The question isn't relevant because the small engine will be in a small car, but in order to get a petrol which pulls 2000rpm at 70mph, it would have to be a bigger car so isn't going to be comparable.

Traditional wisdom used to be that to optimise fuel consumption a car should be driven near the peak of the torque curve or a little higher, before wind and rolling resistance increased too much. Most diesels peaked around 2000rpm, petrols about 3000 - roughly.

So a petrol running at 2000rpm, whatever the road speed, may not give the ideal solution.

<< Not on modern turbo petrols. The VAG 1.4 TSi has peak torque (184 lbs ft) available from 1500 to 3500 rpm. Even the 1.0 TSi in our Fabia has its peak torque (148 lbs ft) available from 2000 to 3500 rpm. >>

I have no doubt that is correct. But how does fuel consumption vary while the management system is making peak torque available right across the rpm spectrum ? There must still be a sweet spot somewhere.

<< In this context, petrols are no different to diesels - when manufacturers quoted a range of engine rpm for maximum torque, it means that the natural torque peak has been electronically clipped to produce a flat torque curve - usually to protect the transmission. >>

Ah, clever stuff. So we are seeing a truncated peak torque, not what the engine is theoretically capable of. And maybe to protect the turbo as well as the transmission ?

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

Exactly my point re throttle opening in both this thread and the OP's previous thread (where I'd hoped we sorted out the obsession with rpm above all else!).

But in order to get a petrol engine which would pull 2000rpm at 70mph, I'd guess you'd be looking at a 2.0 turbo or a n/a of at least 2.5. Which unless you go for a hot hatch, realistically means something like an Audi A4 or BMW 3 Series.

Yes you could go down the diesel route, but given the £5k budget (from the previous thread) and the 100 miles per week spent on a motorway, I doubt the benefits would overcome the potential risks should something go wrong on a modern(ish) diesel. This would almost certainly wipe out (and more) any savings you made over a smallish n/a petrol such as the (very reliable) 1.25 engined Fiesta.

Viktor If you want to estimate fuel consumption across a range of cars you can look at the Real MPG section on the tab at the top of the HJ homepage.

If you are thinking about changing the Fiesta mentioned in your other thread you need to work out if the cost of changing is worth it for what are probably small savings in fuel cost. For example my current car does 55 mpg, my last one did 45. At 10000 miles a year the ballpark savings in my new car are only about £240.........65p a day.

If the Fiesta is reliable changing it to save a small amount on fuel is not worth it.

In carburettor days (before ECU management) the bigger the engine, the more fuel it used - all else being equal - but not by much. I am thinking of Pug 205s, which had 1.0, 1.1, 1.4 or 1.6 engines. Essentially the same car, but its weight increased a bit with each engine size, so that probably explained some of the difference. Cars with bigger engines also get fatter tyres, which will add to rolling resistance.

In theory it should require the same energy to overcome wind and rolling resistance, so if all engines are set up correctly the fuel consumption should be similar. Variations will come with how the driver uses the throttle to make progress.

I would guess an ECU system should limit the differences between engine sizes, but I am certainly no expert. If the car is able to cruise happily at motorway speed (most are these days) it will be up to you to optimise consumption.

The rolling and air resistance of a car is just as important (the second more so at higher speeds as it varies with the square of the velocity) as weight or engine configuration. A high-sided car will do far worse at higher speeds (compared to itself at lower speeds) than a more sleekly designed, but heavier car.

Obviously it depends on how fast they are driven and the actual difference in weight to be able to determine if and when the latter becomes more fuel efficient at high speed. I'd suspect that the chances of a big but sleek barge overtaking the mpg of a similar year designed small supermini or city car being close to zero, as long as they were of the same engine type (can't really compare a petrol with a diesel).

The tyres will certainly make some difference, but only depending on the type/make/model and wear, not speed. The Aero resistance becomes a more important factor as speed increases, hence why for rapid in-car cooling, it is often recommended to use open windows up to around 40mph and A/C above that speed, as air resistance dramatically increases.