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What's the best way? I normally try to get up to speed as quickly as possible and maintain a constant RPM at around maximum torque... but I notice little difference!

In a nutshell smoothness and as little as possible braking/loss of speed (which has to be made up again). As to what gear is most efficient.. depends on models, but generally the higher the better provided it's not a labouring engine.

THink of the brake as being the pedal that uses the fuel and you won't go far wrong...

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During the fuel protests I had to travel for a week on a single tank of fuel - so I kept a close eye on the consumption computer (average consumption, not instantaneous)
I found I got the best economy on the motorway, and maintaining a constant, light, throttle pressure. Driving like this, I was slowing to 50-60mph on uphills, and speeding up to 90+ on downhills - but I got better consumption than sticking to a constant speed, even 50mph. I think a lot of this was because I wasn't having to slow down and speed up to suit traffic conditions, but I suspect it also had a lot to do with better efficiency when the turbo came 'on-song'.
Also, I found that driving into a headwind increased my consumption by 5 to 10mpg.
I also accelerated briskly on motorway on-ramps, taking advantage of gravity.
I also found that I got much better consumption at any speed on the motorway than at 60mph on 'A' roads.

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Depends a lot on the engine - petrol / diesel / carburated / injected...

Carburettor engines have accelerator pump jets so changing throttle position can be bad. Some injected engines have a "full throttle" sensor so accelerating only to "nearly" full throttle when acceleration is needed, is the way to go. Diesels can run much leaner than petrol cars so lower revs might not help as much as you'd think - they certainly do on a petrol engine though! With my big MPV that is heavy and has the frontal area of a house, slowing on the uphill bits and letting it run away with me on downhill bits definitely helps a lot. Some cars have an over-run fuel cutoff and will therefore do infinite MPG on a over-run, which makes it counter-productive to coast down hills! (i.e. will use more fuel at idle than under engine braking)!

Obviously, soft tyres are completely out and turning aircon off helps a bit, as does having the windows up and minimising electrical demand (headlights off in the day, heated rear screen, mirrors and seats kept to a minimum etc.) Avoiding bumpy roads is good too and not carting too much unnecessary junk around all the time helps!

If you are that way inclined: ‘burning and coasting’ is by far the best way to save fuel (coasting with the engine off is extremely unsafe if you don’t know exactly what’s going on with the car; also, doing so will bugger many cars). One of my mates is a tight-ass and he swears by it, I’ve tried it myself and it does work rather well (but I don’t do it – can’t be bothered, the extra wear on the clutch may prove to be a false economy).

Always try to keep to a roughly constant-ish low-ish speed (due to the non-linear drag characteristic of air), 50-70 is fine for most cars.

When on flat roads:
When burning, give it 80% (of naturally aspirated) throttle with the engine at half revs (to get best engine efficiency) to build up speed, then coast until you need to build up speed again. (don’t get in anyone’s way)

When on hilly roads:
Burn uphill, coast downhill.


You could also slipstream behind a truck, the closer the better; however, this has it’s own rather obvious (strictly speaking - hidden) dangers.


Other points:
Ensure your engine has a full amount of nice clean oil.
Ensure the tyres are properly inflated.
Ensure your tracking is straight.

And avoid bends and corners as much as possible!

Smoother cornering will improve tyre wear as well. Finally, take advantage of wet roads - if they are drying out, look for puddles!

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Contrary to what you might think, you are generally better off labouring in a high gear than changing down and using a lighter throttle.

With a modern electronic fuel injection full throttle will only put max fuel in if the engine is spinning fast enough to use it, meanwhile running with a wider open throttle at lower revs is also more efficient, as it reduces "pumping losses" (ie the engine trying to suck air in against a closed throttle butterfly).

Of course this is all "within reason", but if you are climbing a hill the best economy is usually achieved in the highest gear that will actually get you over the top, and allowing the speed to bleed away against a fairly well open throttle, rather than by changing down early and cruising up on a lighter throttle. Feels wrong but it works.

Also, as others have noted, modern fuel injection shuts the fuel off completely on overrun, so knocking it out of gear down hills actually uses more fuel (as it then has to fuel the engine to idle). Leave it in gear and it uses no fuel at all as long as the car is pushing the engine on.

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‘Tis correct.


There’s also engine friction. Assuming frictional forces are constant with revs (not unreasonable): the greater the sum absolute distance the components have to move, the more the energy it will use.


Which that is technically correct it isn’t the full picture. Keeping your engine in gear (foot off the pedal) when downhill will slow the car down, this is ‘natural engine braking’, hence you’ll need to burn disproportionately more fuel to get back up to speed again. It’s far better to knock it out of gear and build up the speed. However, if you need to brake then yes you should keep it in gear.

Only for a petrol engine, though. works the other way for a modern diesel.
In the Mondeo, I tend to keep to constant speed rather than constant throttle on the motorway, keep the revs in the main torque band when accelerating, and keep the 'cruising' speed below/at peak torque. Oh, and don't use the car's cruise control - you can achieve better with a careful right foot. I did a Chesterfield, Southend, Huddersfield and return trip the other week and got 56mpg. (6th gear, 2000rpm gives indicated 74mph, actual 70mph).

I don't see that this can work to maximise fuel economy. After all you have to use extra fuel going uphill to maintain speed, and you can't use gravity acceleration going downhill. One seems to lose both ways.

Surely allowing speed to bleed off when going uphill (converting kinetic to potential energy) and allowing speed to pick up when going downhill (converting potential energy to kinetic energy) is optimal.

With constant speed, you put fuel into potential energy and never get it back. Unless anyone sees it differently?

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Paul Smith
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The only potential improvement I can see is that by maintaining a speed you keep the turbo spinning and therefore burn the fuel more efficiently. but with that caveat I'd still maintain that a diesel should be laboured even more than a petrol as it has a higher compression ratio and therefore much more friction and pumping losses at high revs / closed throttle.

Ride the torque wave!

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Choosing a good line through corners is important.

It's fascinating seeing how often peopletailgaters drop off my back if I take a smooth line through a corner or two, even though I'm not adjusting accelerator at all. They lose momentum by cornering badly.

That fits well with JT's observation that the brake is the pedal that wastes most fuel. A bad line means more braking...

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Paul Smith
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Where do spoilers, body kits and ride height fit into this picture?

I have a strong suspicion that an inch of ride height might well be worth at least 10% of fuel economy.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

I don't think anything like 10%.

I guess most road cars are running so high anyway that any minor change in ride height will have negligible effect on the under car airflow.

Be interesting to see though, transplant Range Rover suspension onto a sports car, take the spoilers off and see if it does 50mpg!

[edited to add]Fuel Crisis Worsens! Lotus release new flagship "Esprit Turbo Big Foot"...

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I would go with that. At speed, drag will be the biggest loss (ignoring engine/transmission losses). The drag energy used to traverse a given distance is proportional to the square of the speed. Think of it like RMS current: smooth delivery of a given charge will minimise overall resistance losses.


Engines are most efficient when delivering a decent amount of power, strictly speaking – more torque at reasonable revs (due to internal friction losses). Operating at that power band will use disproportionately less fuel when hill-climbing.

Knocking the car out of gear when going downhill will minimise the revs from the engine which will directly lead to less frictional loss (hence less fuel used); less benefit will be gained if the engine is kept in gear when going downhill.


This bit throws me, can you explain?


Allowing speed to vary so much will lead to disproportionately more drag losses at the higher speeds, resulting with a greater net drag loss (RMS loss). Also, piddling up a hillside with no torque from the engine will yield poor power efficiency.
It’s better to power (torque) your way up at your desired average speed.

I think Paul has a good point, depending on how big the hill is.

To take an extreme case, let's say we are crusing at 50mph and there is a small hill ahead. It is conceivable that we could turn the engine off and coast all the way up the hill, cresting the top at walking pace. Then lets say the other side falls to slightly lower than our original start point, so by the time we have regained level ground we have coasted back up to our original speed of 50mph. All this and using no fuel whatsoever.

Now if we were to maintain 50mph all the way up then we must use more fuel.

So how does this translate to real life?

I'd say that the optimum approach to climing a hill is to slog up at whatever speed keeps the engine burning fuel at its most efficient and in the highest possible gear, then back off just before the summit, allowing the kinetic energy we have "banked" to temporarily power us over the top, in the knowledge that it will be given back to us as soon as we start to descend the other side.

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Just to add my 2p worth - At higher speeds air con is preferable to windows being down, at lower speeds the opposite is true.

I find my car, a diesel, varies little in efficiency when driving steadily between 50 & 80mph. It did drop throughout winter but now the weather's warmer, efficiency has improved. I think just trying to get into top gear asap is the biggest thing you can do, but there'll always be situations where hard-revving in lower gears is needed - best example I can think of is joining the A56 southbound from J8 of the M65. Turn left off a roundabout onto an NSL DC at the bottom of a looonng steep hill. I can almost watch the fuel gauge drain down to zero when trying to accelerate up to speed along here

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I think JT covered the main points well.



Taking JT's example, on the downhill section we regained cruising speed on the downhill for no fuel use at all. (='gravity acceleration')

The alternative - cresting the hill at cruising speed - means that we cannot gain kinetic energy from the potential energy. The potential energy is going directly to friction losses, and in the case of a steeper hill directly into the brakes.

Engine efficiency is certainly an issue, but I submit that the potential energy / kinetic energy issue is bigger.

Windage losses are hugely significant, and limit efficient maximum speed. They don't limit minimum speed - in fact they favour low speeds strongly. So losing speed on the up and gaining speed on the down seems optimal here too.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety