Ok, I have an R80RT. I am taking a distance trip this summer and trying to gather the best peace of mind possible. I fear a flat. I will bring two extra tires (Front/Back) have two tire repair kits, C02 capsules, foot pump, tyre irons, etc. I would like to carry a tube(s) to cover all situations. My goal, hopefully, is to carry one tube.

FRONT TIRE SIZE: 90/90 -18 m/c 51H

REAR TIRE SIZE: 120/90 - 18 m/c 65V Both are Lasertec Metzeler's.

If I were to get one tube to carry, what size would you suggest that could accommodate both tires if needed? I have an idea, just want to get another opinion(s).

Thank You