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Tires and Passenger Vehicle Fuel Economy
TABLE 4-2 Comparison of UTQG System Grades for All Passenger Tires
and for the Tires in the Combined Ecos Consulting and RMA Data Tires in Combined Ecos Consulting and RMA Data
UTQG Tread Wear Rating
Percentage of All Tires with Grade According to NHTSA
Percentage
Number
200 or less 201–300 301–400 401–500 501–600 601 or more Total
11 21 33 22 8 5 100
3 18 28 23 17 11 100
6 36 55 45 33 21 196
wear grade for the data set is 440. As noted previously, neither the combined data nor NHTSA’s national ratings are sales weighted. Therefore, neither can be used to calculate an average UTQG wear rating for all tires sold. A scatter graph of all 196 tires in the combined data set does not exhibit any noticeable association between RRC and tread wear rating, as shown in Figure 4-3a. Disaggregating the data by graphing only the tires in the data set rated S or T reveals a slightly noticeable, but still weak, pattern (Figure 4-3b). Further disaggregation by graphing only those S or T tires with 15-inch rim diameters (Figure 4-3c) suggests the possibility of a relationship between rolling resistance and UTQG tread wear grade, which warrants more data for thorough statistical analysis involving more explanatory variables.
Explaining Variability in RRC and Tread Wear Grades Multivariate analysis can help determine whether there is a relationship between RRC and UTQG tread wear rating and other variables among the 196 tires in the combined data. The original variables in the data set are • RRC, • Speed rating (S, T; H, V; W, Y, Z), • Tire manufacturer,