I'd probably find an unmolested Honda Civic VX and remap the ECM to run in lean-burn mode for everything but rapid acceleration. Do a complete fiberglass aero package like basjoos has, with a dynomatted sheet-metal belly pan, LRR tires, and FE gauge. Then I'd install a small electric motor at the rear wheels for 0-5 mph acceleration and give the car an auto shutoff capability. You'd let the clutch out and push on the accelerator and the electric motor would effectively bump-start the engine and you'd be on your way. You'd probably have to play with 2nd or 3rd gear starts and then a quick downsift to avoid a jerky start, or maybe size the electric motor so you can start in 2nd. The electric motor would be extremely helpful in slow bumper-to-bumper traffic where having the engine running is extremely inefficient. Obviously I'd give the small battery pack a plug-in capability and unstall a ultra-high-output alternator to charge it while in motion. You could also get a small amount of regenerative braking by using the electric motor(s) to slow you down. Overall, I'd be suprised to not see 100 mpg in the city AND on the highway.
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