When I checked at the local dealer for tubes the supplied info listed a size range. For instance the stock tube fits into the range for my K270 4.00x18. Tubes aren't an exact fit but do need to be close. Because your tire size is between the stock front and my 4.00x18, I would think the stock tube size should fit. Anyone selling the tubes should be able to give you the proper information for your tire size.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.