Question about BMW on-board computer

Voodoomelon

Hi guys.
Just a quick question, when you choose between MPG or KM/Litre as the readout for fuel consumption, is the MPG imperial gallons or US gallons? I would presume imperial, would that be right?

Because either way, based on my calculations, the MPG readout is way off.

I filled up last week when the tank had 44km left in it. I noted the mileage on the odometer while at the pumps and filled up by €40, or 40.45 litres and reset the MPG to zero.

Now this week I let it drop to exactly 44-45km of fuel left in the tank, so the same as the week before. I had done 131 miles on the odometer.

40.45 litres in US is 10.66 gallons, which means 12.29MPG.
40.45 litres in Imperial is 8.90 gallons, which means 14.71MPG.

Computer reads 19.7MPG, which is way off. I would expect BMW to be somewhat generous with their readouts, but I wouldn't have thought it would be that wrong...

peasant

Voodoomelon wrote: »

I filled up last week when the tank had 44km left in it.

Now this week I let it drop to exactly 44-45km of fuel left in the tank, :

That's where your "problem" is. The remaining distance is just a guesstimate from the computer and in no way accurate. You cannot expect a correct reading out of this.

Brimming the tank, checking the miles/kilometers driven and then brimming the tank again is the only way to do it.

Voodoomelon

It may be a guesstimate, buts its the same estimate it made both weeks if you know what i mean. The level of petrol remaining is always accurate, relative to its own calculations anyways.

But I take your point about filling the tank, I doubt it would make any difference though.

Seth Brundle

I take it its on the 728i in your sig. Firstly, I really doubt that its a US import and would strongly presume that its imperial measurements.
Secondly, I think your greateer concern should be the consumption of under 15mpg. Driving unkel's 735i a few months ago I was getting waaaay more than that even in rush hour traffic!

Esel

^Shill Alert!

unkel

It is Royal British Imperial Gallons alright

As the lads said, the 'puter is only a vague indicator of fuel consumption. Fill up, measure distance and fill up again (several times)

Voodoomelon

I do get more than that, that was from a nightmare week of last minute Christmas shopping with traffic from hell in town, bumper to bumper! I get 33MPG on the motorway and 23MPG around town (according to the computer).

Seth Brundle

esel wrote: »

^Shill Alert!

ssshhhh

Voodoomelon

It costs me €1 to go 3.3 miles. Bejaysus!

Voodoomelon

Just to follow up, I filled the tank 2 weeks ago (well, 7/8s lets say) with 72.54 litres.
On board computer read out there was 68km left in the tank before I filled up, 2 weeks later (now) there is 68km left in the tank again.

In that time I did 318miles and the 72.54 litres I filled up with is 15.96 gallons.

Which equals 19.92MPG and the computer reads 21.0MPG. That's pretty damn close, so i'm a happy bunny.

rebel.ranter

The value of miles remaining is based on the previous recent driving. So the 44 miles remaining one week may be a good bit different than the next.

For example if I drive form Cork to Dublin the journey normally entails more efficient driving meaning that the OBC will estimate that the remaining fuel will bring me "x" miles (based on the assumption that I will be driving as efficiently as I had going from Cork to Dublin).

However if I am just doing the city commute which is not as efficient the OBC will calculate that I need more fuel to do the "x" miles.

Finally if you watch the miles remaining while you progress on a journey on open roads you'll actually see the miles remaining growing as you drive along, this is the car taking into account the change in economy.

Robbo

It's worth bearing in mind that the "Range" quoted is usually a very conservative estimate. I've driven an E61 past when it hit 0 and then I managed to make another 5 miles to the petrol station with it reading "---".

Anan1

Robbo wrote: »

It's worth bearing in mind that the "Range" quoted is usually a very conservative estimate. I've driven an E61 past when it hit 0 and then I managed to make another 5 miles to the petrol station with it reading "---".

I'd imagine the BMW Assist service would collapse if they didn't leave a bit of wriggle room!

Voodoomelon

rebel.ranter wrote: »

The value of miles remaining is based on the previous recent driving. So the 44 miles remaining one week may be a good bit different than the next.

For example if I drive form Cork to Dublin the journey normally entails more efficient driving meaning that the OBC will estimate that the remaining fuel will bring me "x" miles (based on the assumption that I will be driving as efficiently as I had going from Cork to Dublin).

However if I am just doing the city commute which is not as efficient the OBC will calculate that I need more fuel to do the "x" miles.

Finally if you watch the miles remaining while you progress on a journey on open roads you'll actually see the miles remaining growing as you drive along, this is the car taking into account the change in economy.

Yes, this is indeed absolutely true. But all of my driving is essentially exactly the same, I only venture onto motorway or even a 100km road once or twice a month, and do 90% of my driving in down, my average speed is only 36km/hr. So the remaining fuel would have been very accurate.