For the ECO Marathon, the students at my Uni used a direct drive on their vehicle without a throttle butterfly. What they did was to run the engine on WOT from low RPM to max, then declutch it and shut it off. When the car slowed down, they re-engaged the clutch, which bump-started the engine and they accelerated to their max speed again.
Keep in mind that each gearmesh that you add to the drivetrain is a 2% power loss. Usually in a gearbox, you've got two gear contacts, so you've lost just under 4% of power (1 - 0.98x0.98).
A good toothed belt with good quality pulleys will give up to 99% efficiency. Toothed belts are however very sensitive to dirt, so you might have to keep it protected from the elements.
CVTs are a lot worse, as their efficiency varies according to the ratio of over and/or underdrive. A belt-type CVT with centrifugal pulley sheaves will vary between 80 and 95%.
Long story short, I'd propose that you should rather go for a single-ratio toothed-belt drive or a single ratio gear drive.
