Internal combustion engine. A little more utility than a steam engine but not as useful as most others.

If it breaks to the point that you can't drive it anymore there's a lot that could be done with its parts, but it's more useful in one piece even if this isn't made for driving.

[D pauses as he considers the problem of a lack of fuel.]

Making more fuel for it would be only a little less difficult than making a whole new truck.