Anyone good friends with a cop?

[Dr. J]

So.... is anyone good friends with a cop? And, could you convince them to "clock" your Clubman?

Backstory - awhile back I noticed that my Clubman's OBDC (speedometer) was off when compared to a GPS unit (OBDC said 72 mph and the GPS said 68 mph). Then I noticed that when back-calculating a distance (using trip avg speed and time, both from the OBDC), there was a discrepency vs. the actual distance (OBDC calc said 60 miles whereas Mapquest said 57.3 miles). Lastly, when measuring MPG vs. the OBDC, the actual (simple division based off pump values) was 39.0 whereas the OBDC said 40.0 mpg (over a full tank).

The simplest answer to all of this would be if the OBDC thinks it's going farther than it actually is (say if the wrong size tire is programmed into the OBDC), it would overestimate anything with mileage in the numerator (e.g. M, Mph and Mpg).

I hadn't thought much of this discrepency but I had my SA on the phone the other day so I mentioned it to him. I was more concerned about warranty/odometer issues (if the OBDC thinks it's going farther than it actually is, then the odometer will be higher than actual and your warranty will expire sooner, in addition to there being "more" miles on the car). He said that "MINI's are notoriously high on the speed" (e.g. they register faster than actual) and that "BMW issued a service notice" that "the observed SPEED can be as much as 14% ABOVE actual". He didn't say if there'd be an upcoming remedy, but he did say that the 14% discrepency only pertained to the SPEED and NOT the odometer. I'm not sure how THAT works - I assume that the OBDC only monitors the RPM of the wheels (or engine RPM and gear ratio), time and gas consumption then calculates all the variables - so if, say, the RPM is off (AFAIK the time is OK), then both the odometer and speedometers will be off as well.

As to the discrepencies I noted to the SA, I did not look at a trip odometer in those cases, so it's still possible that the odometer is OK and the speedometer is somehow out of whack.

The only real way to test this would be to:
- Drive a measured "course" at a set speed (CC), measure the time it takes then see if there's a discrepency
OR
- Have a police "friend" clock you.

I'd tend to trust the vehicle's OBDC, and while I admit there might be discrepencies between "real" values and a GPS or mapping service, it shouldn't be that much, and the data is all pointing in the "same" direction (that is, the data "makes sense" and does not self-conflict).

Has anyone else noticed these discrepencies?

[MarkW]

can't you check the accuracy by timing the car over a known distance o get he time?

Here in the UK we get nice distance markers on motorways exactly 100m apart.

Also in the UK it is illegal for a cars speedo to underead but legal for it to over read as long as it's no more than 10%

[matfrost]

A good GPS unit should be as accurate as a police speed gun.

[Martin F]

To get an accurate GPS reading you need a flat straight road and as constant a speed as possible over a reasonable distance, say half a mile. Cruise control is good if you have it. Provided the GPS has a good number of satelites to use for calculation ( I believe 5 is ideal) the accuracy should be around +/- 0.2mph.

A speed gun is unlikely to be better.

[Pom Down Under]

To answer your original question - I have lots of Cop friends, but alas none over your side of the Pacific!

A Radar would certainly not be more accurate than a good GPS, however a Prolaser III or similar may be.
When I was a traffic cop I 'error checked' speedo's for motorists on a few occasions, but I would always warn the motorist that there are numerous factors which can influence the speed reading on the vehicles odometer.
If you are that concerned about your speedo being accurate I recommend you find a measured mile and use a unit similar to the Traffic Safety Systems VASCARplus (Traffic Safety Systems, Inc. - VASCAR-plus® home page) which you calibrate over the measured mile and use this as your speedo - of course this would be a very expensive option!

[schatzy62]

I did a lot of testing with this when i first got my car because I had heard of this in the Mini and my BMW is the same way. The speedometer reads faster than actual.

What i found out is that the USA does not actually have a standard on this as long as the speedometer does not read lower than actual speed. Many other countries such as in Europe have a reading allowance of minus 0 to plus 10%+2.5mph/kph. This is to ensure that when people travel on the roads the are not exceeding the speed limit.

Also through doing a bunch of calculations I found that if you take a new tire and measure it and use that as base figure and calculate the number of revolutions per mile and then check it against the same tire when it gets down to its wear bars there is a .8 to 1.15 percent difference in the actual speed shown.

So if we take a Dunlop Direzza Sport Z1 at say 215/40R17 and measure it we come up with 876 revs per mile when new but when used an ready to be replaced at the wear bars it now does 883 revs per mile. This equates to a .85% difference in mph.

Now lets take and compare a a few tires ofthe same size from different manufacturers and see what difference they have to the standrad we set from the Dunlop Direzza Sport Z1 at 215/40R17

Tire Revs Mer Mile

Dunlop Direzza Sport Z1 876

Hankook Ventus R-S2 873

Continental ContiSportContact 2 859

Yokohama Parada Spec-2 869

Yokohama S.drive 908

From this we can see that there will be a difference of +1.019% to - 0.96% just by changing the tires when new. now add the fact that the tire wear on continental goes down by 1.03% when calculated for ful wear tothe wear bars we find we can have as much as 2.049 % difference.

Now take a look at what happens when we go one size larger

225/35/R18

Continental ContiSportContact 2 853

Continental ContiSportContact 3 850

Sumitomo HTR+ 848

So by changing tire size we also have a range that can add to to the error.

This is why BMW/Mini and many other manufacturers build in to have the speedometer read slightly high.

As for odometer readings, I have also tested that in my mini against state highway markers in Maine. To test this I called the state of Maine and asked how accurate the mile markers had to be. I was told that they measure them in the right center of th right hand lane and they must be with 3 feet of the actual mileage (per mile) . So with that over 100 miles the distance of 528000 feet can vary from 528300 to 527700 of a .99 percent maximum.

I found that my odometer when measured against these file markers was off by more than one percent to my favor.

Measure distance 60 miles.

Odometer reading 59.6 miles

SO what i came up with on this is that my 50,000 mile warranty by the OBC will actually let me travel 50335 miles before the warranty is up.

As this is in my favor i am okay with that. If it was the other way around then I would probably be upset if it was more than a 300-400 mile difference.

So in conclusion could the speedometer be more accurate Yes. Do i see it as a problem no. I just set my car to read 60 and the car does 56 or set to 72 and the car does 65.

[Pom Down Under]

Wow...thanks

[Dr. J]

schatzy;

Thanks for the informative post. On the way to work this morning I reset each of the trip values and marked my time. Google maps indicates the trip is 56.7 miles and that's exactly what the ODO read at the end. Using time/avg speed gets me 58.6 miles which is about a 3.35% difference (increase), meaning my average speed was more likely 56.64 mph rather than 58.6mph.

I don't trust mile markers very much on highways here in CT and NY; in the past I've found them (with numerous vehicles) to be inaccurate. I think what I will do is a series of test trials with a GPS unit, traveling different speeds (CC) and see what the GPS says. Theoretically in the MpH vs rev (tire) is linear (both lines must share the same intercept at 0!) so we can decipher a correction factor for the speed (likely just a %).

Regardless, on my first attempt it appears that my SA was correct in stating that the speedo is out of whack whereas the odo is spot on.

[ravioliworld]

I bought a scangauge obd tool. The speed it displays is a few mph lower than the one displayed on the digital speedo on the tach. So, definitely the speedo on the car is reading high on purpose. On my Volvo, the scanner read the same speed as the speedo.

[Hairdresser]

Sorry to jump on your thread here guys. I have recently bought one of those Apple iPhones, (absolutely love it), and along with it bought a very fun and I think worthwhile program called Dynolicious-about 7 quid. The phone has a built in accelerometer in it and the program uses software algorithms ot work out speed/accelerations and if you input the cars weight, peak horsepower.
Don't know how accurate it really is but here are the screenshots:

Look quite spot on don't they?

S for speedo readings both my sat nav and my phone should prove to be more consistent than the car speedo then. I think the car makers do it to avoid lawsuits. I mean could you imagine some fool using it as a defence in court?

[Dr. J]

Quote: Originally Posted by ravioliworld (original)

I bought a scangauge obd tool. The speed it displays is a few mph lower than the one displayed on the digital speedo on the tach. So, definitely the speedo on the car is reading high on purpose. On my Volvo, the scanner read the same speed as the speedo.

Hmmm from Hamden eh? Maybe there is another Clubman in CT that I just haven't seen yet!

Anyway, I have the scanguage in the car now - it DOES read 2-3 mph lower than the OBDC but there are many calibration factors in the SG so I'm not sure how accurate it is (there is a calibration to adjust the speed displayed so I'm not sure what to trust!).

[Snowie]

Quote: Originally Posted by ravioliworld (original)

I bought a scangauge obd tool. The speed it displays is a few mph lower than the one displayed on the digital speedo on the tach. So, definitely the speedo on the car is reading high on purpose. ....

Same on my GP - Scan Gauge reads the same as sat nav which is about 3% lower than the car.

[ravioliworld]

Quote: Originally Posted by Dr. J (original)

Hmmm from Hamden eh? Maybe there is another Clubman in CT that I just haven't seen yet!

Anyway, I have the scanguage in the car now - it DOES read 2-3 mph lower than the OBDC but there are many calibration factors in the SG so I'm not sure how accurate it is (there is a calibration to adjust the speed displayed so I'm not sure what to trust!).

Nope, no clubman here. I wish, though, because then I would dump the volvo winter beater. I think the scangauge reads and displays the speed directly from the car. It has no other source of info to figure that out on its own. But, it also allows you to adjust it to account for differences in tire/wheel size and for other errors, like the built-in offset.

[Dr. J]

Well all I know is that there's something fruity going on - I can't understand how the ODO can be correct but everything else incorrect? I mean - presumably the OBDC only "knows" the wheel RPM, which with the wheel diameter it can calculate a speed. Then either through time or a wheel rev count it can know a distance. Everything else is calculated off of these values - including MpG. So, if the scanguage is giving us the same info that the OBDC is using to calculate the various values, then why is there a discrepency? Just very odd...

[Snowie]

Now I am confused

I think you got your OBD and OBC sssss drunk