It's hard for people to cut back because they don't know how. Case in point, my dad has a 2000 Silverado truck with the 5.3L engine. It got 19MPG when he got it. When I drove it home to borrow while working on my own truck I drove it 130 miles, got 19MPG driving it easy, right in line with what fueleconomy.gov claims at 14/19. How many people would be able to get better mileage in that truck? Not many. Just by reprogramming the computer (bought EFI Live to do it) I was able to get it up to 24MPG for the drive back. 5MPG difference just off programming, and it's a helluva lot faster now to boot. If my dad follows my suggestions on parts to change, then lets me get back in it for a couple more runs, I'll be able to get him to 29+MPG at 75MPH average speed with an investment of 1500 dollars in parts (electric fans, tonneau cover, long tube headers with true dual 2.25 inch exhaust). This will be about a 60 percent improvement in mileage, and what's really bad is GM could have made these changes themselves. Just with the programming I netted him a 26 percent improvement in MPG with no other changes and no effect on durability, reliability, and only a positive effect on power. The truck still runs on 87 octane, too. If GM were to have made the same program I've got in there now in all their trucks (and believe me, if I can get 24MPG with simple tools they can get 30MPG) then a million trucks on the road going 20K miles average would save 220 million gallons of gasoline per year. A co-worker with a 2003 or thereabouts supercharged Grand Prix claims to have tuned his to the point that he's pushing 40MPG and is faster than he was when he started. These cars get 16/25 according to fueleconomy.gov. He's currently rebuilding his engine to gain power and thinks that with the parts he selected 40MPG should be an easy target to hit.
1 million trucks * 20,000 miles / 19MPG = 1,052,631,579 gallons used
1 million trucks * 20,000 miles / 24MPG = 833,333,333 gallons used
1,052,631,579 gallons - 833,333,333 gallons = 219,298,246 gallons saved
1 million trucks * 20,000 miles / 19MPG = 1,052,631,579 gallons used
1 million trucks * 20,000 miles / 24MPG = 689,655,172 gallons used
1,052,631,579 gallons - 689,655,172 gallons = 362,976,407 gallons saved
Unfortunately most people don't even know how to change their spark plugs, much less reprogram their rigs, so they have to depend on those that do know how to sell them cars that get the better mileage from the get-go. Unfortunately, it would add about 300 bucks to add the right parts to most cars, and the manufacturers up to now have had no incentive to get better mileage when it's a lot cheaper to just announce loudly and often that they are already at the theoretical limits of what they can achieve.
What I'd like to see is true diesel electric cars, not hybrids, out on the roads. If a General Electric locomotive can move thousands of tuns while getting 3 gallons to the mile, then surely a small car or truck would be able to see 100+MPG while having no acceleration or tow issues at all.
