Riders I know that have used tank bags complained about the gas smell. If you find a bag that has a non absorbant bottom on it, it might be okay but as pointed out you may run into running issues with putting the bag over the gas cap. The cap acts tow ways, first it vents excess fuel vapors from the tank and also allows air to enter tank so that a vacuum doesn't keep gas from flowing. If you have a backrest I would look at getting something like a T-bag or other backrest bags. A lot of them have backback straps so that you can carry it around easily.Jpr1968--is it likely to be bad if the bag is only on when I'm riding? I have no intent to store the bag on the bike. part of the reason for looking at magnetic bags is so I can take it off easily.