Fuel - ROLLING DOWN THE WASTELAND

This open world racing game published by Codemasters and developed by Asobo Studios (creators of the upcoming The Crew) had the biggest gaming map and also got a Guinness World Record.

I couldn't find a reason why this game isn't available on Steam anymore. However, many keys are around the web. I got mine on Tremor Games. I recommend this game, but don't spend too much money for it.