The rim width will be a single number, so I'm a little bit confused by your mention of "1.75/2.125" as the width.

There's a good blog post here with a chart in it that might help:

THE RIGHT TYRE WIDTH ON THE RIGHT RIM WIDTH - Engineerstalk : Engineerstalk

Look on your rim for two numbers in the form 559-23. Your tire should have a similar set of numbers. The first number, the 559 part, needs to match for sure. The second number goes to width, and you want your tire to be reasonably wider than your rim.