Plus that .50 on the disabled Humvee which I'm not so sure driving them over spikes disables them that easily... Some of our military vets on here might be able to elaborate but I thought their tires didn't deflate so easily. Maybe I'm wrong though.
Even if the tires are all shot, roll it on up (tow it, push it, drive it, whatever) up inside the inner fence and fortify the thing so it's easily accessible for defense. Like you said, the Woodbury folks had a lot of stuff stockpiled and when they acquired that, I'm sure they gathered all the ammo for that .50 they could. Next time a van carrying zombies get's driven through the gate open up the .50 on it.