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Official fuel economy tests in Europe and the USA are based on setting the dynamometer to simulate the vehicle roadload at steady speeds. Roadload is determined by a track test, generally coastdown (USA), or a drive test using wheel torquemeters (Europe). Computer models of these procedures were constructed, mainly to show the effects of ambient weather conditions. It was concluded that the accuracy of both procedures could be improved, particularly in respect of correction to reference ambient conditions.

The rolling resistance of fifteen different types of tire was determined on-road by coastdown tests, using several vehicles variously fitted with 14 and 15 inch wheels. Corrections for tire pressure, and for external temperature, were deduced by data regression. The rolling resistance of the same tires was measured on a flat-belt tire test machine, and correction for tire pressure was determined in a like manner. The results, in terms of the characteristic rolling resistance, were compared between rig and road, and the relation was found to be very close to one to one, over the normal ranges of tire pressure, load, and external temperature. The various test procedures are discussed in some detail.

Three passenger cars were subjected to relatively high speed coastdown tests, with six different types of production tires. Analysis yielded a non-linear expression for the rolling resistance coefficient of each tire, depending on road surface temperature and tire pressure. The influence of speed and load was found to be negligible. The same tires were then tested on a flat-belt tire test machine, at constant belt temperature, over the same range of load, pressure and speed as on-road. Analysis yielded a similar expression for rolling resistance on the flat-belt machine, though the power law relation for pressure differed from that determined on-road. A simple correlation was found, from which the rig measurement could be transformed to within about 13% of the on-road force. There was some correlation between tire tread temperature and the characteristic rolling resistance coefficient of each type of tire, which was independent of ambient temperature conditions.