The wheels are pretty small (compared to even the rear wheels of the zero turn), so I doubt filling them would make much of a difference. I've used a Gravely ZT XL 42 on hills, and never had the feeling that it might tip over backwards (other than the time when a rear wheel when into a small ditch next to a ravine, and then pulled the mower and me down the ravine, I was VERY lucky it didn't flip over on me and just stopped when the rear of the mower hit a stump).
Since you're considering a steering ZT (vs one with caster front wheels), adding some front weight would work for you. If it has a bagger option, those typically also have a front weight to counterbalance it, you might get that weight if you don't want to fab up something yourself.
Since you're considering a steering ZT (vs one with caster front wheels), adding some front weight would work for you. If it has a bagger option, those typically also have a front weight to counterbalance it, you might get that weight if you don't want to fab up something yourself.
