IDK my brother bought the truck brand new in 1987 first year for 4.0,and it had 178k on original motor before I swapped for an Exteme machine long block from Lake Tahoe ,Cali.,its a 1/2 ton truck and I`ve had over a ton on it .It`s a long bed i have platinum plugs ,new injectors . also the pigtail wiring and ballast resistor was missing from the factory for the auxiliary fan ,so i have it rigged to a switch. more than likely lower rear end gears,and is probably running rich. 235-75-15 Bridgestone Deulers on stock steel rims for winter