I listened to the segment of the show regarding mile markers and realized that Tom and Ray really do not know everything there is to know about cars! Why this was a sudden and recent revelation to me I do not know because, now that I think about it, there’s been more than enough evidence over the years to justify my conclusion.
What is the true answer to the mystery of the odometer and the mile markers you may ask. Well, it’s remarkably simple. The displayed speed (and consequently accuracy of the odometer) is governed by the laws of various countries. By agreement, Europe has a more or less common standard for instance. Almost all countries have a minimum requirement that the indicated speed must never be less than the actual speed of the vehicle. US Federal law allows for a maximum 5mph error at 50mph but since the error must overstate rather than understate the speed and given that no instrument is perfectly accurate, the speedometers always, by design overstate the speed and distance.
Let me add another voice to verify what Mike found. I too have the same model BMW, a 1998 540i, and I have obvserved the exact same difference between the speedometer and what I believe was my actual speed. My standard for the actual speed is a fairly new Garmin GPS. I never measured the difference down to the tenth of a mph, but my calculated difference was always about 5% regardless of the speed. I also believe that the car has the same size tires as when new. From what others are saying it seems BMW and other manufactures may do this intentionally to be sure they meet legal requirements. I just find it surprising that BMW which prides itself on great engineering ( it is true) would do this. They could design it to be accurate. Also this is not the case in my other family cars - an 2007 Acura TL and a 1998 Subaru Outback. (And yes I too have an engineering degree but that doesn’t mean I am crazy on this issue - it just increases the chances. )
Anyone else find a significant difference between their GPS speed and the odometer?
Adding to the conspiracy theories, suppose the state of Montana had hired a private construction company to build the road, and suppose the private company were paid by the mile. It would behoove them to “shorten” the miles and claim the road is actually longer than it really is, thus getting paid more.
George, if Montana paid them based on the mileage from a road vehicle, any road vehicle, I’d say that the Montana commissioner of highways should be fired and investigated for corruption. That should constitute the largest corruption investigation in Montana history. Oh, and I’m sure representatives from the federal Highway Transportation authority would want to have a sit-down as well, as there’d likely be federal funds involved. A federal investigation would, I’m sure, ensue.
From what the comments here suggest, the odometer and speedometer are separate. That’s fine. What concerns me then is how accurate are the computer functions that make use of this data. The MPG reading of my car computer (2010 MINI Cooper S) is regularly far above what the MPG I calculate using the trip meter and the gallons purchased at a fill up. This makes me less than trusting of the “miles to empty” readout.
On a modern car, the speedometer and odometer both operate by the signal from the Vehicle Speed Sensor (VSS). In olden days, there was one “speedometer cable” that ran both, but the gages themselves were mechanical, and one could break and the other not. .
This VSS signal may, in fact, be used in engine and powertrain controlling functions such as the Cruise Control system and the transmission. But the errors we’re discussing are normal errors that will have no effect whatsoever on the operation of the vehicle. All measuring devices and systems have some error to them. Even laser systems. Even the atomic clock. Error only becomes important when it adversely affects system operation. The ones we’re discussing don’t.
By the way, the “miles to empty” readout has error intentionally built into prevent those who actually use it from running out of gas. It’s a strange fact of humans that the “empty” has to be adjusted so that it isn;t really “empty” at all…
Uh, this is what you said: “The more squat, the less diameter, the less circumference, and thus less distance traveled per revolution (i.e. speedo “error”).”
well, I am almost certain that a low tire will travel a shorter shorter distance in any number of revolutions. And give a higher speed reading on the speedometer.
@MG McAnick ,
Not to be difficult but as a result of a GIS system implemented ten years ago in Mass My highway mile marker went from 51 to 50. Same old strip of asphalt. Do not trust mile markers much, however many areas still have what is called a measured mile marker, one at the start and one at the end. Those are exact.