trying to work out diesel MPG

[Starbuck88]

Did that familiar journey in the BMW today.

34MPG going... 32.1MPG returning. Same Speed. Less Weight (down a passenger and 150 miles worth of fuel burnt)... just goes to show, you may not notice anything and it may feel the same to us squishy things behind the wheel.

[ollienoclue]

Originally Posted by bidderman1969

bear with me on this.....

been getting fuel in the oil, so i had to try something as someone had said it could well be something to do with it trying to regen more times than normal, but i have never had any DPF issues yet, so thought i'd try something out

i did an airport run sunday, reset "average fuel consumption" drove normally to airport, 42.5 mpg, trip starts off rural, fairly hilly, for around 10 miles, and about 40 mile in total would be the "run", fairly consistent 70 mph slowly for some traffic, but only to 5th at one point, M25 stayed in top gear, varied between 50 and 75 mph.

on the way back, i thought i'd try the "keep revs up, lower gear" technique to see if i could thoroughly clean the DPF system out, or see if it might help, so from heathrow to home i avoided 5th and 6th gears very weird experience having the car roaring away at around 3000 revs all the way home, mostly 65-70 mph and had to discipline myself not to change up regardless. back to the area i started of at, 45.5mpg



how do you work that one out?????

car does seem to be smoother a little, might try this routing a few more times

It is to do with engine load. Fuel consumption on a graph is always quoted as grammes per kWh but these figures are at maximum engine load which would be a rare thing in a car's life.

Although on board fuel consumption meters are always a bit dubious, I would not expect huge differences in fuel consumption when driving a stretch in 4th gear or 6th. Yes you are burning fuel to keep the engine at a higher speed but the load is largely the same because your vehicle mass and speed were largely identical.

There is a lot of computer tom-foolery going on in a modern car, none of us have direct access to fuel delivery any more- we are all used to thinking of the pedal as a throttle; you pull on a bit of wire and more fuel enters- at least in engines of old- but today we have no such authority. A computer decides how much fuel will be metered out and injected based on engine speed and the amount of air coming in at that moment. I presume in some cars engine load will also be calculated.

It's all clever stuff.

[Summit Detailing]

Download the Roadtrip app to give you actual an actual mpg per fill up, longterm mpg and other graphs.

All car trip displays are way off, my car shows long term mpg of 39.4 on the dash, actual mpg is 34.15.

Happy driving.

Cheers,

Chris

[ollienoclue]

The best way to establish actual MPG is brim the tank, drive a known route over a known distance and then brim the tank again so you know how much fuel was used for certain. No messing then.

I've experimented with the real-time MPG display in many cars and you will be surprised to to see how the various engine speeds and gear ratios marry up when it comes to their fuel consumption.

I was taught that optimum fuel consumption was usually where the engine produced peak torque but modern engines are far too clever for that and often have their torque curves artificially constrained.

Each engine will have a specific map on which there will be an area where the best fuel consumption is obtained. I cannot remember what that measurement is called but it is often shown as a graph with engine speed on one axis and engine load on the other. On that graph there will be a region where fuel consumption for a given engine 'thump' (torque basically) is best. It is not a single point but more of a blob or circular shape, so it is entirely possible that you could get the same fuel consumption at say 2000 rpm and 3000 rpm for a given engine load according to how much power is needed- the factors are all interrelated and change each other.

[PugIain]

I do full to empty, when filling up let the pump click off. Usually get about 35mpg which is alright enough for me. Some people I've read on the net are getting 20 something, which either means I'm driving too steadily or they're unsympathetic oafs.

Only ever given the estimated MPG read out a few glances, but it looks to over read a little.

[NorthantsPete]

Originally Posted by ollienoclue

The best way to establish actual MPG is brim the tank, drive a known route over a known distance and then brim the tank again so you know how much fuel was used for certain. No messing then.

I've experimented with the real-time MPG display in many cars and you will be surprised to to see how the various engine speeds and gear ratios marry up when it comes to their fuel consumption.

I was taught that optimum fuel consumption was usually where the engine produced peak torque but modern engines are far too clever for that and often have their torque curves artificially constrained.

Each engine will have a specific map on which there will be an area where the best fuel consumption is obtained. I cannot remember what that measurement is called but it is often shown as a graph with engine speed on one axis and engine load on the other. On that graph there will be a region where fuel consumption for a given engine 'thump' (torque basically) is best. It is not a single point but more of a blob or circular shape, so it is entirely possible that you could get the same fuel consumption at say 2000 rpm and 3000 rpm for a given engine load according to how much power is needed- the factors are all interrelated and change each other.

The problem ive always had with brimming is on a car that holds 80 litres of fuel ,thats a lot of weight... if you did two 40 litre jaunts it would be the same amount of fuel but better mpg due to less weight.
THATS if the fuel station is on your route, if you need to drive elsewhere it can negate any benefit.

If you drive by the instantanious mpg calculator, youll see how different syels affect mpg, i notice a big decrease after 55mph, no matter what gear or load, and when a hill comes, its down to 15mpg before returning 99mpg ont he incline averaging out to 35mpg.

[Christian6984]

I use an app called Fuelly on fiesta. Don’t let the fuel drop much below a quarter and then always fill full. Put in details over the last year and tip computer and average are the same at 53mpg. And the one that got wrote off with a lot more miles on was doing nearly the same.






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[PugIain]

I use fuelly too. My 86 is averaging 35 ish.

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[NorthantsPete]

i cant use fuelly, it asks to brim the tank
On av70 volvo, that's 80 litres im luggign around!

I instead refill when the ecu says 50 miles left and the light comes on and fill up with 30 litres and do this 3 time to make 90 total.

[Christian6984]

Originally Posted by PugIain

I use fuelly too. My 86 is averaging 35 ish.

Sent from my VFD 710 using Tapatalk

The Fiesta ST has averaged 35, but not sure if it because its low mileage

Originally Posted by NorthantsPete

i cant use fuelly, it asks to brim the tank
On av70 volvo, that's 80 litres im luggign around!

I instead refill when the ecu says 50 miles left and the light comes on and fill up with 30 litres and do this 3 time to make 90 total.

yeah definite disadvantage of it, if you select part fill doesn't give the mpg until next full tank