M12XC = Intermediate Terrain. S12XC = Soft Terrain. The sizes are as follows: 100/100-18 = Michelin size of 120/90-18. 100/90-19 = Michelin size of 120/80-19. 80/100-21 = Michelin size of 90/90-21. Note: Please check your current tire size to ensure you are ordering the correct replacement size.
"About 7 years ago I tried a set of S-12's at the recommendation of a friend. I replaced a set of OEM Dunlops on my 250X. I primarily ride Michigan's tight, twisty, technical sand single tracks. These tires added a level of confidence I'd lost due to the Dunlops. I went up one size from the recommended in the rear to a 120/80x18 to suit my riding style. Other than a tendency to shed knobs when running in rocks and roots when running low (sand) pressures the handling is impressive. After 7 years it remains my tire of choice."