A performance review is presented for ethylene glycol and propylene glycol coolants in a heavy duty fleet test. Vehicle maintenance and testing specifications are highlighted. Aspects of regular filter change intervals and glycol segregation are underlined. The effect of glycol type on system hardware is examined. Comparisons are made by examining radiators, thermostats, water pumps, and wet-sleeve liners. In addition, inhibitor concentrations and glycol degradation products are monitored using LC, GC, and elemental techniques. Improved water pump seal life has been demonstrated using propylene glycol coolant. Comparable performance is shown for the other components.