Which trucks are these?
Daihatsu Hijets, Suzuki Carries, Mitsubishi Minicabs etc all can get 40+ (30+ is easy even if you aren't driving for economy) if you aren't going too fast, which is why I say they are better in town, on slower roads or off road. You have to remember that mini/kei trucks have tiny engines. If you are driving on freeways they are not ideal, these are designed for Japanese highways and cities which have slower speed limits. This is also assuming you are actually taking care of the carbs in particular, as a lot of these are not injected and your mpg tanks like CRAZY if you get jetting etc. Grab some kits

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Worth noting also besides the folding beds a lot of these trucks have dump beds as well. I just really can't universally recommend these unless you can tolerate right hand drive, can deal with not going on freeways, or you live rurally. I can think of too many situations where I would want a pickup at least, when I retire though it's on!