The Gr.4 Swift is the best in terms of economy. not just at Laguna Seca, but in the other Gr.4 races, it's always the last one to pit (or in the case of Laguna Seca, not pit at all).
Laguna Seca was one of the few races in this game that I didn't win it on first try. I even had to watch the replay to see what strategy that Suzuki did, and yep, no pit at all. Worst of all was that Suzuki is also the fastest AI and starts in pole position. I won with a high RPM Turbo tuned Supra Gr.4 with fuel map 6 all race, and still finished almost out of fuel and passed the Suzuki on the last sector of the last lap.
As for Gr.3, Gr.2 and Gr.1, I have no idea which cars are best in terms of fuel economy. Some detuned high level Gr. cars can have good fuel consumption, the Chaparral 2J being one such example, or the Tomahawk X but that's just cheating.