mpg indicator

[pirate952]

how acurate are these? and how does they work?
i keep finding myself watching it more and more, seeing if i can keep it over 25mpg. then i normaly get bored and see if i can keep it under 12.
i know its just instant mpg, just wondered if anyone takes any notice of them.

[leeparkes]

i think it just works out an avererage over a journey

[pirate952]

what the one under the rev counter? it cant work out a average its the instant one.

[Simon13]

and it's a very good guide too

[e30topless]

i do keep an eye on this just lifting off the pedal ever so slightly especially on hills can give more mpg not sure how accurate it is though ?

[FlappySocks]

The ECU knows how long the injectors are open for, for any given moment, and how fast you are going, and what the fuel pressure is. Stick that into a formula, and out pops the MPG.

How accurate? Not very. Too many inconsistent variables, such as tyre tread depth/inflation, fuel regulator accuracy, and how clean the injectors are.

[leeparkes]

say your doing a constant 70mph and keep it there what you see on the gauge is your average mpg,

useless round town though

[pirate952]

i see what you mean, yeh average if you keep at that speed, i thought you ment average over a few miles.

[Felix79]

It's a good guide I find over a longer journey. It's one of the litte things I have always liked about BMW's

[papercutout]

I always thought that having a little guage in the fuel pump and having a flow meter would be a far better idea for monitoring MPG, but thats just me being logical!

I thoroughly despise mine, I've only got a 4 cylinder, but whenever I boot it it drops to 15mpg in higher gears, 12 or 10mpg in lower gears. And I find myself sitting there on the motorway trying to get it to sit as close to 50mpg as possible, and inevitably find myself doing 60mph instead. Still, I drive more gently because of it...

[FlappySocks]

I always thought that having a little guage in the fuel pump and having a flow meter would be a far better idea for monitoring MPG, but thats just me being logical!

Ah, but then you would need two flow meters...one for the fuel in, and one for the fuel return to calculate the difference. Two more components to go wrong, and extra expense for the manufacturer.

[pirate952]

ive always loved the gauge cus i can remember when i was a nipper and my mum had a few bms with the gauge, used to watch it fly from 50 to 0 constantly lol
and lol, i only have to look at the go fast pedel and mine drops to 10! but then again if you watch the instant mpg on other cars they often drop to the same. anyway owning a bmw is to enjoy the quality engineering and you gots to pay for that i find myself picking up the 320 keys alot more often then my fiat ones, and the fiat can do 3 times the mpg.

[fingerman]

I think the gauge's on the bm's are great as I do a fair bit on the motorways and try to keep it over the 30mpg - never really worked it out per tank but it does make you (well me anyway) drive more economically

[zimmer-320i]

I thoroughly despise mine, I've only got a 4 cylinder, but whenever I boot it it drops to 15mpg in higher gears, 12 or 10mpg in lower gears. And I find myself sitting there on the motorway trying to get it to sit as close to 50mpg as possible, and inevitably find myself doing 60mph instead. Still, I drive more gently because of it...

on a motor way doing 80 mph at 4ks with about 5-10% pedel pressed i get the needle just over 50mpg!!

[fingerman]

Hhhm - not that accurate on your's then

In mine I can do that at 80 going down a steep hill ! - on the flat I can do just over 30mpg (well it says on that on the real time thing!) and do 70mph

[djk]

The ECU knows how long the injectors are open for, for any given moment, and how fast you are going, and what the fuel pressure is. Stick that into a formula, and out pops the MPG.

How accurate? Not very. Too many inconsistent variables, such as tyre tread depth/inflation, fuel regulator accuracy, and how clean the injectors are.

It won't be very accurate to real world MPG, but it is useful for comparing different techniques, such as which gears to use when going up hill - say you were going up a hill that required you to change out of 5th, sometimes 3rd is better than 4th despite the increased revs because you're not using as much throttle. I had assumed it worked off intake vacuum divided by speed as this seemed a simpler way to get a reading. I got 37mpg on a long motorway journey in one of my 320s in part thanks to this little gauge, saved a few bob.

I always doubt the average consumption readings of trip computers as well - they're really there so that if you notice a drastic change you know there's something wrong somewhere - I doubt they are even close to true consumption in a lot of cases as they would have to be much more sophisticated.

[Brianmoooore]

The econometer on an E30 is reasonably accurate, within the constraints of an analogue gauge. It knows the road speed of the car from the pulses from the diff, it knows the fuel consumption rate by measuring the width and frequency of the injector pulses, and calculates the instantaneous fuel consumption from these.