I am shopping for tubes for my new tires. I am having an issue finding a tube that would fit the tire I ordered for the front which is a 120/80/18 Kenda K270. I haven't seen a tube in that exact size. I did find the one for the rear from Bridgestone (kinda guessed it would have to be their tube or am I wrong there, because they want $ 30 for it). Well my main question is how important is the tube size, or rather how close does it need to be (in relation to tire size) to function properly.