That's if you somehow managed to average 60mph. My typical average speed is closer to 25-30mph so it's more reasonable to expect half that, unless you drive almost exclusively on the autobahn to raise that average.

 

No not really, If the number used for the tire OD is indeed the culprit for the incorrect readings, the error will be constant on the odometer at the same percentage of error no matter the speed. I just used a number and a speed that's easy to fathom. In effect, a larger number placed in the algorithm for the tire OD would be reading as if you had larger tires on the car. Covering more, but equal distance with each revolution than a smaller number/tire. Your tires are going to go around the same number of times to go 10,000 miles, or one mile no matter how fast you go. (straight road, no spinning etc.)

 

Did you guys with the accurate OBC find that it needed a couple of tanks to learn, or was it accurate right off the bat?

 

Just to add to the confusion (if there is any) I stuffed 14.1 gallons in to the tank once. My OBC ssaid I had 4 miles to go before DOA time.

 

You "mine's accurate" fellers are assuming that your odometers are telling you the truth then...?
If the odo says 205 miles and you really only went 200 miles... ...ya follow?

I went **** testing mine against gps on loooonnnnnggggg stretches across CO, UT, and NV. It was interesting; try it.

 

The law allows the speedometer to be over by a certain percentage, but I do not think this is true for the odometer. AFAIK, the odometer should be very accurate or one could easily sue the manufacturer if they could prove it was reading too high. There'd be too many legal issues.

 

I mean if you put in "another" sensor to do what one could do, Why? just so speedometers could be wrong and odometers right? Huh?

Ok, maybe you could just write software to manipulate the data from one sensor to make the speedometer wrong, and keep the odo correct. That maybe a litle cheaper to do it that way.

If this is the case, what is the point? Slow us down? Or give us bragging rights?"I did 140 MPH today" when really all you "really" did was 120?... Maybe.

 

All I know is that it is not hard to make the speedometer accurate. A very cheap American car like a Chevy cavalier that we use for company cars is dead on with my GPS. Seems like some car manufacturers have their speedometer reading a bit high by a certain percentage. I can only conclude that they intentionally do this. Why they do this I do not know, but there'd be too many legal issues with the odometer being off by that same percentage.

 

I understand from my VW days that it's a law in Germany. The speedo has to accomodate different possible wheel/tire combos and is therefore a certain % optimistic per bureaucratic decree. The odometer is right on though (with the OEM tires). That's the way it was on my VW. I don't know how fantastically accurate GIS is for odometer use.

 

What do you mean?

 

Topography. I'd think a wheel would be inherently better in calculating miles traveled, up hill, down dale, left turn, right turn, etc.

Those mile markers are everywhere on the roads. I don't know how accurate they are, but that'd be worth a shot.

 

Both the speedometer and odometers use the same wheel-speed sensor, but as you suspected, the sensor data is processed separately for each gauge. So, there's no reason why a speedometer error would imply a corresponding odometer error, unless the speedometer error is from using a non-stock tire diameter (which *will* affect both the speedometer and odometer).

And I suspect that the intentional error on the speedometer is to avoid a class-action lawsuit.

 

If you're sure they use the same speed/distance sensor and they're just processed differently, then I suspect the error on the speedometer is perhaps to avoid a lawsuit. Like i said above...no point in having the odometer off too as there's another lawsuit right there.

 

I'm sorry, I can't see where there is a potential lawsuit in having an accurate speedometer. Like many have said, It's been done accurately for years with cables and gears in the differential, why would the electronics be more difficult? It just doesn't make sense to change to innacurate readings anywhere if they aren't making money at it. The only way I see them making money is by "shortening" the miles under warranty.

 

It's not about money. The problem is, speedometers are not precise, lab-quality instruments. Because of the tolerances involved, if the automakers tried to make them read spot-on, then ~50% of the speedometers out there would read high by some amount, and ~50% would read low by some amount.

In our modern-day litigious society, all it would take would be for someone to claim "I got a speeding ticket (or got into an accident) because my car was moving faster than the speedometer said it was", and we'd be looking at a class-action lawsuit against the automakers.

Silly? You bet - but it would happen. That's also one of the reasons I can promise you that there's no intentional error in the *odometer* system.

I also don't believe that the speedometers in the older cars were necessarily more accurate than those in modern cars. I just think that the proliferation of consumer GPS units has made a lot more people aware of the inaccuracy, compared to the small segment of society that actually tested their speedometers with mile markers and a stopwatch before GPS.

 

I did a search.
Bear in mind that I'm no expert.
The European Community seems to have a code relating to speedo accuracy.
There is a formula at the bottom of the attachment.
There can be some discussion of the formula following this post.

 

 

Correct. Europe requires that the speedometer CANNOT read under.
Since all systems have a +/- tolerance, manufacturers will bias the speedo so that even if everything else adds to an under-speed reading, the speedo will not show under. Hence, most MINI speedos will read over (and likely, most European vehicles).

The internal speed measurements are not biased (you can read them out with OBC readers) and show true speed - and the odometer is calculated using "true speed" without the bias.

However - errors due to tire size / inflation will affect everything, including odometer.

 

Thanks for posting this. Translated to English (and English units), it means:

The speedometer is not allowed to read low *at all*, and it's allowed to read high by as much as 10% of your actual speed, *plus* an additional 2.5 MPH.

 

As I said earlier a mechanic told me a similar thing about US motor vehicle laws.

The issue is variance. I agree that our speedometers aren't laboratory grade, but we're not measuring atoms either. Like many others have stated, older cars seem to have relatively accurate speedometers, or should I say have little variance.

My 2002 Jaguar was always off by 3-4 MPH on going through those "this is your speed" radar guns along highways an construction zones etc. But my 2001 Dodge Truck, then my 1994 Ford truck, my 1986 Porsche, my 2001 Honda VFR, and prior to the Jag, my 2000 Dodge Stratus ALL read the same speed on the speedo as the SAME radar sign.

Now it's been a few years since that sign has been around. However, there are different one's on other routes I travel, the MINI is off by 5-6 MPH, the '94 Ford is right on, my friends '89 RX7 is right on.

We are talking modern cars with factory tires and wheels that are off by pretty decent margins compared to these "old cars".

When I noticed the large difference on the MINI, I wondered about the odometer. Like I said earlier, I checked it against the tenth-mile marjers along the interstate and it seems the MINI Odo was consistently short of the posts. I know the signs may be off, I know this isn't a scientific measure. But it does seem an odd coincidence, if that is what it is.

 

Here's what the values would mean, I think.
Our VW's and BMW were off.
Our Mazda is right on.

 

I was contemplating the "why" of these "modern variances. Seems odd to me that the new tech would be "less" accurate. The root of all things is usually money, and I set forth another proposition that is still based in money as a cause.

It was relatively cheap to have a chart at the factory telling a worker what color little gear to put in a diff on a certain build of a car than it is to change the software. for each variance. I mean look how much it costs to have them reset your computer if you want to install the iPod hookup.

I think they pick the OD of the tire/wheel combo they plan on selling/buildoing the most, and let the chips fall with the rest. Plus there are alot more choices on tire wheel combos now than there were back in the day, however, you could pick your own gear ratios alot easier then.

I'm probably not right, but I'm done speculating, (big Hooray from the crowd)
See you guys on other posts.

 

FDOT regs allow 10% +2mph optimistic for US-spec cars; the guidelines for the odometer aren't watched for the same reasons. That would affect resale and warranty.
Use your GPS some time and roll for 200 miles straight on your odo; now look at your GPS. Bring that up at your next session with your MA; you're getting shorted on your warranty, or at least my cars are (about 2.5% off).

 

I haven't done it for 200 miles, but I've done it for ~100 miles in West Texas on a straight line.

My speedometer was off by about 4% the whole time, but the odometer was within half a percent at the end of the run. I attribute the speedometer error to the intentional error inserted by MINI, and the odometer error to normal odometer tolerances and variation in tire diameter.

If you're getting 2.5% odometer error with stock-size tires, that *would* be worth bringing up to the dealer.

 

Someone here mentioned that their Mazdas were right on. Funny you say that as my 2004 Mazda6 was not right on. It was 2-3mph off which is better than MINIs, but still it was not right on. That being said, when I use our company cars, they are Chevy Cavaliers that are just a couple of years old and they are right on!!!