Over the years I've seen all kinds of weird and wonderful devices that are supposed to give extra MPG. I doubt if any of them work.

The idea of nitrogen in the tyres instead of air is so you dont need to check your tyre pressures,(but I suspect you already knew that) ;)

That is complete nonsense. Nitrogen will not stop you getting a puncture or slow puncture thus you still check your pressures as you would normally. Nitrogen is supposedly less likely to pass through the rubber. But since common air is approx 80% nitrogen its only a small difference and unless you ensured there was a vacuem inside the tyre before inflating with Nitrogen you would never get a 100% Nitrogen fill.

As for a 10% improvement in fuel economy earlier this year I discovered a free way to achive this. We go to Scotland twice a year, always the same route (430 miles) and it normally takes the same time within 1/2 hour. Over the trip we do about 1200 miles and the mpg always works out at approx 55 mpg. After our May trip I did the calculation and it worked out at 60 mpg, a 10% improvement.

Was I delighted, suppose I was but there was a simple explanation. On both the outbound and inbound trips we had a following wind.

As the website (debunking the Nitrogen myths) says, the main reason why Nitrogen is used in some vehicle tyres (planes, the Space Shuttle and, of course, racing cars) is because they then contain no Oxygen, so are less of a fire risk in the event of a crash etc.

Having the tyres at the correct pressure makes a reasonable difference, as well as to the life of the tyres (given how expensive they are nowadays as most of us have low-profile tyres - replacing them prematurely due to excessive wear because of over/under inflation [under is worst as it can cause blow-outs]) will help reduce fuel consumption, as will not carrying heavy items needlessly (including roof racks/box which probably add 20%+ to the air resistance).

I have found that the following works for me (I achieve an average of 41-42 mpg on my Mazda3 [combined mpg should be about 38 on the official figures]):

• I find (as it appear the F1 drivers do now as well) that slowing down earlier when coming up to junctions using engine breaking (i.e. lifting off early rather than braking from 25-50m later) and not actually stopping at junctions (unless directed/need to do so) helps. The brakes will use engine power (fuel), whereas the engine braking does not (hence why HJ says you shouldn't coast [and for safety] going downhill by either taking the car out of gear [it then uses fuel to keep ticking over and to provide electrical power, as gravity is not helping keep the wheels and engine turning over) or even turning the engine off (dangerous - the steering lock could engage). Staying in 2nd gear by keeping at a few mph as you turn at a junction will save fuel.

• Anticiptation of taking action - e.g. the above (lifting off/slighting slowing so you can reach the junction [especially on dual carriageways/motorways where you can see a long way ahead] when its clear) or chosing the right time to overtake on dual carriageways/motorways so you don't need to brake (too close to the slower-moving vehicle ahead you will be overtaking) then floor it to avoid other fast-moving vehicles. I also try where I can to overtake on dual carriageways/motorways on right-hand bends so I have to travel less distance/fast to overtake (see lane draws on athletics tracks for why).
• Use open windows to rapidly cool you car down up to about 40mph - this has been proven to save fuel over using A/C as the extra fuel used by the engine to overcome the extra wind resistance is less than it uses to power the A/C. As a mechanical engineer working on A/C and ventilation system design on a daily basis, I would concur that having a large amount of air does rapidly cool areas heated to above the outside ambient temperature by the sun.

• Having a light right foot generally - not slow (labouring in higher gears uses more fuel and can be dangerous when you need to speed up quickly [anticipation again]), but using the correct gears and not thrashing the car. Its also amazing what planning your journey and leaving enough time (including in case of hold-ups) to reach your destination helps - no need to rush at 80mph down the motorway to that important meeting (easier said than done these days when we're expected to rush everything).

Edited by Engineer Andy on 21/09/2014 at 13:50