Fuel Saving Tips

Accelerate, or keep it lean

To completely burn 1kg of petrol takes 14.7kg of air – this is called a stoichiometric mixture. Your engine management computer has to create this kind of air-fuel mixture when you accelerate hard, or even moderately hard. Modern engine management systems also use a 'lean burn' mode at more modest throttle openings and when there’s a smaller load on the engine. In this mode, the engine is only using about 50% of the fuel that it needs when running stoichiometricly. The worst efficiency (and therefore the worst fuel consumption) is when your throttle opening is just big enough to move the engine out of its full ‘lean burn’, but not big enough to accelerate the car very much. So I find it helps to think of my car as having two distinct efficiency modes. In other words... (1) If you’re not going fast enough then accelerate efficiently (and quite quickly) to your cruising speed: don’t be too gentle, because you need to get back to that ‘lean burn’ mode as quickly as possible. But don't completely floor the throttle either. (2) Once you’re at the speed you want, hold the throttle at the lightest setting needed to (more or less) maintain speed. Trying to hold speed up a hill will often push you out of that super-efficient ‘lean burn’, and holding back down the hills just means your failing to capitalise on all the momentum and speed that you could have had (whilst staying away from stoichiometric) – so be fairly relaxed about being a little bit slower on the ‘ups’ and gaining some speed on the ‘downs’. The mpg ‘killer’ is actually that middle ground, where the car is accelerating rather gently and inefficiently for ages and ages to get to a sensible speed. To me, it seemed counter intuitive to accelerate a bit harder to save fuel, but it works!