Interestingly, this very subject was raised by the man in my local bike shop today.

Most inner tubes are very stretchy. It is generally possible to use one that is slightly too small. A 700c wheel is designed for a 28 or 29 inch tyre and tube. This tube will fit a Coker which has a 36 inch tyre. Thus, we know that you can “stretch” a tube by at least 24% (36 is 24% bigger than 29).

A 24 is 20% bigger than a 20.

A 26 is 8% bigger than a 24.

A 28 is 7% bigger than a 26.

Thus “in theory” you should generally “get away with” a tube a size too small, whatever wheel you have.

In practice, it may not be this easy, and I guess that you might struggle to stretch a 20 tube onto a 24. I’ve never tried, though.

Is it worth it? You’re looking at a small saving in weight - a few grammes. However, look where that weight is situated: at the furthest point from the hub, which means that it exerts considerable leverage. An ounce on the tyre will take more accelerating (and braking) than an ounce of chocolate in your Camelbak!

(Incidentally, if you think you see an ounce of chocolate on your tyre, be suspicious - you may have ridden over a dog’s egg.)

If I’ve got this right, at any given “rpm”, the extra weight will make more difference on a bigger wheel. At any given speed, I think the size of the wheel is less relevant. For most of us, who are not speed specialists, that means that the weight saving will be more significant on a Coker or 29 than on a 20 or 24.

As it happens, I bought a lighter tyre for my 28 today. I’ll see if it makes any difference. Report follows in a few days.