A purely SWAG here, about 175 - 185; trucks were way under-rated, and HP was and still is not nearly as important as torque and the appropriate application of same. In 1966, that one ton, even if it was a pickup, most likely had a "granny" fourspeed and at least 4.10 rear end gears, and was maxed out at 60 MPH on the highway. It was a TRUCK , not sport vehicle.