I've noticed a difference too. I haven't kept track of the numbers, but the gas lasts longer when I drive. I try to be more smooth with the gas and brake while he on the other hand tromps both. It's a wonder I don't have whiplash somedays. Of course it's because he's not used to the pedals .

The gf gives me that too. Somehow she never notices the negative gas mileage, when the gauge goes up from where it was when I hand her back the keys. Well, sometimes she does. But she always notices when she gives me the car with the needle just barely touching the pin and I give it back to her with the needle just barely off the pin.

Sorry, that's not true at all. Engines produce different rates of power per unit of fuel consumed depending on RPM and torque output. This is charted on a brake specific fuel consumption or BSFC map. Most engines do best at around 60% load at low RPM. However, because most cars sold in North America have more power than they'll ever need, highway cruising happens at around the 15-30% power levels depending on incline and speed. Working opposite to the BSFC map is that aerodynamic drag has a exponential relationship to speed. This is why driving at 80mph uses significantly more fuel than driving at 55mph. I do have MPG instrumentation and my small 2 door coupe does its best of 43mpg at 53mph.

Spoiler drag never helps push a car. It robs some of the forward energy and directs it downwards to keep the tires planted on the road surface. This is why formula one cars can pull about 2G's on high speed corners where they'd only be able to pull around 1G at lower speeds.

"Spoiler drag never helps push a car" Subsonix
Never said it did. I'm referring to the push on the flattish backend. Why do you suppose most new cars have the trunk deck so high you can't see out the back window? To take advantage of that. Ever see a duck swim upstream?
You have done the numbers for your combination, good. That will not work for my combination. I have read plenty, ever heard of Smokey Yunnick? 60% load at low RPM? I'm talking V8. Engine efficiency greatest at about 3/4 RPM ability of the engine, whatever the load. Overcoming frictional losses inside the engine is the game.
I use the middle grade, because that's where I get the best mileage per buck. I have carburetors on both my vehicles. I'm not crying about gas, as I long ago figured it as cost of doing business. Now, my "wants to roll at 60" car comes up behind your "60% at low RPM" driving style.
Solitary, I consider speed limits arbitrary. I am licensed to operate the vehicle, insured, can make a number of choices that impact the safe operation, and now they want to tell me how fast I can go? All of a sudden my judgement is no good? Baaaaahhh!
The car I'm building will have a "highway" gear ratio to get some better mileage.

Has anyone tried acetone?

Engine torque (volumetric efficiency) might be greatest around 75% of redline, but fuel efficiency is not. The goal is to keep load as high as possible while maintaining speed, and you have to do that in a high gear at low RPM.