With gas and diesel prices currently hovering around the 40 to 45p per litre mark in the USA, you might wonder why drivers are that bothered about fuel consumption, but amazingly the hypermiling fuel-saving movement has taken off strongly and has spawned a good number of websites where you can pick up some very interesting facts and figures. One (Ecomodder) offers an app-like calculator that works out a car’s power requirements at any speed, when you dial in the critical data for your car. You need your car’s drag factor (cd), its frontal area, kerb weight, and co-efficient of rolling resistance, which is around eight to twelve per cent of your car’s weight, depending on the tyres fitted. And it delivers some eye-opening figures! Who would think that most of today’s European family cars need little more than 10bhp to cruise at 40 to 45mph? Naturally the numbers rise swiftly as aerodynamic drag becomes significant, with 40 to 50bhp needed for cruising at 80mph, but it makes you wonder if we really need 100 to 150bhp engines, or even more.
But of course we need a decent power surplus to accelerate reasonably rapidly from rest, and to overtake slower cars, and the performance levels perceived to be acceptable are determined by an increasingly demanding market. The result means that many cars today are condemned to running at less than half-speed, like Mo Farah training with the keep-fit joggers that you and I regularly see out on the roads, rather than covering his usual fast paced, gruelling 20 miles a day. With only that modest level of exercise, Mo would very soon lose his performance edge, and the same danger faces your car’s engine if it rarely has the opportunity to stretch its legs. So you should really take advantage of any opportunities to get your car’s heart beating fast and lungs gulping for air, without which its combustion chambers, oil passages, and gas-carrying pipes will gradually gum up like arteries clogged with cholesterol. Fuel additives and modern oils do help, rather like statins can do in the human body, but regular exercise is the better solution.
Those steady speed power figures of between 10bhp and 50bhp for motoring within the UK legal limits mean that most cars have ample power surpluses for overtaking, but harnessing them is all very much dependent on engine characteristics and gearing. For example, in many cars, any attempt to overtake another car at 45mph in sixth gear is going to be somewhat laboured; it may well be the best gear for cruising economically at 45 to 50mph, but it’s not the right gear for safe or economical overtaking. Dropping down into fourth gear at 45mph means that the engine is probably pulling strongly at 3,000rpm, but it could rapidly become breathless and run out of power surplus as the car’s speed increases. Fifth gear is the obvious best choice and, if we were to examine individual engine figures closely, we would see that it offers ample surplus power and that the engine spends most of its time around the efficient peak torque speed of anywhere from 1,500 to 2,500rpm, and this gives the engine a good work-out. These figures are mere examples, not etched in stone, and you need to get to know your engine, and its gearing, to know your own best gears for given situations.
Overtaking at maximum power, when you may need to brake to slot back into a small traffic gap ahead, or merely hope for one to appear, is rarely that safe, and inevitably fuel inefficient. When overtaking is difficult and there are few safe opportunities, perhaps when stuck behind a long HGV, wait for something like a roundabout to appear and, if the road’s clear, overtake as you accelerate back up to your cruising speed, when your power surplus will be bigger, the exercise quicker and safer, and when you’ll also use very little extra fuel. Accelerating positively, in the right gear, out of any roundabout from 20 to 25mph up to 50 to 60mph represents a good opportunity to exercise your engine efficiently without wasting fuel. Using anticipation and a measured approach to the roundabout will often set you up with a clear road ahead on exit, so that you can accelerate briskly, rather than in a more leisurely manner, as some do, mistakenly thinking that this is great for fuel economy. The same goes for attacking most hills; get into a gear that sets the engine pulling fairly hard, probably at around 2,000 to 2,500rpm, and climb the hill positively, swiftly, and efficiently. The time for the light touch on the throttle is in town and when you’re cruising at steady speeds but, when there’s work to be done, it’s usually best done positively, quickly, in the best gear, and with a firm push on the right pedal!