Does tube size matter?
 

Why do manufacturers make so many tube sizes? What is the difference between say a 700c 18-23 and a 700c 23-26?

23-26 weighs more and is bigger. Tube sizes aren't really set in stone, they have quite a big leeway - they're just air bladders.

Your girlfriend will lie and tell you no

If I can blow a 18-23 up to fit a 38 why should there by so many sizes offered inbetween?

You've stretched and weakened the tube. Just because something works doesn't mean it's the best application

Because a 38 won't fit in a 23 tire and a 23 stretched to 38 isn't going to last long- try it and see!

If it's that or walk I'd try it, but otherwise buy something close to what you want.

Is this another thing your girlfriend says?

Oh, and as of today I'm running 18-23 at 90ish PSI in 35s. Let's see how long they last. Want to start a pool?

Assuming you ride every day, one week tops is my bet.

I've kind of wondered if running noticeably undersized tubes results in faster air loss? It seems like the "pores" would be stretched a little bigger?

That is based on the assumption that the air escapes through pores. If the air instead diffuses out through the solid material, then nothing is gained...

Unfortunately all I have is a wife of 20 years

As a 'get-you-home' any tube that can be folded or stretched to fit inside the tyre casing will do, BUT I would always advise changing it out for the correct size once you do get home.

I tried a patched 18-25 tube in my 38 tire to save a half mile walk. It blew in 200 yards.

The rate of diffusion through solid material is controlled by, among other things, path length. A small tube stretched to fit a larger tire will have it's wall thickness reduced and, therefore, the distance the air has to diffuse will be shorter.