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Technical Varying MPG on Computer

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Technical Varying MPG on Computer

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ActiveEz

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Hi all again,
Just another niggle on my 1.6 stilo. I don't normally play about with the computer while I'm driving but as I was on the motorway doing a steady 70 as one does :). I thought I would see what my mpg was doing.
It was all over the place one sec showing 50mpg then 26 mpg, 40 mpg then upto 60 mpg. I'm taking a stab in the dark here and thinking to myself this isn't right :confused: so as anyone got any ideas ??? or is it back to Fiat ?
kindest reagrds as always.
Ezz
 
Hi T, yes that is what I selected as I thought thats the one to check to see how much MPG I was getting at a steady 70 mph mile after mile. I was supprised to see it fluctuating so much and thought I would have got one reading at a steady 70 then maybe drop or increase as my speed varied. If this variation is the norm then what is the point in having it in the first place??
:confused:
 
Viewing the instant MPG is a good exercise in keeping a steady right foot! :D

If you want an average MPG at any given speed then simply reset trip 'B' once you're at the speed you want.

If you don’t trust the fuel flow device then compare the fuel you put in the tank against what the computer says you’ve used (best to do it from tank full to empty).
 
Yeah, I find when I get to hills I have to be really gentle, and even then consumption typically increases from 7.5L/100km to about 9.5L/100km.

Getting less than 10L/100km driving around town with the Selespeed takes tremendous willpower, like trying to eat just one or two chocolate biscuits. The moment that glorious engine note makes itself heard, that's over 20L/100km...

I just realised that you guys will have your trip computers reading in MPG because your distances will be in miles. I need the distances in km, but otherwise I tend to remember MPG easier... basically, 7L/100km is about 37mpg and 20L/100km is something near 20mpg...

Two more questions though...
- Does anyone else try and use 5th gear like I do, when driving at any speed over 50km/h or so, to get the consumption down from 8.5 to 7.6L/100km... do you think it's harmful to use 5th at such low speeds even if driving gently (in an Uno 70 or a Tipo etc. I'd never use 5th below 80km/h - oops, 50mph, 50km/h is 30mph)

- Is there in fact a fuel flow device? I've always wondered how these trip computers work...


-Alex
 
Yes the instant MPG is the one to use to train your right foot for featherlike touches and steady speed. A very useful tool it is too.

Like you Alex i try to whop it into 5th whenever i can but the curve goes the other way if you're at too low a speed and having to use gas in 5th gear, better off in a lower gear with less gas. It's not so much about gear or even revs it's that gas pedal! Keep the petrol engine between 1500-2500rpm whenever you have the time and you get the best power v economy. (Even lower rpm for diesels). Modern engines can climb hills at much lower rpm than older cars, again though, it's the gear that enables you to keep the smallest amount of gas on that wins the prize

Going downhill with no gas the fuel is cut off so it doesn't make much difference what gear you're in economy wise so you might as well be in a gear that gives useful engine braking

Block changing (or selective changing) is the key, boot it up to speed and block change into the econmy gears. Light footing around going gently up through each gear dosen't cut the mustard


Active
If your instant mpg readout is fluctuating wildly when you're doing a steady speed then something's wrong, if it's moving every time you move the gas then that's correct

Measurement? Well there's a direct relationship between intake pressure MAP and fuel flow and it's already reading out the miles so with pocket sliderule you can work out the mpg. It uses instant mpg figure too to work out how many "miles to empty"

The only bit i don't get is why there isn't an indicated hike down in instant mpg when you put the aircon on. All this grumbling from green peace that the aircon wastes fuel and it just doesn't appear so

You waste more fuel in drag at 50mph with the windows open than having the aircon on and windows shut (Mind you i tend not to have the windows down if I'm in drag, there's some funny people about:) )
 
Last edited:
Deckchair5 said:
The only bit i don't get is why there isn't an indicated hike down in instant mpg when you put the aircon on. All this grumbling from green peace that the aircon wastes fuel and it just doesn't appear so

You waste more fuel in drag at 50mph with the windows open than having the aircon on and windows shut (Mind you i tend not to have the windows down if I'm in drag, there's some funny people about:) )
Click to expand...
I don't think there can be any doubt that aircon has quite an effect on fuel consumption although I must admit pushing the Stilo aircon button does not produce quite the lurching affect you can feel on some larger cars (a bit like pulling the handbrake up lightly).

The story about the open window at 50 mph vs aircon is also often quoted but I'm not sure I'm convinced. Having said that, it's worth noting that the extra load from using aircon has no bearing on speed which means there MUST be some crossover point at speed where you're definitely better off shutting all windows, the sky roof and getting the aircon on.

The opposite argument is that at low speed you should ALWAYS shutdown the aircon and get those windows open.

Would be quite a nice touch if we had little tables in the backs of our handbooks stating at what speeds we should shut the windows. I love having my sky port open at speed but I sometimes wonder if I should close it and switch on the aircon :(
 
The effect on fuel useage can be tested easily,
Put the air con on when driving at speed. Do you need to add more gas to maintain speed? No? Then it's neglible effect. The internal fan you would have going already so it's just the compressor and the pulley for that is already going around on the Stilo.

Small engined cars will be noticeable, i've even felt like turning the air con off just to overtake when driving a few older cars, but on larger or more powerful engines the cut is miniscule

Driving at speed with the windows open though has caused rear screens to shatter and even blow out a sunroof as it's like dragging a parachute behind you so that is easy to see

I'm all for economy driving and do it whenever i can but you have to put it into perspective. If you drove in a maximum economy way for the rest of your natural life, light foot, keeping revs down, turning engine off in traffic jams, cycling whereever you can, getting the bus, walking to work then you would save enough energy to run the whole of the UK's power requirements ........ for 1 second

Meanwhile in the Phillipines, 2 million litres of crude floats out to sea

So off i go now to wind in my outside mirrors for less drag co efficient:)
 
Last edited:
Deckchair5 said:
The effect on fuel useage can be tested easily,
Put the air con on when driving at speed. Do you need to add more gas to maintain speed? No? Then it's neglible effect. The internal fan you would have going already so it's just the compressor and the pulley for that is already going around on the Stilo.

Small engined cars will be noticeable, i've even felt like turning the air con off just to overtake when driving a few older cars, but on larger or more powerful engines the cut is miniscule

Driving at speed with the windows open though has caused rear screens to shatter and even blow out a sunroof as it's like dragging a parachute behind you so that is easy to see

I'm all for economy driving and do it whenever i can but you have to put it into perspective. If you drove in a maximum economy way for the rest of your natural life, light foot, keeping revs down, turning engine off in traffic jams, cycling whereever you can, getting the bus, walking to work then you would save enough energy to run the whole of the UK's power requirements ........ for 1 second

Meanwhile in the Phillipines, 2 million litres of crude floats out to sea

So off i go now to wind in my outside mirrors for less drag co efficient:)
Click to expand...


Couldnt we get a group buy on some of that crude we could offer the petrol at discount prices after you have turned the lead to gold [notice I have given you the easy job Decks ]:p
 
Deckchair5 said:
I'm all for economy driving and do it whenever i can but you have to put it into perspective. If you drove in a maximum economy way for the rest of your natural life, light foot, keeping revs down, turning engine off in traffic jams, cycling whereever you can, getting the bus, walking to work then you would save enough energy to run the whole of the UK's power requirements ........ for 1 second
Click to expand...
:nono: That's the favourite reason for NOT making an effort to do a lot of things.
.........but if we all tried?

I must admit I've made hardly any checks on the Stilo in connection with use of aircon vs windows, sky roof open etc. The effect of using aircon is bound to be noticeable although as I've said before far less so at high speed. If I only had some nice long runs along the same route then I could do some testing.

Changing the subject. Are you sure the Stilo isn't fitted with any form of fuel flow device?

I remember some of the top end very early Golfs had them but tended to be very unreliable as the sensor would often stop at low flow levels. I had one and it was one of the very few times a ran out of fuel (damn computers :mad: )
 
Now we can all do something to lessen the impact by being aware and not doing needlessly damaging things. But you have to analyse the reality from the hype.

Biggest impact is to stop needless flying although that's largely been taken care of now

Yes those old flowmeters were abysmal. The MAP sensor gives almost all the information required to calculate fuel useage along with a few marginal parameters like temperature, atmospheric pressure etc. It does a good job of providing data to the ECU to calculate accurately what fuel to shovel into the engine so it just needs to do a bit more maths to work out the fuel per mile.
Going from memory here
Engine mass flow =rpm x Manifold Absolute Pressure/Absolute temperature
Fuel mass = Engine mass flow/ 14.7

So you just need to do that very second in your head. Who needs computers?:)
 
Hi Boys,
as always its a pleasure to read all your comments. So to my original question. If when I'm on the motorway doing a steady 70 with my foot not moving on the gas pedal. Should I get a constant value as I thought I would and not a wild variation. If it is a fault then is it a quick fix or something you think I should live with ?
 
"If when I'm on the motorway doing a steady 70 with my foot not moving on the gas pedal. Should I get a constant value as I thought I would and not a wild variation. If it is a fault then is it a quick fix or something you think I should live with ?"

It should stay pretty much the same level although it will sample evey few seconds, there shouldn't be anything wild about it

Maybe MAP sensor showing problems. Does the "miles to empty" stay steady?
 
Deckchair5 said:
The MAP sensor gives almost all the information required to calculate fuel useage along with a few marginal parameters like temperature, atmospheric pressure etc. It does a good job of providing data to the ECU to calculate accurately what fuel to shovel into the engine so it just needs to do a bit more maths to work out the fuel per mile.
Going from memory here
Engine mass flow =rpm x Manifold Absolute Pressure/Absolute temperature
Fuel mass = Engine mass flow/ 14.7
Click to expand...
Decks, That's a fascinating insight into how the on-board computer works out fuel consumption. I had no idea that the consumption display is quite literally showing what fuel is going into the engine.

My only thought on the subject is that instead of relying on the fuel/air ratio to calculate the fuel used - would it not be more accurate to measure the pulse duration to each injector (along with fuel temperature and perhaps pressure if it's measurable) ?
 
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