An Abrams has a 289 mile per tank range (on a 500 gallon tank). Assuming you find one above ground diesel storage tank it will have somewhere around 10k plus gallons of diesel fuel in it. Even assuming they picked up the tank at Fort Benning (when in show they've been shown abandoned at random intervals like in the middle of downtown Atlanta) and that Woodbury is actually where Senoia (the town its filmed in is) then its only approximately 90 miles from Benning to the Prison. If the fictional Woodbury is actually in the location of the real Woodbury its only about 60 miles away.
That is the one thing people forget about a ZA, other than perishable goods (and in real life non stabilized fuel is a perishable good but in the Walking Dead it isn't apparently) people truly underestimate the economics of scale. Other than growing the stuff that will rot, a ZA scenario (where the population is reduced by say 99 percent) is pretty much the end to physical scarcity. Now of course service industries, innovation, and other human capital would be hard to come by.