Maximising Fuel efficiency

The Jaguar V12 engine is not known for its frugality, and lugging around an 1800kg car makes it worse. Absolute best case (50mph on a motorway) might just tip over 20mpg. Around country roads one might expect 16mpg and in town rather less than that.

The 3.6 & 4.0 litre 6 cylinder engines are quite a lot better, especially the manual gearbox cars, and getting over 35mpg on a motorway is not a big feat, and in town 20mpg is normal.

The V12 suffers from inherently poor consumption, and this is mostly due to the friction of 12 cylinders rubbing up and down the bores, and all the extra cam gear, though note that with 4 valves per cylinder, the 6 cylinder engines have the same number of valves as the V12.

In addition, the V12 is a 1960’s design and does not benefit from modern design and manufacturing improvements such as variable valve timing and direct injection.

What can we do about this? The important thing with the V12 is to keep the revs down. Friction in the bores increases disproportionately to speed. The 3 speed gearbox is not good in this respect since at 80mph the engine is doing around 3000rpm which is high and noisy. It could easily get away with 1500rpm in top gear for relaxed and more economic motorway driving.

Driving style is important. Every time you use the brakes you are in effect wasting energy and fuel, so the trick is to minimise braking by getting to the optimum speed for the road, and sticking to it. The Speed or Cruise control is invaluable for this. Of course, be careful about taking bends too fast!

Contrary to widely held opinion, using very gradual acceleration uses more fuel per mile than using full engine power. This is because the engine is at its most efficient with both throttles wide open – but if this causes one to be in first or second gear longer than usual then the benefit is more than cancelled out. The only way to master fuel consumption is to have a manual 5 or 6 speed gearbox.

Make sure the tyres are pumped to the maximum permissible. These are heavy cars, and cause a great deal of tyre compression as they roll along. This continuous compression takes a lot of energy, but if the tyres are at a high pressure there is less compression – but less comfort!

LPG is still an option for reducing fuel costs, and it is widely available in the UK. At July 2022 LPG is £0.75 per litre, compared with £1.89 for petrol. It’s a bit less ‘powerful’ but for routine driving this is not an issue, and if you want max power for a period just click the switch to go to petrol instantly.

There are some tuning techniques for the V12 which release a bit more power, or improved economy – but not both! Call KWE for more information. 01635 30030 or email us at [email protected]