Gallaway et al. (20) showed that the Mu-Meter and an ASTM E274 tester had a good correlation when both testers used tires without tread and both operated with the pavement wetted by sprinkler truck (Figure 27), but the correlation was not very good when the ASTM E274 tester strictly followed the specifications. In either case. The average maximum deviation from the correlation line was +5 SN at 64 km/h (40 mph) for tests on the same pavement.