Originally Posted by
mstateglfr
I said a 23 in a 40 is dumb because there is no need for it. Want a smaller tube in a 40?...use a 32. Tubes come in multiple sizes for a reason and a small difference like 32 in a 35 which is the OPs question is nothing to think twice about. But there is a reason why tubes come in different sizes.
I use 35-43s for my 40mm tires so clearly there is a safe and acceptable variance in tire size for tubes.
If I had a bike shop inventory in my garage I'd probably always use the nominally 'proper' sized tube. But since my most-used bike has 23mm wide, 622mm diameter tires it's more convenient to sometimes just use that size tube even in my touring bike that currently has 38mm wide tires since the narrow tubes do work as well (albeit needing slightly more frequent filling). For that matter, I sometimes just carry a 622mm diameter tube for use on my folding bike with 451mm diameter tires since again they work fine and I don't always have the smaller tubes in my personal inventory. Using what you have available at the time doesn't make it 'dumb' if a trip to the store at that time would be inconvenient and what you do have available actually works just about as well.