Find Eva the mechanic and test drive the custom built monster truck with built in jump jets that she has been working on. Once you've run a check-point race you'll be able to purchase the car from her.

If you want more firepower in the game then complete the following tasks to unlock the corresponding equipment.

Unlock MOAB Bomb:
Complete ALL HVT missions for the Allied Nations.

Unlock Ambassador Gunship:
Complete level 3 of the Helicopter Flying mission.

Unlock Diplomat Heavy Tank:
Destroy the targeted buildings for the Allied Nations.

Each Character has 4 costumes (incuding the default costume). To unlock them for each character beat Fiona's Level 3 challenges: Gun Emplacements, Grenade Launcher, Pistol and Sniper. The Pistol challenge is the easiest to complete while the Sniper challenge the most difficult.

This is accomplished by completing ALL the Outpost missions before you initiate the final mission in Story mode. The Outpost missions must be done first because they can no longer be accessed when the game has been completed and will prevent you from getting 100% completion.

This glitch which will come in useful during the game is performed by parking your Vehicle outside of a building that you can enter. Go inside the building and then exit it and your Vehicle will now be completely repaired with full ammunition.

Whenever you win a level 3 weapon challenge bet at the PMC Villa with Fiona one of three alternate costumes will become unlocked. Bet the maximum money for the level 1 and 2 challenges then save and reload the game until you win. On the level 3 challenge only bet the minimum money.