The MPG needle on my BMW

[eltax91]

Hi guy

Recently picked up a 2006 e61 5 series for the daily motorway trudge. First impressions are its perfect for the task!

I have a quick question. The car has the traditional 'needle' mpg indicator as well as the info available on the in dash display and the iDrive system.

What I'm wondering is what are BMW using as the input to measure this mpg? How's it it calculated. I hadn't expected it to give and meaningful info, since my car is lpg, however it seems to swing away from left to right whenever I hit the loud peddle and hovvers around where I expected it to on a cruise.

So, my question boils down to; how reliable are the dash readouts, given I'm running on gas 99% of the time? Intruiged to satisfy my curiosity by knowing how its calculated.

Cheers all

[carlingofblack]

That MPG needle in my 330i just depresses me... Wish the bloody thing would just break!

[yorkshireegg]

I've wondered the same since getting my 3-series, not really looked into it much so this is just speculation based on my own observations.

As far as I can tell it reads engine speed, road speed and throttle position / engine load. Then there's probably a fudge factor based on the mapping of the engine.

I'm not sure how accurate the reading is, an instantaneous mpg is pretty useless anyway! Think of it more as a 'power reserve' needle rather than mpg, it will upset you less that way when you floor it

[BFG TERRANO]

The needle in my dads used to just wave about, I often wondered what's the point.

I think the older ones were operated by vacuum, the newer ones probably electronic. As for accuracy the one in my company Seat is not far off to be fair.

[littleredrooster]

On petrol models, I'm fairly sure that it's a simple vacuum gauge off the inlet manifold. The early ones definitely were.

[blondini]

It will be driven by the ecu using road speed against real time fuel usage which is calculated from the length of time the injectors are open. Something like that...

[andy43]

littleredrooster said:

On petrol models, I'm fairly sure that it's a simple vacuum gauge off the inlet manifold. The early ones definitely were.

That's how the E46 one works.

[littleredrooster]

blondini said:

It will be driven by the ecu using road speed against real time fuel usage which is calculated from the length of time the injectors are open. Something like that...

The computer may work like that, but not the wildly-swinging needle!

[0a]

I'm not entirely sure of the point of the needle in my merc, it tells me the exciting fact that using throttle is bad for mpg.

[yorkshireegg]

littleredrooster said:

On petrol models, I'm fairly sure that it's a simple vacuum gauge off the inlet manifold. The early ones definitely were.

Makes sense actually! I stand corrected.

I still try not to pay too much attention to it though.

[philmots]

Iirc some of the US tuning equipment can turn these otherwise useless gauges into slightly less useless gauges showing boost etc

[eltax91]

Thanks for the responses. Nice and basic then! So, relating this to the fact the vehicle has lpg, if ky computer reads an average of 32.4, as it did yesterday, this is basically calculated how? Surely it can't be working it out on the lpg used? It can't measure petrol used, since it does use any and still show in the 30's, so its basically measuring air mass on intake and 'guessing' the rest?

[McSam]

littleredrooster said:

On petrol models, I'm fairly sure that it's a simple vacuum gauge off the inlet manifold. The early ones definitely were.

Yep, they are (the dials). I can't see why they'd have changed it from that, works just fine - but it is of course corrected by the roadspeed too. It also knows when fuel is cut off on the overrun, you can see it step from high 70s to just rest at the "infinite" end when that happens.

It gives a pretty good reading at a cruise, and I quite like mine for that, useful little thing.

It only "waves about" if the driver is incapable of applying a constantly load.


In any case, OP, your computer will still be telling you how much petrol you would be using, were you just using petrol. It can't account for the LPG. It simply knows how much petrol should have been injected to do a particular thing, if you see what I mean - the fact that you actually require a bit more LPG to do the same means that the gauge won't give you a correct LPG economy reading either.

[CraigyMc]

carlingofblack said:

That MPG needle in my 330i just depresses me... Wish the bloody thing would just break!

If you sellotape it to the 50mpg mark, your car will have a much greater range than it does now and it'll also be much cheaper to run.

[carlingofblack]

That is a fabulous idea!

[CraigyMc]

carlingofblack said:

That is a fabulous idea!

I suppose a better idea would be to tape the fuel gauge to "full" - then you'd not need to fill up at all, and you wouldn't have to care about MPG either.

It's all in a day's work for Mr. Logic

[laingy]

CraigyMc said:

carlingofblack said:

That MPG needle in my 330i just depresses me... Wish the bloody thing would just break!

If you sellotape it to the 50mpg mark, your car will have a much greater range than it does now and it'll also be much cheaper to run.

I keep reading how every 330 averages 30mpg so it shouldn't be too bad

[Nigel Worc's]

As suggested by others, this seems to be a vacuum guage.

They were very popular in the 70's & 80's, when we used to buy consoles and fit extra guages, back in the days when a lot of cars didn't have clocks, radios, temp guages etc etc.

They can be quite educating in a way, if you wish to drive economically.

[Riff Raff]

McSam said:

Yep, they are (the dials). I can't see why they'd have changed it from that, works just fine - but it is of course corrected by the roadspeed too. It also knows when fuel is cut off on the overrun, you can see it step from high 70s to just rest at the "infinite" end when that happens.

It gives a pretty good reading at a cruise, and I quite like mine for that, useful little thing.

It only "waves about" if the driver is incapable of applying a constantly load.

I admit that I use the needle for economy (not something that a PH'er should admit to I know)...... but

When on the motorway at a constant throttle opening, the gauge reads a constant number. Quite often you'll find that if you lift just a fraction, you can maintain the same speed, but the economy gets better.

It's like a game to me now, to see how high I can get the needle pointing whilst maintaining the same speed. I dare say it's a game I wouldn't have played when petrol was 70p a litre, but now it's double that......

[Mastodon2]

The fuel needle in my Type R works like an MPG gauge. If you've been clogging it, it drops right down, and if you start taking it easier, it starts to refill, as if its regenerating petrol, or telling you how far you'll get if you drive more gently.