The Type A durometer as specified by ASTM D-2240 is the gauge used by tire manufacturers to measure hardness of tire rubber. The gauge gives readings of hardness in durometer points on a scale of 0-100, with 100 being the hardest reading. Some race tires come from the factory with the durometer hardness noted on the side wall, but factors such as ambient temperature, track temperature, usage, and age of the tire can change the actual hardness of the tire from the factory hardness.

Rex Gauge Company's 2100 Tire Durometer allows racers to quickly and accurately make checks of tire hardness. Unlike other "racing durometers" (which are just relative hardness gauges), the Rex 2100 is calibrated to the ASTM D-2240 standard for durometer hardness and is traceable to N.I.S.T. Thus, the Rex 2100 allows comparison with the factory tire specifications.

The 2100 is a handheld gauge weighing only 4.2 oz. The indicator case, bezel, and barrel are all billet machined aluminum. Because of its billet construction, and the fact that the durometer is manufactured in the USA by Rex Gauge Company (an industry leader in durometers for over 60 years), the craftsmanship that goes into the 2100 is unsurpassed. The gauge is furnished with a carrying case and a one year warranty.