An engine’s fuel efficiency at any time is related to the engine speed, the power and torque demands, and the inherent characteristics of any particular engine. If you’re in a high gear for cruising steadily at 50mph, then you are unlikely to be in the right gear to commence an action requiring significantly more power, such as when overtaking, or climbing a significant gradient. Alright, some may argue that diesel engines are very flexible and will offer better overtaking and slogging up hills than many petrol engines at lower engine speeds, which is part of their appeal; but even a diesel engine that’s supplying maybe only 20bhp to cruise economically at 45mph and around 1,300rpm cannot offer enough power output in sixth gear for safe overtaking. As soon as you depress the accelerator to demand more power, the engine’s efficiency falls and, if you don’t select a more appropriate gear, then you will make a bad job of overtaking and waste fuel in doing so, or be forced eventually into a down change when climbing a hill, as the car struggles to maintain its speed in the wrong gear, even with more accelerator input. That should be obvious to our readers, but it’s not an uncommon sight to see overtaking manoeuvres undertaken in the wrong gear, subjecting a driver and any occupants to increased danger, annoying other road users, and wasting fuel in the process. How many times have we toiled behind another car that gets slower and slower up a long hill, when it’s quite obvious that the driver is just in the wrong gear?
It’s a base principle of good, safe, driving to drop a gear (sometimes even two, with today’s six-speed transmissions) when, having positioned yourself accordingly on the road, you prepare to overtake another vehicle. Dropping down into fourth gear may offer you more immediate power though, but there’s a danger of then being forced into an up-change mid-overtake, as the engine starts to lose its breath, whereas fifth gear will usually comfortably take you back up to 70mph and beyond, before you drop back into sixth. Whichever is the best choice depends on you knowing your engine.
In contrast, a long, leisurely overtake in a high gear, even when seemingly perfectly safe, with no approaching traffic, is not good for fuel economy because it takes a lot longer in the wrong gear, and it doesn’t use your engine in its most efficient speed band; exactly the same thing applies when accelerating out of a corner in too high a gear, with the engine bogged down well below its best operating speed. Choose the right gear as you approach any corner and use enough accelerator input to keep the car nicely balanced through the bend, without any significant (usually rear to front) weight transfer that might initiate loss of road grip on a wet or slippery road.
It’s worth pointing out that just as in golf, the more clubs you have in your bag (rules permitting), the better equipped you are for all situations, and the same applies with gearbox ratios. Basic model variants of some cars only offer five gears, rather than six, which means you can’t get the best economy and performance balance out of the engine in all circumstances. Having one less gear makes quite a difference with some cars and it’s often worth upgrading to a model with an extra gear ratio that usually offers more economical cruising, and a better choice of ratios for varied range of road conditions.