The Fuel Tank upgrade increases how much fuel the Drifter Bike holds. Fuel Tank I costs 500 credits at Trust level 1 and Fuel Tank II costs 800 at Trust level 2. It is the best first upgrade for most players.
You ride further between gas stops and run dry less often in the middle of nowhere. Fast travel also costs fuel, so a bigger tank makes it more usable.
Buy Fuel Tank I first. Running out of fuel early is the most common problem on the bike. Get Fuel Tank II as soon as Trust allows.
Derelict Vehicles
Derelict Vehicles are the dead cars and trucks across Oregon, and nearly all of them can be searched. They give crafting parts, ammo, fuel and Medkits depending on the type. Loot them as you ride, and use them as cover.
Engine
The Engine upgrade raises the Drifter Bike's top speed and acceleration. Engine I costs 1,200 credits at Trust level 1, and higher tiers add durability. It is one of the first parts worth buying, since speed is how you escape Hordes.
Exhaust
The Exhaust upgrade quiets the Drifter Bike so you can ride near enemies without being spotted. Exhaust I costs 600 credits at Trust level 1. Buy it early if you like stealth approaches to camps and nests.
Forks
The Forks upgrade helps the Drifter Bike absorb damage. Like the frame and rear suspension, it means fewer stops to patch the bike with scrap. Buy it after the speed and fuel parts.