Confused: Fuel Economy

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First question is how accurate is the onboard computer when it comes to mpg?

According to my calculations the figure stayed on my computer is more than 10% off what I think I'm actually getting.

I drive an 04, 2.2. The avg mpg is showing at 33.2 mpg but I don't think that can be quite right.

£25 of fuel gets me around 150 miles of mixed driving. At today's rough rate of £1.08 per litre that means around 23.1 litres of fuel. Divide by UK gallon measurement of 4.545 litres gives me around 5.1 gallons of fuel.

That works out an mpg of about 29.5 and not the 33.2 the computer states.

Is this something I should be worried about, is there potential fuel loss somewhere? Or do I just need to sit back and enjoy the ride? :)
 
Hi, what you need to do is reset the obc by pressing the stalk-you could be looking at a long term average that doesn't reflect your driving style :thumbsup:
Rob
 
Over a 2 year average of readings mine is about 1% out. The only way for you to really find out is to fill the tank until the pump cuts off, record the mileage, reset the obd and note the fill volume next time you do the same, then do the maths a bit more accurately.
 
Scooba_Steve said:
Over a 2 year average of readings mine is about 1% out. The only way for you to really find out is to fill the tank until the pump cuts off, record the mileage, reset the obd and note the fill volume next time you do the same, then do the maths a bit more accurately.
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x2

Beat me to it.
 
Also, have you got standard sized tyres? different width/profile can upset the obc calculations.
Rob
 
I'm sad enough to keep an Excel spreadsheet for each of my cars. :cry:

Anyway my first Z4C had an average of 32 on the OBC and my spreadsheet showed 32.3 so not much in it (this was for 8k miles)! Don't forget the OBC doesn't change every 0.1, it does it in jumps of 0.3 or so. And my calculation doesn't take into account it having more in the than when I sold it than when I picked it up! :scratchhead:

You could brim it, reset the OBC, do some miles then brim it again and see what numbers you get - but you are still dependent upon the car recording mileage accurately, unless you use GPS on a sat-nav!

But to be honest I'm quite happy to have a 3 litre Z4 that always averages better than 30 mpg, given the fun it gives me when I get a chance to chase the red-line!

Your 2.2 will have an M54 engine though - like my daily 325ti Compact, but that seems to manage very similar economy even though the OBC is showing 34.6 mpg!

Just enjoy it! :driving:
 
Thanks for the responses. I'll giving brimming a shot and see how it goes :)
 
http://www.sust-it.net/miles-per-gallon-mpg-fuel-efficient-cars.php?type=coupe&manufacturer=bmw&engine=2.2
You are not far off what the car should be averaging. Basic things already said about tyres/ spark plugs can make a difference. But really the style of driving and the conditions make a difference. Short journeys will result in low MPG. Also check you don't have the air con all the time by mistake !
 
So I brimmed the tank last week. As of my journey home tonight the tank was showing just under half a tank.

The computer, on the other hand, has conflicting info...

Miles done since brim: 154.9
Miles left in the tank: 187

I could understand a small difference if the gauge showed it on or just above half a tank, but not on its way down.

Any suggestions? I'm at a loss here.
 
brazenskies said:
So I brimmed the tank last week. As of my journey home tonight the tank was showing just under half a tank.

The computer, on the other hand, has conflicting info...

Miles done since brim: 154.9
Miles left in the tank: 187

I could understand a small difference if the gauge showed it on or just above half a tank, but not on its way down.

Any suggestions? I'm at a loss here.
Click to expand...

Yes, enjoy it more and stop fretting about economy - that's the domain of those who drive cars with tractor engines! :lol:

FWIW every car I have ever owned had a gauge that dropped slowly from full, then from half-full down dropped quicker and quicker the nearer it got to empty! Oh, and your gauge looks to be slightly below half anyway.

The clue is in the word "Gauge" - it means an indication, not a true measure (which the OBC seems closer to being from my experience). I don't think of the gauge as anything more than a rough idea of what is in the tank ti give me an idea of whether I need to fill up or not.

Now go and drive it as a Z4 was designed to be driven!! :driving:
 
Hi Brazen, the previous post is right - my gauges have also always been slower from full to half empty and then half to empty seems to fly down! That's on all three of my Zeds.
The discrepancy between the first half of your tank and the remaining mileage could be down to this or to the suggested mileage being based upon your most recent driving style.
From my reading of this topic on the forum over the years, I'd say most of us have found the computer read out fairly accurate. There are so many variables such as price per gallon, mileage covered and amount used in the tank that errors will occur when paperwork is used to work out figures - need to be very careful how it's done - brimming the tank and then noting mileage covered before brimming again can work but only if your calculations from litre to gallons is accurate.
Now I'm running the M I've tended to go for 'smiles per miles' instead of mpg - otherwise I'd spoil my runs out!
You have a Zed which is capable of chasing high 30s to the gallon on a steady throttle on the motorway which can be very useful on a long journey but once you bury that throttle mpg plummets to the low 20's. As others have said - the Zed is not really bought for its economical capabilities but as mentioned above, surprising numbers can be achieved with a light throttle foot, early changes up the gearbox and careful driving.
Either way - enjoy it :thumbsup:
 
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