[dx87]

It's probably because if you're low income, you likely can't afford the lower MPG or expensive things like tire replacements, and if you're upper income, you just hire people to do work that requires a pickup. I think middle-income people are in the sweet spot for trucks and SUVs where they can afford the extra costs, but can't afford someone to do manual labor for them, so they actually use the truck to haul things so they can do the work themselves.