There have been post saying that the speed odometer can be alittle off. For example, when you are doing 125, it might really only be 122.

Anyway, I just came back from my first long road trip from Chicago to Pitts. A 9 hours drive. The Mini did great.

So I had time to play with the on board computer that shows things like temp, miles left and so on. But it also show average speed.

This average speed... What I did no the highway is put my cruse control on at 60 MPH, then reset the average speed. It stayed at 59. So I tried it at 70, reset my aveage speed, it says 68.

So my thinking here is that the on board computer is more accurate? Or is it?

 

I've done the same thing that you just described, and came to the conclusion that the onboard computer was indeed more accurate. That's where the mpg numbers are computed, and presumably where the odometer resides. The physical speedometer on the dash, on the other hand, is deliberately set to read slightly high, with the effect increasing with speed. It's been posted here before that this is a trait common to all new BMWs, with the reason being to ensure that the speedometer never reads low, which could lead to speeding tickets.

 

The OBC is notorisouly inaccurate.
When I check my MPG compared to what the OBC says, it's off by several percent.

Also, the speedometers in ALL cars are mandated by the federal government to comply with the SAE standard for speedometer accuracy. I would trust the gauge on the dash more than the OBC.

 

Let's see if someone can confirm the answer to this. Is the average speed calculated only through the distance and time the engine was running? I believe that is how it works and it doesn't factor anything from the speedometer.

I know for a fact that the odometer in my MCS is inaccurate. It is reading low by 2.3%, so when it says I've driven 100 miles, I've actually driven 102.3 miles. I've tested and that's what I measured. Your results may vary depending on wheel size and tire choice.

I also know for a fact my speedometer is inaccurate. With the assistance of several "your speed is" meters about town, it seems a fairly consistent 4 mph off. It's strange, but it does not seem to be a percentage. If it says 34, I'm going 30 (more that 10% off). If it says 74, I'm going 70. So it gets more accurate the faster you are going, but never quite right.

If we assume that the average speed is calculated as I believe, we can conclude that the OBC average speed is more accurate than the speedometer.

 

The best way to calibrate your speedometer is to borrow a friend's GPS system and let the US Military's satellites and atomic clocks figure out how accurate your speedometer is.

Let's say that your speedometer is reading 97mph and the GPS states 100 mph. You now know that your speedometer is off by 3%

 

>>The best way to calibrate your speedometer is to borrow a friend's GPS system and let the US Military's satellites and atomic clocks figure out how accurate your speedometer is.
>>
>>Let's say that your speedometer is reading 97mph and the GPS states 100 mph. You now know that your speedometer is off by 3%


Sure, if you have a friend with one. I learn to work with what I have. Fixed radar displays or mile markers and a stopwatch. It might not be perfect, but if you take enough samples you get a pretty good picture.

 

I was using a friends GPS in my Miata once and it read right on the money. Comparing my MC speed with my wifes car it seems to be right on the Money. Also those speed devices always read correctly.

I have heard that GPS speed is accurate. I will have to try this sometime. As far as average speed and MPG mine is always right on the money. The most I have ever been off was by 0.8 MPG. I consider this to be very good. Those that have problems do you fill the filler neck with gas? However I doubt that could cause that much discrepency SP???

 

Dont know if anyone said it yet or not, but no spedo or obc is perfect, every single one will have an error, I forget what the MINIs % is. They get your speed by how fast the wheels rotate, and depending on the size of wheel and tire your speedo/obs will be off a certain ammount. I used to have a link to a tire size thing that would ask for stock tire size and new tire size and then it would tell you the difference between stock and new.

 

I have been a BMW/MINI owner for a decade now, and have been following several BMW and MINI discussion boards. This topic certainly comes up often.

Here is the deal - your OBC, when it comes to speed, should be dead accurate. The car's odometer reading is basically as close to accurate as it could get. It will always be off, because the diameter of your tire changes as the tire gets worn. However, the error is certainly much smaller than speedometer error.

Our speedometer on the other hand, is almost never accurate. This is because of German/European regulation dictating that the speedometer never read below your actual speed. Since BMW has to take into account variations, they simply moved the displayed speed up to ensure indicated speed never falls below actual speed. In fact, BMW's acceptable speedometer reading range is 0 below actual speed, to 10% plus 5 (don't remember km or mph) above actual speed. Therefore, at 70mph, BMW's acceptable range is 70 to about 80mph. Typically at 70mph our speedometer would read about 72-74.

I didn't come up with this info; this is what I learned from reading several discussion boards over the years. Hope this helps!

 

>>The OBC is notorisouly inaccurate.
>>When I check my MPG compared to what the OBC says, it's off by several percent.<<<<

Yes, but I think that this is because the OBC has no way of accurately measuring how much fuel has been consumed. As far as distance and speed go, I still think it's probably more accurate than the dash speedo.

 

>>I was using a friends GPS in my Miata once and it read right on the money. Comparing my MC speed with my wifes car it seems to be right on the Money. Also those speed devices always read correctly.
>>
>>I have heard that GPS speed is accurate. I will have to try this sometime. As far as average speed and MPG mine is always right on the money. The most I have ever been off was by 0.8 MPG. I consider this to be very good. Those that have problems do you fill the filler neck with gas? However I doubt that could cause that much discrepency SP???

Just beware that with a GPS you must be traveling on flat pavement to get an accurate measure. Using my GPS (on flat roads) I have determined that my speedometer is off anywhere from 2 to 5 mph (depending on speed and whether I'm on 15 or 16 inch wheels) e.g. speedometer shows 70, gps shows 67.

Just an interesting observation. Having read about speedo errors early on I did an un-scientific experiment with my friends assistance. He drove in front of me in his VW Jetta and we matched speeds. Our speedometers were spot on. Hmmm - is this a German thing???

Motor on!!!

 

>>>>>>Just an interesting observation. Having read about speedo errors early on I did an un-scientific experiment with my friends assistance. He drove in front of me in his VW Jetta and we matched speeds. Our speedometers were spot on. Hmmm - is this a German thing???
>>
>>Motor on!!!
>>

Yes, as KatanaPilot said above, it is a Germany/European thing! I have measured mine against the roadside "your speed is" signs a number of times and it is 4mph fast at 70mph. I therefore run at about 5 mph over the posted speed limit and don't worry much about it.
Accuracy of the Speedodometer, size of cup holders and lack of an arm rest are not very high on my list of what is important.