Rolling Resistance, Traction, and Wear Performance of Passenger Tires
97
a B grade have a 5 to 19 percent lower RRC than all other tires, all else being equal. Variables not included in the model are tire weight and dummies for UTQG temperature grade, neither of which was found to be statistically significant. Retail price was not included in the model, since it is not a parameter that can be changed directly to affect RRC in the same manner as a physical property.9 Nevertheless, to the extent that tire prices reflect tire manufacturing costs, price is an important consideration. Analyses of the selling prices of tires in the combined data set are presented in Chapter 5. A dummy for OE tires was tested, but the small number (eight) of OE tires in the data set limited its significance. The data set consists almost entirely of replacement tires. To the degree that OE tires are constructed on the basis of technologies not common in replacement tires—for instance, by using alternative tread compounds to reduce rolling resistance—the kinds of relationships reported in Table 4-3 might not emerge from an analysis of large numbers of OE tires. Whether differences exist in OE and replacement tire technologies is an open question that is considered further in Chapter 5. Explaining Variability in UTQG Tread Wear Rating Table 4-4 shows the results of a regression explaining UTQG tread wear rating as a function of six variables. Once again, speed rating is a highly significant variable; tires rated for higher speeds tend to have lower tread wear grades. Compared with all other tires in the data set, tires with a speed rating of W, Y, or Z have average tread wear ratings that are 175 to 275 points lower. Tread wear ratings of tires having speed ratings of H or V are lower by 50 to 130 points. As might be expected, there is also a statistical relationship between a tire’s tread wear rating and tread depth. The regression coefficient indicates that each increase of 1/32 inch in tread depth results in a 1- to 39-point increase in the tread wear rating, with a midpoint of about 20.
9
It is inappropriate to include price as a regressor in the RRC regression because it is not a predetermined variable and would thus bias the results.