I bought a mountain board from a second-hand shop, knowing full well that I'd have to replace the tubes on it, problem is, I don't know what size tube to get. The tires are approx. 10" in diameter and the tubes i pulled out are approx 9" seam to seam and 4.5" inner seam to inner seam. I still am not sure if I should get a 9" tube though. There is no identification on the tube except the numbers: 300-4. If anyone knows anything, it would be greatly appreciated.