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.