Improve your fuel economy by 20%

In any top 10 list of how to go green, the obvious one always crops up: drive more slowly.

But how realistic is it, and can it actually lead to significant gains?

Our 308 long-termer has a speed limiter function. What else to do, but put this theory to the test?

GALLERY: getting 74mpg at 60mph

Few commuters will never exceed 70mph; even the self-proclaimed 'slowsters' will at times creep over the UK national limit. Capping the Pug's snazzy digital gadget to this would, therefore, probably have some sort of effect. But we decided to go further. Mindful of my £25 daily fuel hit, I went all-out.

Cap the speed to 60mph. They thought I was barmy in the office, but it was bound in clear logic. Lorries are capped to 56mph, so it would realistically be foolhardy to go any slower - you need some sort of differential, for safety's sake. This is therefore the 'slowest' go-slow speed you can drive on a motorway. Does it work, though?

Experts reckon that every 10mph increase can lead to a 10-20% reduction in fuel economy (and, thus, emissions). But this doesn't seem to be bound in any practical evidence, just as official fuel economy figures are lab-conducted tests, not what you actually get from the car.

The theory is sound, mind. My long-termer weighs around 1,400kg. Dragging that up to higher speeds, and maintaining forward motion despite the added influence of aerodynamic drag, requires but GCSE physics to see that it will demand more energy and, thus, fuel. Less speed, less energy equals less fuel.

F1-style rev limiter

So, to the test. I set the speed limiter to 60mph, which meant the car's electronics would not let it exceed that. Hit 60 and it's like an F1 pit limiter - you can press the accelerator further, and it will have no effect on forward motion.

Surely this is dangerous? Well, there's a get-out safety valve. Jam it to the floor and full power is re-engaged, acceleration available and higher speeds allowed. It meant that, in case of emergency, I could still squirt out of danger. Suitably reassured, I reset the fuel computer, and ventured out onto the M1.

At first, it felt really slow, sticking to the inside lane and venturing into the middle but rarely. The speed differential between the speedies was huge, so I kept well away from them. But, you know, half an hour in, I started to quite like this new, relaxed, stress-free gait.

I was certainly far from bored. Unable to escape from trucks, I sometimes felt as vulnerable as a cyclist in Central London. But there was not a trace of aggression. This was my speed, and if others went faster, so be it. I simply trundled along in my own good time.

Alert yet slow

Quickly, I became the most alert driver on the road, scanning both front and rear for miles to plot my path. Truck-passes were timed to perfection, slotting into gaps I spotted minutes ago and waiting patiently to arrive alongside. Often, my speed was below 60mph; go with the trucks, rather than dangerously baulk the speeding Passat.

Besides, the amount of cars I saw hurtling up, then remaining in badge-reading view for many miles in front, was staggering. That's busy motorways for you: go as fast as you want between the waves of traffic; but it probably won't get you much, if any, further up the road.

I budgeted to be back home really late. In reality, my key was in the door 15 minutes later than normal. Bear in mind this is a two-hour journey, and it's hardly a disaster. Particularly as I was buzzing, thanks to a sneaky look at the trip computer as I switched off.

Verdict

74.2mpg. How does that grab you? Yes, really! That's an electronically-corrected 20mpg more than I normally get when driving at little more than 70mph. This was amazing stuff. I'm a pretty steady commuter - but, capping speed to just 10mph less had improved my economy by... over a third. Later, I did the test again. Got 78.3mpg...

Not bad, not hard and yours simply by lifting off the throttle. It won't cost you much time and will mean you're paying two thirds what you currently are on fuel - in exactly the same car. In my case, that's a saving of over £8 a day. That's it. From now on, I'm on go-slow...

New £55k green Brit supercar
Diesel v petrol: which is best?
Smart fuel saving tips
Three countries, two tracks, one tank of fuel
Do you really need a car?
Save the planet - get up early

All our Peugeot 308 long-term reports

308 first service
Gallic charm
Long-term arrival: Peugeot 308 1.6 HDi