Need to replace front tire - Dunlop 606 90/90-21 seems to be available at many sites. Tubes for 21 inch tires seem to be in 100/80 and other sizes (everything but 90/90) . Are they interchangeable or how close do I have to get? Any recommendations to prevent flats - had a good day of riding cut way short due to blown front tire - feels kinda squirrelly when those things go. Was running stock wheel.