On Wed, 09 Nov 2011 16:42:01 +1100, Rob wrote:

Both the same pressure. my 27" rim bike had a narrow tyre which I
couldn't replace unless I used a wider tyre. The size is think it went
from 3/4 to 1" and it did make a difference.

There are other factors - trad, construction, thickness, bead and material
- which all contribute to what for want of a better word I'll call a
tyre's "ride". As a general rule, and by no means absolute, wider tyres
are aimed at more casual riders. They're cheaper, so use lower grade
rubber that is less elastic, might have tread that looks good but does
bugger all but absorb energy, and quite probably weigh a truckload more.
All of that contributes to a much slower tyre.

If you can find a high quality 38mm tyre you can chuck 100psi in and have
a pretty good ride that'll be marginally faster than a 23mm tyre of
similar quality and construction at 100psi. This is a good option for the
back wheel of a commuter which tends to be harder to unweight due to
panniers, etc. Maxxis do something in that size, because I've got one on
the back of my commuter right now [1]. Anecdotally, the day I put it on is
the first day I've broken 20 minutes for my commute in both directions in
a single day in ages. That would have nothing to do with traffic lights,
feeling OK, wind, load, or any other conditions, of course.

Narrower tyres are probably quicker up hills by a similar margin as the
fatter tyres are quicker down and along the flats though, due to their
lighter weight. In all cases the difference is going to be 3/5 of bugger
all though!

[1] well, the width is probably a touch greater, and the height a touch
lower at this exact moment. Bloody bogan droppings!

--
Dave Hughes -
We are, in fact, at the cutting edge of cocking about
- Richard Hammond, Top Gear