When you say truck do you mean big truck? I carried along a folder when I was trucking, but mine was a 20" - I did not carry it in a side box, but it lived in front of the passenger seat in the cab of the truck. That cheap little singe speed big box folder was one of the main things that got me back into bicycles after so many years of being distracted by motorcycles - and it saved my sanity a couple of weekends when my dispatch stranded me far from home for the weekend.

Point is, if you can find room for it, opening your search to include 20" folders gives you more options, as well as making the gearing choices less critical to get some top speed out of the bike.