how many miles do they average before rebuilding time? got my truck used 67000 miles i now have 94000 on it. i had no problems other than water pump and brakes. dont use or leak any oil.or smoke. seems like it was tooking care of. i was told it had new timing gear in it motor is clean untouched steel looks like factory this is a 305 are they in 3/4 tons? why is this motor so bad on gas to much truck?