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An Overdrive may be most easily described as an extra, or normally fifth, gear, and although it may technically be used to make a vehicle go faster, that does not realise its correct potential. Indeed, with some cars being equipped with more engine power than stopping ability it may even be dangerous to misuse it in this way. If an overdrive is used to reduce engine revolutions when cruising speed has been attained in top gear, the benefits are numerous. Probably the most obvious of these is the fuel saving. In normal use this can be expected to be of the order of ten percent, but with care may be pushed up to well in excess of fifteen percent and if the fuel consumption / engine speed graph were linear, a comparative steady speed test should give twenty eight percent saving - but of course life does not consist of linear graphs and steady state conditions! A less immediately obvious benefit is the potential engine and transmission wear reduction. As the car travels further, with an overdrive fitted, for the same number of engine revolutions as without, the engine and gearbox must wear out more slowly. |
This not only extends the life of valued components, but puts off the ugly day when a full power train strip down makes inroads into the bank balance. The third benefit is even less quantifiable, but is perhaps the most pleasing aspect of using an overdrive. It depends less on theory and calculation and more on the discerning driver and the local County Council. It is not only the cars that have developed over the years, but the roads that they run on are now straighter, less interrupted and smoother, and although even early cars may be capable of a respectable turn of speed, they possibly had little chance of doing so for more than short bursts on most of the roads of their time. Today, however, what with junction modifications and the straightening out of bends, even British "B" roads will usually allow a reasonable cruising speed for a reasonable period of time - relatively. The use of an overdrive then allows the engine to relax on the longer straight runs and, hopefully, enhances the pleasure of driving. |