SYMPATHIZING with the engineer who finds that the high efficiency designed into his engine fails to produce the anticipated increase in average miles per gallon obtained under normal driving conditions, the authors touch upon some of the factors, many of them out of the engine designer's control, which may completely mask the expected improvement.
In so doing, they start with an engine of known specific consumption and show the effect of air resistance, chassis friction, gear ratio, and car weight on constant-speed road economy, comparing calculated values with actual test values available.
Also discussed are such factors as climatic variations, traffic operation, cross-country driving and the individual driver, which have a definite effect upon economy, but over which the designer has little or no control. The effect of these factors is illustrated by the spread in tank mileage shown by a number of cars of similar model in fleet operation.