I drive a 1991 Dodge E 250 with Cummings 3 speed automatic trans and 3:07 differentials. I live where the access road has an adverage grade of 25%. The engin doesn’t hold the truck back when I go down the hill. At 5 mph the engin at idle pushes me. I think the best way to fix this problem would be lower differential gearing. I hear that they made 4:11 differentials for the truck.