Actually, the warthog does not use petrol or gas or whatever you want to call it.
"The hydrogen fuel is burned at very high temperature with a synthetic carbon/silicon catalyst to achieve even better fuel consumption. This hydrogen engine is the standard among all UNSC ground based vehicles, replacing fossil fuel-based engines some 400 years ago. The M12 features a Graf/Hauptman solar/saline actuator, and will convert up to twelve liters of fresh, brackish or salt water into hydrogen on the fly"
-the halo wiki
What about a grenade launcher or a flamethrower 'hog, those would be cool.