If your Comets are labelled 1.75 they are probably getting up there in age. Several years ago Primo switched to calling what used to be 20x1.75 with the new name of 20x1.50". I've read that the 1.75 was actually about 1.5" wide - thus the switch to a more accurate name. Therefore, I would suggest a tube labelled for a range that includes 1.5" as the max. Important tip: You have to make sure not to buy tubes intended for 451 tires. 451 is the "other" 20 inch wheel/tire size and is different from 406-sized wheels/tires. I have no favorite tube brands.