incoherent design.
fuel flies down rapidly even with all but last upgrade in it. don't know what the last upgrade does for it. i just know that i was so disappointed in how NOT empowering the fuel upgrades were that i lost any incentive to play.
but i will go play motherload. cuz this reminded me of the genre. and motherload actually knew how to do it properly.
Contrary to what you may think, the fuel upgrades are balanced out to match the depth of the game. You'll find it difficult/impossible to beat the game without upgrading. The fuel levels are pretty significantly spaced, so if you didnt feel a difference, perhaps it was because you were running out of money staying up near the surface? I think you prefer a slower paced version of digging game to this, the fuel runs out faster in this game because the game is much shorter than motherload 8V.