They pick up a really fat hitch hiker because Ian feels sorry for him after they get some direction from him. Wow is amazing how you can get such a large person in the car. Their fellow traveller is a person of few words other than his copious wind he is almost silent…The wind effects their journey. They use the great aircon to evacuate the smells, Ian gets technical and scientific with talk of physics external airflow and aerodynanics. The man continues to even guff odiously and repeatedly in his sleep they try and loose him at a service station. Ian baits him with massive greasy lunch he reluctantly succumbs and as people faint around him, they flee.
Fart is good, but it could be done simpler and hitcheing used elsewhere. Maybe Ian is a bit of trumper hisself - if he lives with just his Dad, I think 'whiffy little monster habits' can get well established, but if they sleep in the car (or share a hotel room?) maybe he's been holding it in for ages and has to find an excuse to let it out.. He needs to go for a jog, or back into the woods. Or he waits until they're near an airport or a noisy tour of a cheese factory. And then he makes a long bunch of wind.