Referring to some past fuel consumption records of several new cars over a number of years, there’s generally a picture of their fuel consumption improving over the first 30,000 miles, by something of the order of four to eight per cent, although there’s one exception where the fuel economy of a garage ex-demonstrator, bought at 6,000 odd miles, failed to show any significant change over the next 25,000 miles. We’re guessing that it was run-in enthusiastically by the dealer!
It’s rather a funny business though, this running-in, and there are still some intangibles that don’t appear totally supported by conventional explanations of reduced friction in critical engine areas, as rubbing parts bed in. It has always been suggested that a nice thin coating of carbon, rather than bright shiny new metal, aids combustion efficiency – so where do detergent fuel additives fit in with this, for example? Maybe you’re best advised to stick to un-additised fuel for the first tankful? We’ll check that one out and get back to you on that! But many cylinder walls and piston rings now have special coatings to reduce friction, and a far better quality of micro-surface finish than in times past, and with roller bearings (such as in-valve activating cam followers) where in past times simple hardened metal bearings were used, all lubricated with (usually synthetic) oils of vastly better quality.
Yet that running-in period when future engine efficiency is determined still undeniably exists. One valid argument for enthusiastic and rapid running-in says that the piston rings will not bed in with the cylinder walls if the gas pressure during cylinder compression is not high enough to press the rings against the cylinder, and you won’t achieve that without a decent bit of throttle, and some good turbocharger pressure to assist! But the service life of a good diesel engine, regularly serviced, properly run in, and driven with due mechanical sympathy, is probably well in excess of 100,000 miles – and there’s plenty of evidence that such engines are actually in their prime at 40,000 miles and beyond, all the way towards the 100,000 mile mark. This all suggests, then, that alongside the significant savings in depreciation, a good used car with a full service history bought with around 30 to 40,000 miles on the clock is a great investment. It offers better fuel economy than a new car, and has also had any new car niggles sorted out by the time you buy it. This is particularly relevant for those who may not be able to easily do those longer runs that help run in a new engine properly, and an ex-fleet car that’s been given healthy exercise and serviced regularly makes a very good buy. You’ll want to be fairly selective mind, check those service records, and make all the usual checks that we advise in our used car buyer’s guides. You might need to accept the almost inevitable minor bodywork marks that you won’t get with a brand new car; but you won’t have to suffer that new car stress every time you park up at the supermarket. You’ll be able to immediately go out and drive it as you wish, slow or fast, and with a genuine fuel economy bonus over any brand new car is pretty well guaranteed.