The M109A6 isn't a tank, it's a cannon. It comes with a tender vehicle that ties into it. They're both armored and you're able to transfer ammo from the tender to the 109 without getting out.
These things gulp gas like nothing you've ever seen, but they're economy cars compared to the M1A2 tank.
Go to the fas.org website and look for US Land Systems. You'll find a listing for the M1000/M1070 Heavy Equipment Transporter there. They're big trucks that are used for hauling these things for long distances prior to hot operations. That's how we save on gas. Further, each heavy battalion is supported by a forward support company (what used to be the bulk of an HHC) that has a fleet of fuel trucks, cargo vehicles, medics, cooks and associated log types.
A heavy division uses a ton of fuel, but it's a bargain when you consider the firepower that it brings. Each M1A1 battalion has 58 tanks - about the same size as a comparable east bloc battalion. The difference is that our tanks rarely miss their targets. Each tank carries nearly enough ammo to take out an entire east bloc battalion.
This won't even be fair. I really doubt that the Afghanis know what they've gotten into.