I think the engine has more to do with the drop in gas milage. Back in the early 80s a friend and I both had cab over campers. He had a Chevy with a 305 and I had a Ford with a 460. Without the camper, I got 14 to 15 mpg on the highway and he got 18 to 20 mpg. With the campers on, I got 13 to 14 mpg and he got 11 or 12 mpg. I think you loose more milage if your engine is staining.
I have a 3124KS that I tow with a Ford E-150 with the 5.4L. I only have a drop of 1 to 2 mpg from my normal 17 to 18 mpg when towing.
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