Regarding speed, the primary reason that fuel efficiency drops off at higher speeds is because of increases in air resistance. If the cars were traveling at high speeds and very close to one another, that air resistance would be dramatically reduced, and therefore the efficiency penalties for high speeds would be similarly reduced.
The drag reduction isn't that great. Bump drafting, where the rear car actually pushes the front car, can gain you maybe 15 mph at NASCAR speeds, the equivalent of perhaps 50 to 100 HP per vehicle. But then to go that fast they need 750 HP engines and get about 5 mpg. Drafting might conceivably get the mileage up to 6 or 7 mpg. If the cars were as streamlined as a Prius that might drop to 300 HP or so and get maybe 10 to 15 mpg.
And of course to travel at those speeds with even marginal safety we'd all have to put on our Nomex fire suits, helmets, and crawl into the roll cage through the window because our doors would be welded shut.