My tires are 700x45 (47-622). Bike shops here all stock either a 700x35-42 or 700x37-43. Nothing larger. All say these will be fine. At least a couple of times when I'd encountered this in Europe the shops were very much against using undersize tubes and would recommend going to another shop if they didn't have the right size. They said that stretching an undersize tube causes much faster natural air leakage (instead of having to add air once every three months you'd need to add it about once per week) and seemingly they flatted easier.

Any truth to this?

I've always assumed an undersized tube would be stretched more and puncture easier, but I got nothing to back that up.

700 is also 29r size, so you might try shopping around for 29 x 1.75-(whatever) or 29 x 1.9-(whatever) size tubes.

But I'd not hesitate to go with the 700x37-43 tubes if that's all you can find.

Schwalbe shows the size range each size of tube is suitable for. Bicycle Tubes | Schwalbe North America

I agree on all points, except that I have personal experience that backs up the first assumption. A too-small tube can fail very suddenly, like a balloon landing in freshly cut grass. They are also harder to patch.

Some, but I would assume the effect is proportional. A tube designed for tires up to 42mm doesn't have much further to go to fill up a 45mm tire (only ~7%), so I'd be completely confident in using one. You probably wouldn't want to use a tube intended for 20-23mm tires, though.

An undersized tube may leak more air & lose inflation faster, otherwise it should be ok, as long as its not some crazy size difference.

- Andy

Those warnings about undersized tubes are true, but in your case, the tubes available to you are not very undersized, so you'll be fine.

And from my experience, probably impossible to patch. The rubber has to stretch so far that the glue cannot hold.

I have been using AV17 tubes from Schwalbe for years in the U.S. with no problem and seldom (maybe once every two weeks) add any air. Previously in Germany I used tubes with Dunlop valves that covered a wide range of sizes (about 28-47mm) in my 622-47mm tires, also with no problems at all.

I have used very narrow tubes made for 700x23mm tires with 700x32mm tires and never had a problem, I have also used tubes made for 28mm tires with my 38mm tires and it was always ok. Tubes stretch. Actually I prefer to use smaller tubes because I carry 3 spares and smaller tubes take up less space. In my experience smaller tubes also make tire removal/install a little easier with some types of tires. I think that using tubes which are too big would be a more serious problem then using the smaller ones.

My exact experience. I was given several smaller tubes and have all but two of them left. The first two that I used are still in the tires.

I have a pair of 45s I use on one of my bikes and I have a set of tubes for them that fit a large range of tire sizes (like 35 - 47). I probably got them online but I don't know for sure. It might have been at "One on One" downtown or at the Hub Coop.

FWIW I had a tube sized for 28-32mm tires inside a 35 for quite awhile. It was supposed to be temporary (like a few days) but it was probably more like a few weeks. Since tires sizes are not very exact I'm guessing that a few mm isn't a big deal. However, I personally would rather stick with the recommended ranges if at all possible.

Thanks for all the input. I went ahead w/ the 37-43 and ordered a couple of proper size tubes so I'll have them on hand since these seem to rarely be in stock at LBS's in the U.S.

I find the range of sizes at my LBS varies noticeably whenever I need to buy them. If they have the size I want, they'll have ungodly long valve stems etc.
Sometimes they have exactly what I want and sometimes I have to "fudge" a bit.
Now, if they have what I want, I buy 2-3 spares. Murphy's Law says IF I have way more than I need, I probably won't need them.

The diversity in sizes is pretty crazy. It's hard for a bike shop to stock everything nowadays. And it's hard for us, as customers, to even know what we need and what substitutes are reasonable.

I always shoot for close in size....700x 20-23 will have no problems going into a x25 or x28. I wouldn't put it in a x32, though. With high pressure tires, you are going to have to top off the air every day or so anyway so I just use what is on hand or available. I even am using 700x35s in 700x32s right now and have had no problems

Slightly undersized iz no problem. Slightly oversized is. I wouldn't sweat - go for it.

These make all sizes:
Schwalbe North America | Schwalbe North America

It will be lighter, many people want that.

i still have a tube that's 700x23 in my 700x25 it's been that way since Oct 2014 and i've put several hundred miles on it.

based on what my more knowledgeable friends have told me, i agree, having a smaller tube is preferable to having a larger tube if you don't have the size available to you.

I've used undersized tubes for easily hundreds if not thousands of miles with little real harm. I don't think the stretch ( and subsequent loss of durability and ability to hold pressure) is a real problem. None of my bikes experience any loss of pressure beyond what is to be expected. I just top them up every week or two, whenever I remember, and they're all fine. The tube isn't a major barrier to puncture anyway, so it shouldn't affect the rate at which you get flats at least.

Neither has my ability to repair a puncture been at all impeded. I have a bike with 23mm tubes inside of 45 mm tires that has been patched 10 times (at last count). If it weren't for the occasional pinch flat, or hole right on the valve stem, I don't know if I'd ever actually buy new tubes.

However, if you really disobey manufacturer recommendations and use a wide tire on a narrow rim with a narrow tube, you do get more flats (ask me how I know). The tube around the stem seems to stretch out and wear thin, and I got a string of holes right around the reinforced grommet. Switching to a tube that was closer to the right size helped out a bunch with that particular problem.

That is some of the best advice I have seen in this thread. The weak point when using undersized tires is the valve area. Just pump a tube up with no tire and you'll see how it expands. We are not talking thin like a balloon here, but tubes do stretch a fair amount. If you are going to have a problem going undersized, it will be in the area of the valve.

I've used slightly undersized tubes for years and don't get many flats. The last time I stocked up on tubes was during a buy 3 get 1 free sale and I ended up with several spares. I'm hoping that is going to negate their need.