I am thinking of changing my MTB knobby tires and inner tubes(26x1.95 on a 26x1.75 rim)to a narrower and smoother Tioga City Slicker 26x1.5 tire.
My question is what size innner tube should I use? Do I need to stay with a 1.75 to match the rim or should/could I use a 1.5 tube?
You could use either size, but generally one that's designed for the tire's actual size will be easier to mount and won't tend to get pinched as easily as one that's too big. You can go "too small" as well, at the cost of having a thinner inner tube wall between thorns and the air inside.
Quoted message said:
One other question, is what is the small narrowest tire I can put on a 1.75 rim. I would like to go down to a 1.25 if possible.
1.25" tires work just fine - it's what I run on my own MTB in "urban mode". Don't try muddy trails with 'em though...