A recent mission introduce a new boost item which allows unlimited fuel for 60 minutes !!
Perfect item for some quick cash boosting or part collecting.
Only downside is that to purchase another of these items I need 120 gems. Thankfully the slot machine & the tier 2 tracks reward you with quite a few gems so if I wanted to save up 120 it would not take too long.
With the recent new Jackal bike and quite a few upgrades to the earlier bikes I'm now revisiting the earlier tracks to get some gold medals.
There is of course the time clock cheat to refill your fuel tank by advancing the system time by an hour to recharge your tank but I don't get more than 15 or 20 mins to play during the day which I prefer to spend on the slot machine tracks for earning more money/gems or the missions tracks.