On highway and proving ground vehicle tests were conducted in the Southwestern United States to compare the performance of propylene glycol and ethylene glycol based coolants. A cross-section of vehicles encompassing pickup trucks, large and small cars, a minivan and a sport utility vehicle from various manufacturers were included in the program. Test protocols were completed with vehicles ballasted to curb weight with a four passenger load, with and without maximum recommended weight trailers. On highway driving schedules were run simultaneously with each vehicle pair. Two sets of tests were completed with each pair; once with ethylene glycol/water coolant and once with propylene glycol/water coolant. This was done in an AB/BA test sequence to reduce the impact of different ambient conditions between test. Ambient temperatures during the testing ranged from 35°C to 50°C.The results compare the heat transfer performance of aqueous propylene glycol engine coolants to aqueous ethylene glycol coolant in high ambient, low load service. Under these conditions, the small differences in heat transfer characteristics between propylene glycol and ethylene glycol based coolants are not significant to the customer. Variations in coolant concentration were shown to have a small effect on operating temperatures under high ambient conditions.