In the coliseum, you can bet on monster fights.
With the results of your bets, you can get an engine that does not consume gas (giving infinite gas)
Thing is, that engine is 3.7 million medals (from memory, can't see the exact number), so even if you bet and win the max amount every time, it's gonna be a while. The counter only has 4 digits, making the max bet 9999 medals.
Is there a way to upgrade and make bigger bets and advance faster?