Thanks a lot Tony. When I tried to search on conti site thy have differwnt dimeters like 3.75" or 4" ... Do you know if these were anything related to type B or C ?
These are just conversions (to inches from metric).
Example:
A 110/80-18 tire is 110 mm wide (approx 4.4 inches), the height of the tire from the bead to the tread is 80% (called aspect ratio) of the width, so it's approx 88mm tall (3.5 inches) and the wheel is 18 inches in diameter.
Tubes stretch a certain amount and therefore one size tube will fit several different size tires. A tube that says fits 3.25-3.50 X 18 means it will fit tires with a width of 3.25-3.5 inches with a wheel diameter of 18 inches. Tubes have a round cross section, so the aspect ratio of the tire isn't all that important.
I realize its all very confusing, so you would be better served finding a supplier that specifies "Type A, B or C" - which has become a more widely accepted standard.