It really depends on what you mean by hauling groceries. I have 2 fixed bikes for grocery. One just has rear panniers and is meant as a short/small run getter. If need, I can use my Chrome Backpack for the extras. For big runs, I have a full pannier(ed) front basket(ed) bike that can take a grocery trailer. In both instances, they are 26"er's. Mind, with the former, it really doesn't matter much what ti(y)re size you use. Light loads or small runs don't put much extra weight on the bike to matter much. For me, with all the weight and pulling of all the grocery on a big run, I prefer 26"er's. Allows me to run more comfortable with less chance for puncture and the extra width of the ti(y)re helps overall with the pull of the load. Specifically for me though, I also get grocery by bike year round and it snows here. So for me, knobby 26 is always the answer.