I would be surprized if it's more than 1%.
If you read the published articles about the development of the Insight, they fought for every tiny improvement possible. What about the little air dams in front of the front wheels? The door seals? The underbody panels? The taper of the rear window?
Aerodynamic effects start taking effect at higher speeds, so I would concentrate on the rolling friction. For a regular car, my guess would be that the following are the most effective steps to improving mileage.
- Drive more slowly, in a higher gear. Lug the engine.
- Use the Insight LRR tires and pump them up. (Look at the specs for this particular tire size on http://www.tirerack.com/tires/Spec.jsp? ... tenza+RE92
to see how it's a completely different tire from the others sold under the same name: It's much lighter than the other RE92 tires. You can't just use any old RE92 tire; it's got to be the 165/65R14 size.)
- Get a zero degree toe alignment. Front and rear.
- Use lighter weight oil, and synthetic, especially in the gearbox.
- Reduce weight of car by taking out all the junk you're carrying in the truck!
- Back off the drum brake adjusters a bit more than specified, including the handbrake.