O/T (Sorry) But I was wondering Is this true?

50mph is borderline 5th gear? I'm in 5th by 30mph on the flat and certainly by 40mph up steady inclines (and this is the 5th car I've owned with 5 gears over a 20 year period and all the same basically). The road conditions you indicate suggest to me that anyone with half a brain would be in 5th by 50mph.

Basically, there is a point that cars are most fuel-efficient (56mph if the advertising blurb is to be believed) but obviously this varies by maker and driver!

On two points I disgree with your reasoning.

Point one is that everything depends on the vehicle and the drivers' sensitivity. Thinking to the cars I've driven recently, they've all varied. Some cars are happy in fifty at 40, some not so happy, some have a sixth gear (did you consider these?) and some only have fourth. What about automatics?

Second, lumping up a hill at the minumum speed for that gear may well use more fuel and chuck out more gasses than using fourth.

Of course they did.

There you go then. What are the figures for opting for 80?

56mph (or 85kmh), is just a standard speed used for measuring fuel economy.

What the most efficient speed is, varies greatly between cars.

So you obviously like wasting fuel?

Yeah...and you're a s**te one and deluding yourself if you think you've got half a clue how to drive economically.

It does. I've done a proper economical driving course and all that Dave is doing is wasting shitloads of fuel whilst not even being in decent control of his car. I bet he finds maintaining 30MPH in 5th gear quite hard to do.

Depends on the car a bit. My Carlton could happily hold 20 in 5th on the flat, 30 in 5th on almost any hill.

But in most cars 30 in 5th would have the engine labouring on a slope, admittedly.

Indeed. There was a big debate about this on rec.autos.tech a year or so ago. My old Fester XR2i (modified with big valve head and hotter cam) varied its consumption quite markedly with speed. My average over 10k miles was 34 mpg which was also what it got at normal motorway speeds of 80 ish. Dropping that to 70 would see high 30s. 'A' road runs at 60mph would just get to 40 mpg. However in the fuel crisis in 2000 I did a trip using 40mph on A roads and 50mph on motorways and managed to squeeze 51.5 mpg out of her. It was boring as hell going that slowly but pretty damn good consumption for a tuned engine.

By way of contrast the economy of my current Focus 2.0 is much less speed sensitive. My average to date over 7k miles is 30 mpg which like the Fester is also what it gets at normal motorway speeds of 80 ish. I've tried doing a steady 70 on motorways and also mad dashes at 100 plus in the early morning and it only varies between about 29 mpg and 33 mpg. For the pitiful fuel saving I might as well go fast and make the journey quicker. Even babying it on A roads won't get me above 36/37 mpg.

Quite why the two cars behave so differently I couldn't say.

Actually on a flat road in very steady traffic (ie very little throttle) my

2.0 Avensis will happilly tootle along at 1500RPM. I certainly wouldn't suggest trying to climb any kind of incline at that speed and in that gear. The fuel computer on the car would suggest that I'm doing things correctly.

And labouring your engine to hell surely.

I have a good torquey diesel and it doesn't like that.

Lower speed = lower fuel consumption and noise (above about 30mph, anyway). Ye cannae change the laws o' physics!

Your main bearings must be oval by now, unless you drive something that actually does pull cleanly at 30 in 5th (e.g. a huge V8).

Up to a point. There is a constant consumption (overhead) by just keeping the engine running, so if you are stuck in a queue and don't make any headway, the mileage rated consumption will be high. My average fuel meter will drop to 15mpg, but it quickly recovers since little fuel is used when idling.

Which isn't far above what the ECU would try to maintain as an idle.

Wrong.

Really? Odd that my car is rated at an idle speed of 700RPM +/- 50RPM which rings true as it idles at 650RPM when warmed up. As I said, it's quite happy at 1500RPM on a flat road in steady traffic in fifth gear but it'll go straight down to third should I have to suddenly make progress.

where not far is approximately double?

Can anyone point at a fairly authoritive source for a nice graph of fuel consumption vs speed?

my starting point.

Don't ever doubt Conor as he's done some fuel economy course and he's allowed to cuss anyone that has their own point of view.

Lower speed = lower fuel consumption and noise (above about 30mph, anyway). Ye cannae change the laws o' physics!

At least quote the entire sentence to which you give your one word response.