needforspeed

Better than I ever got from my first car a 'brown' MV4 ford cortina - RNL 713Y. But then I think 4-star was about 40p a litre then (1990).

God rest her she was a lovely car and great for handbrake turns

KAF

If you run a tank by tank consumption test you will indeed see variation, which is why I suggested running it for about a 1000 miles.

I filled up at my local Petrol station and recorded to mileage and fuel needed to refill for 3 refills from near empty. Taking care to use the same pump.

This meant I used a total of some 171 litres of fuel, even if in the final fill I was a litre out from the fill up level of the first tankful, the margin of error is only 0.6% error in total fuel used.

I know the pumps are giving accurate readings of the fuel dished out 'cause they are calibrated and checked by law.

I checked my odometer with my Tom Tom Sat Nav, over 25 miles it was accurate to within 0.1 mile. So that was OK.

Now over 1100+ miles and 171 litres of fuel I now know my calculations are going to be pretty accurate, it worked out at just over 30 mpg.

The OBC which I reset when first refilling and left untouched until the end showed 27.4 mpg.

That is a big error. I know which I trust, the computer is only as accurate as the calibration set in the first place. And as it does not have to be accurate by law, unlike speedo's and odometers, why should BMW bother.

Now if you have no interest in your fuel consumption, fine, everyone to thier own. But if you are, you might as well have the OBC displaying accurate figures.

For info, the OBC displays the average fuel consumption since the time you last reset it, so you control the period of time it is looking back at.

The range left, uses the average fuel consumption since (I think) you last switched off the engine, so moves around a bit more, it is looking at your recent fuel consumption which is a better guide to how far you can go before you have to push the car.

But if the calibration in the OBC is wrong in the first place, the range will also be wrong.

Finally, trust the computer as it is set up by BMW to be accurate? Guess that's why they built in a system to adjust it then

new-kid

My 530d only gets 20-22mpg around town. BMW not interested after sold - stated in writing that "this is commesurate with the vehicle".
Checked physicaly brim to brim for 10k miles - overall only 26.5mpg.

Around town this is worse than previous 2.8l petrol, which had heavy quattro transmission to boot.
Even very gentle motorway potter never averaged more than 36mpg on any trip

needforspeed

Each to their own and I quite understand if you wish to have your OBC reporting exactly right consumption. However, are you certain that the above is correct.

It's just that (for instance) in my last car I probably did 20,000 miles without resetting the fuel consumption, are you certain that the 'average' would look back that far? That would be a very large calculation.

My suspicion is still that it is a rolling average with a limited data set, which might explain the discrepancy you have found.

In respect of the other points - I guess trading standards are fairly hot on the petrol pumps, but given that it is pretty much accepted that the speedo runs fast (by approx 2mph) doesn't it follow that the odo does also (this would certainly have been the case with an old fashioned cable driven system). I can;t imagine this is a legal issue for BMW.

I have tomtom satnav as well and I am not sure that it is an entirely accurate check of the odometer, although interestingly it always places my speed between 2&5mph below what my speedo reads (and it gets me where I need to be).

Nopointsyet

Thank you again. I will look at this. One obvious question is will fitting a DMS invalidate the BMW warranty?

NPY

colejl

In theory yes. However, I believe DMS provide their own 'guarantee' to protect you - discuss it with them... (Actually I believe it's a no-detect and reprogram warranty)

KAF

1. I am certain.

2. This only applies to the 'range to go' function

3. If the ODO and Tom Tom, give the same mileage, then it's a pretty good bet that they are both accurate. The Odo has to be pretty accurate to pass Type Approval Inspection. They only get a very small margin of error, similar to speedo.

4. GPS is location accurate to about 15-25m at worst. Maps are also pretty accurate also, so it should be good.

Your not a very trusting person are you......................

needforspeed

Ok then - you win I'll trust you

I spose this has just never hassled me enough to pay attention. I think it probably stems from the fact that I have had free company fuel for the last 5 years

Now that I've opted out of company carship and am paying for my own derv, perhaps I should start checking a bit more (I've already stopped filling up at the motorway services :thumbsdown: )

At the end of the day though - does it matter what the computer says - you know what you've paid for fuel and how far you've been so ...... isn't that it ?

gIzzE

I am not sure if the mpg reader does continue to work out the figures for the life of the car if you don't reset it.

I know with Audis when they get to 999miles they start a new loop, in other words it only looks at the last 999 miles of travel, it does not reset so if you have got 35mpg for 999 miles and then you floor it from 1000 to 1005 and only get 15mpg that is all that shows, it just no longer uses the info from 999 miles ago.
Does that make sense??

KAF

It does, but having left mine untouched for several thousand miles, the MPG readout remained consistent. Suggesting it does not reset in the BMW.

After all it would take something strange to alter a figure that was averaged over 5000 miles.

PS. If I had company gas I wouldn't give a stuff over mpg either !