You didn't say what kind of truck you have, but I checked the specs on a Honda Foreman ATV (2015/2016) and the overall length was 82.8 inches and the overall width was 47.4 inches. So a 5'-5" bed is 65 inches. As long as the tailgate is 18 inches high (i.e. long when folded down), it would likely just fit in lengthwise. The width between your wheel wells might be more of a problem.