I recently purchased a single speed that came with a 42t chainring, but its a commuter/road bike frame with 700x28c tires. A 42 would work, but you may need a 17 or 18t freewheel on the rear to make up for any extra weight, or rolling resistance from fatter tires. My bike has a 16t freewheel, but I'm considering swapping it with an 18t to enable me to climb hills easier. I've only had to get off and walk it up one steep hill so far, but I haven't rode up long hills yet either.