Abstract A question arises as to whether the wear rate of tires is constant under constant conditions and whether the tread life of tires may therefore be projected by linear extrapolation from low mileage tests. A second question arises as to whether commercially produced tires are uniform enough to serve as monitors of environmental change. A third question concerns the specific method of estimating the wear rate. This paper reports the results of experiments conducted on the National Highway Traffic Safety Administration test course designed to supply data relating to these questions. Regression of average groove depth against mileage for each of 27 tires gave a straight line which accounted for over 99% of the variability with a standard deviation of about 2 mils. Tread life projected from a 6400‐mile (10 300‐km) test gave on the average the same tread life as projected at higher mileages. A bias tire built under stringent quality control specifications and the radial tire selected for monitoring environmental effects were very homogeneous. The bias and belted bias course‐monitoring tires were more variable. Various methods of estimating the wear rate and use of the multiplicative model (requiring geometric means) for analysis for factor effects are discussed. The regression line method is chosen for tire rating.