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.
Well some modern road cars can have top speeds over 200mph, so the technology is there. As for fuel consumption it is true that you do pay a peanlty for those speeds. But if we look at F1 engines which from 2014 will be 1.6L V6 turbo engines around 600HP and they'll have to be a baout a third more efficent than the current 2.4L norm ally aspirated V8's as the fuel tank reduces from 160kg to 100kg.
Now whilst those cars that are following might recive some fuel benefits the car in the lead won't as it'll have to work the air so that in essence others can draft them. Do you want to be the lead car?
For it to work you woul in essence have trains of cars, where a highly aerodynamic lead car sets the pace. This might have to be run by the state. You could have seperate ones for slower moving traffic such as lorries. Which have there own uniquelt designed lead vehicle.