters. The government removed the deliberate inaccuracy in GPS years ago, everyone now can get true readings.