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A few years ago, I modified a '73 Buick 455 in a Riviera for the "Silver State Challenge" road rally in eastern Nevada. It involved mildly ported aluminum heads with thinner head gaskets for a slight compression boost, a more aggressive cam, headers, re-worked Quadrajet carburetor, and Mallory electronic ignition. I also added a Gear Vendors overdrive, even though the Riviera has a 2.93 axle ratio already. I also put an air dam on it to cut drag and had the tires pumped up to 50 or 60 psi, as I recall.
The "run" was 90 miles one way, and my "target speed" was 105 mph, which I missed by about 2 seconds (slow) at the finish line. After the end of the "race", we had to go a few miles down the road and wait for them to open the road so we could return. There was a gas station a few miles further down the road they suggested we go and top off our tanks at, but I didn't go to it, just turned around and headed back. Now, about halfway back, I realized that might have been a mistake... I may not have enough gas to make it back!!! But I did, and then began calculating. From the gas station in Ely where I filled-up, it was about 30 or 35 miles to the start of the course. Then 90 miles down at 105 mph, then another 10 or 15 miles to the turn-around. Then that 10 o 15 plus 90 back to the start, where we drove the car onto the trailer for the ride home. I figured it had to be at least 240 miles, and I only had a 20 gallon fuel cell. So that's at least 12 mpg in a car that when stock and going 55 or 60 gets only 12-13 mpg.
The engine modifications were for the ability to run at speed, not for mpg, but we think it increased the efficiency some, or we would have surely run out of gas on that deal. I know the air dam and high pressure in the tires cut drag too, though. - and I had "salt discs" on the wheels too, which probably also helped the "aero" some...
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