I also don't want to hijack or derail this thread too much, as I am enjoying it thoroughly and learning a lot. Here in Hawaii, the trucks you see are pretty limited. Mostly skeletal trailers with 20, 40, or 45 foot containers; flatbed low loader equipment haulers; and fuel tanker trailers. No sleepers, twin steers, snowplows, or other cool trucks like that.
My question is about a tandem axle tractor I saw that was hauling a fuel tanker. Although the tractor had two rear axles, only the forward one was powered, and the rear one was freewheeling (like a trailer axle). I made a guess that having only one driven axle was either to save on fuel or maybe allowed the tractor to have a tighter turning radius than a normal tandem axle tractor. I asked the driver, (who was a young kid) if he knew why his truck had only one driven axle, but he didn't have any idea, (reminded me of Oddball in Kelly's Heroes, "I just drive 'em; I don't know what makes 'em work").
This was a couple of years ago, and it's bugged me ever since. Thank you. -Mike