Economy

[McSam]

roachcoach said:

That's odd, or more specifically surely only part of the picture as my car gets VERY similar fuel economy at 70--85

Sort of. The fact that it needs that much power is correct, but that doesn't mean it takes that much more fuel, as a car's engine gets the most power per unit fuel burnt at full throttle peak power. So while you may need 1.8 times the power to do 85 rather than 70, odds are the engine will be burning far more efficiently and therefore not use as much as 1.8 times more fuel. It does give you some idea of how much difference the speed makes, though!

[Welshbeef]

McSam said:

ort of. The fact that it needs that much power is correct, but that doesn't mean it takes that much more fuel, as a car's engine gets the most power per unit fuel burnt at full throttle peak power. So while you may need 1.8 times the power to do 85 rather than 70, odds are the engine will be burning far more efficiently and therefore not use as much as 1.8 times more fuel. It does give you some idea of how much difference the speed makes, though!

For my car looking at the instant reading on a flat road 55mph to 85mph in 6th makes no noticable difference i.e. its well over the 50mpg on the guage.

Isnt the best mpg at peak torque? Not talking about theory but in the real world highest gear you have and then drive at the quoted peak torque of the engine?

[Andrew_M]

Depends on where you live as well. I bought a 1990 Jetta GTI 16v off my mate who lives in Norfolk. He got top 30's and an indicated 40.4 mpg average out of that car he had for 3 years. I drove it back to Derbyshire and ran it as a daily for 4/5 months averaging 27mpg. The cars we have at work, Saab 93 TiD, BMW 120d M-sport, Volvo V70 2.4 to name a few manage 37, 42, 20mpg respectively. Derbyshire/Yorkshire is very hilly and roundabouts/traffic lights are seemingly every 50ft!

[McSam]

Welshbeef said:

For my car looking at the instant reading on a flat road 55mph to 85mph in 6th makes no noticable difference i.e. its well over the 50mpg on the guage.

Isnt the best mpg at peak torque? Not talking about theory but in the real world highest gear you have and then drive at the quoted peak torque of the engine?

Sounds reasonable. Obviously you can't measure it very accurately, but 55mph in 6th will be quite an inefficient area of the rev range (in terms of specific output, power per unit fuel, rather than miles per unit fuel), whereas 85mph will likely be much better for that.

The best MPG is an absolute nightmare to quantify, though. Differs completely from car to car, and won't always (ever?) be at peak torque. For MPG, if we're talking about level ground and constant speed, a low speed in as high a gear as possible is the way. The minimum possible amount of throttle you can be using and still moving. None of this 56mph being optimum stuff.

For a rough idea for one particular car, here's a graph I have shamelessly nicked from someone on here, who I hope doesn't mind:


While it's not a cast-iron certainty, it does seem reasonable enough to expect similar trends from other NA petrol engines.

Prizes if you can guess what it's from, might surprise you

[Welshbeef]

Its a car which cannot do much more than 40mph in 2nd.

Its a diesel
Is it a Mk4 Golf Gt TDI 1.9 115bhp PD???

[McSam]

Very logical, but I think second just wasn't tested any higher, see that it does 80mph in third..

It's actually from an RX8

[slipstream 1985]

5lab said:

compare that figure to what you'd get out of a 320 ed (I think its ~80mpg extra urban). people get 45mpg by driving in stop/start traffic, or at higher speeds on the motorway

the 3 series is also quite a bit smaller than a lot of 2 litre cars. A mondeo for example has good room for 5, and a large boot, whereas a 3 series (especially the coupe) is compromised in the back and in the boot - the passenger\luggage dimensions are probably much closer to a focus, if not smaller

Edited by 5lab on Tuesday 14th June 13:31

3 litre engine and all that drivetrain to the rear not make it a fair bit heavier though?