How do I know this for sure?
In the halo wiki for the warthog, it states is is available in 2553
The most recent 2553 edition features a Graf/Hauptman solar/saline actuator capable of converting up to 12 liters of fresh, brackish, or salt water into hydrogen, with increased efficiency allowing the vehicle to get a range of around 200 kilometers after an overnight condensation capture. It has advanced computer systems that allow a perfect distribution of power to every wheel over any surface and is only prone to flipping in hard-bank turns. Thanks to an increased structural rigidity and gas-inflated restraints, this vehicle holds the honor of having the highest passenger safety rating of any recreation vehicles in 2553.
The standard model starts at a hefty 86,000 UN Credits. A manual-transmission version stickers at UNCr 81,000, while the limited edition wood-trimmed Classic Edition with winch, tow and luggage rack tops out at a price of UNCr 91,000.