I will try to make this post short as I wouldn't want to offend Rusty Rod Boat.
Back in 1983 I moved to New Mexico. There was an old geezer there that I met and spent quite a bit of time with. He would build these metal boxes, fill them with copper tubing and then route them to the hot water on a motorhome engine and he would cycle that hot water inside the tubing in that box underneath the carburetor. He used to sell this gadget of his to the people with motorhomes because it would improve their fuel mileage almost double. So if they were getting six or eight miles a gallon they were getting 12 to 16. He put it on a few trucks for different guys but he didn't put it on a car because it raised the carburetor up too much and it interfered with the hood an the air cleaner.
A little side note on old Jim, he took some old radios and invented some kind of gizmo that would confuse the radar in the local cop cars. He would fly through town but their Radars never picked up what his speed was.