BEFORE YOU BUY BICYCLE TIRES OR WHEELS MAKE SURE

THEY WILL FIT!

20 x 1.75 is not the same size as 20 x 1-3/4. There are 5 different diameter tires and wheels with a size designation of “20 inch”. If you are replacing tires whos size is expressed in decimal, like 20 x 1.95, 20 x 1.75, etc. then as a “rule” only a tire with a decimal as part of its size description will fit. They are generally ISO (international organization for standardization) 406 (bead seat diameter in mm). One exception to the "rule", though rare is 20 x 2, which might be ISO 428. If you are replacing tires whos size is expressed with a fraction like 20 x 1-3/4, 20 x 1-1/8, 20 x 1-3/8 etc, then as a “rule” only a tire with a fraction as part of its size description will fit. They are either ISO 451, 438 or 419. There are exceptions to the “rule” but identifying the correct ISO size you are trying to replace and making sure the tire (or wheel) you buy has the correct iso diameter is critical to getting a tire or wheel that is sure to fit.

Having said that the most common sizes in the USA are 451 and 406. 

406 being the most common.