There are people who believe that it is not possible to improve the fuel economy of a vehicle. Yeah just like it's not possible to make a vehicle go faster, go off road better or tow more. Some gear heads think you can't increase power and fuel economy on a gasoline engine. This is some what true but represents a general lack of imagination and obsolete thinking. The engine is a big block chevy 454, mark 4 series, it's a smog motor from the 1980s. The vehicle is a 3/4 ton 2wd suburban with a 3 speed non lock up auto and full float axle with 4.11 gears. First I had to establish a base line, find a sample group to make some realistic expectations. Base line numbers I could find tell me that 1985 was probably the worst year for this engine. Making as little as 190 horses and 380 torque and had the worst fuel economy of all years. Since it was considered a "heavy duty vehicle" no EPA fuel economy numbers were ever published. So it had no power and shit fuel economy. Looking up the next closest thing, a half ton suburban with a smaller 350 engine all indications were it probably wasn't going to be good. Looking up fuel economy on the new suburban that has fuel injection (obviously), a 4 speed lockup transmission, a much smaller engine and aero dynamics. EPA says they get 12 city and 16 highway, problem was people were reporting around 10, usually less in the city and up to 14 highway. Reading and talking to people I found out quickly not to expect even as little as double digit fuel economy on the highway. Actual testing which I didn't do a lot of it was getting around 6.6mpg driving it around town. So I figured I could get the fuel milage up to maybe 8 or 9 in town and maybe 12 on the highway. So I took a wide ban oxygen sensing meter out of my camaro. This instrument tells you in real time what the engines air fuel ratio is. It was running way to rich. The obsolete quadajet carburetor had to go. The cast iron intake had to go to too. I put a edelbrock carb on there which I have had experience tuning and an aluminum edelbrock intake manifold, because the cast iron one sucked and square bore carb wouldn't fit on the original spread bore manifold. I checked the ignition timing before I pulled the top of the engine off it was pretty close to where it needed to be. When I got the intake manifold I saw hints that it was starting to develop a leaking cylinder head gasket. The engine would likely need to come out, but later. I put the new intake and carburetor on and started working on my mad max tune. Lean burning cruise, lean burn idle and fuel dumping wide open throttle, removed the cold air intake so the air cleaner would suck in warm air from the engine bay. This worked very well for fuel economy, but the warm air intake seemed to reduce full power which I didn't like. In town fuel economy went from 6.6mpg to 9 to 10mpg. Driving like a normal person. Then I could milk up to 13mpg out of it when I would cut the engine off at stop lights and use the Mexican overdrive which is where you get up to speed, put it in neutral and cut the engine off and roll to your next stop sign or turn. A Mexican I know told me about Mexican overdrive so it's not racist. I developed a switchable thermostatic and cold air intake for last winter. This would take scorching hot air off the exhaust manifold and mix it with out side air to maintain a consistent 95°F, then I could switch it to cold air for mad max power. I liked the way this worked. So now I had greatly improved fuel economy with no sacrifice in power. The holy grail. I never took the suburban on the highway when it was getting 6.6mpg, so I have no idea what it's original highway fuel economy was. I never have taken it on a long road trip, but I would take it to the next town over which was a 55 mile round trip. I would filler up, drive the round trip and filler up again at the exact same gas pump this would always net between 16 and 18 mpg. Remember I was only expecting 12mpg, new modern 2015+ fuel injected suburbans only get around 14 on the highway. By this time the cylinder head gasket was starting leak a lot and I was starting to wonder if my lean burn tune was burning the valves. I pulled a cylinder head, found the leaky head gasket and found the leaky gasket cylinder was very badly tapered, the engine would need a rebore, it had been leaking coolant for a very long time. So the engine had to come out and it was not going going to be the same when I got done with it.