Today I had the pleasure of dismounting a very old Coker 30X3 tire, from the nicest Kelsey wheel I've seen. I was surprised to find a 30X3 1/2 tube inside. I'm surprised that it held air, as it was folded and kinked in several spots, and has never been patched. I've heard talk lately of a tube to fit both sizes, but from what I just saw, I'll never do it. The tube is a real old Wards Riverside, like we used to get when I was in my teens.
Back when things were made to LAST!! Not Chinese junk you get today.
I have successfully used 30X3 tubes in 30X3 1/2. I did that a long time ago ant they are still doing good. I agree that 3 1/2 in 3s is not good.
Larry and Hal both point out a no-no in tube fitment. Undersized tubes can be inflated to fit a larger width tyre, but the reverse is fraught with problems.
An illustration is the multiple sizes printed on some 21" tubes. 4.50, 5.00, 5.25 x 21" The tube is made to fit 4.50 tyres, but will expand to fill a 5.25 tyre. Just don't try refitting one that has been in a 5.25 tyre into a 4.50 tyre!
Allan from down under.
I know of one vendor who doesn't carry any 30 x 3 tubes. They claim that the 30 x 3-1/2 will work for both. Just because you can get it in the tire doesn't mean it will "work."