I just bought a new (to me) 2008 F250 4x4 XLT Supercab with 4.10's gears and I'm trying to figure out what wheel/tire options I have without having to lift or level the vehicle and something that won't screw up the speedo badly or anything like that. Right now it has the base 17" steel wheels on it with LT 265/70R17 Pirelli Scorpions (pic below).

What I'm wanting to know is basically how big of wheel/tire can I go without having to add a lift or leveling kit. This is almost entirely for cosmetic purposes but that said, I don't want reduce functionality either. While this truck will spend %90 of it's life on paved roads in the city I will use it for off-road towing (medium load, nothing crazy) in the mountains several times a year so it still needs to be able to do that as well. Thanks for any and all help!!