I normally use 23c Conti GP4000s, but I recently put some 32c CX tyres on my road bike and found that over the rough road surfaces they feel much better.
So
Would 25c tyres run at a slightly lower pressure be quicker than the 23c tyres. Or is there actually no difference at all?
The tyre width you can run is a function of the internal width of the tyre. Measure that and multiply by 2 and you have the maximium width you should run. Run wider and the sidewall is not properly supported. Wider tyres are more comfortable but make sure you have the right rim first! Tyre pressure is also a function of tyre width as well as rider weight.
23c tyres are commonly run at 110 psi but 25c tyre should be run at a bit less maybe 100-105psi.
when i run 23's (rarely), i run about 100psi
25's (sometimes): 80ish.
28's (most of the time, the roads round here are terrible, a bit of give is welcome): 60ish.
i've arrived at these numbers through a rigorous process of 'trying stuff, and trying to remember what worked last time, or at least wasn't an epic fail.
hth.
Would 25c tyres run at a slightly lower pressure be quicker than the 23c tyres
To answer this:
I'm certain that a study by a tyre testing center (I assume this can can be found using google) which is used by tyre manufacturers concluded that a larger volume tyre (consequently ran at a lower pressure) was more efficient over imperfections in the road surface because the deformation of the tyre wasted less energy at the lower pressure. Though I am sire that there are some finer details that I may have mis-quoted or just missed, it basically said 25c were faster. I believed the science, bought 25c tyres and I swear that it's true - lower pressure is better on anything but perfect surfaces (although more comfort and more cornering grip are more noticeable than any increase in speed!).
the difference (in size) between my gp4000s (23) and gatorskins (25) is negligible.
a friend has specialized ones and they look much bigger.
i prefer large tyres; have been running 35c, but they are a bit too heavy. paneracer pasellas 28c look like a good comprimise...
Depends...25s will be heavier, but you can run them softer, which is more comfy and smoother on rough roads.
Go wide I say.
can be challenging to get top end road tyres in 25mm in this country (especially discounted) meaning that the benefits of the width of the 25 are offset by having to use a cheaper lower performance tyre which rolls more slowly
Common wisdom these days is that wider-and-softer is faster/easier on rougher surfaces, which is fairly easily explained.
I still find 23s (on a nice light bike at least) give a psychological sense of urgency that 28s don't, though: the faster you pedal, the better you skim across the surface; whereas larger tyres, being more comfortable at lower speeds, give you less incentive to drop the hammer. So they end up making me faster even if they're theoretically inferior. YMMV. (I've not tried the compromise 25, though my 28s seem to come up as 25 anyway.)
And personally I don't think the weight difference is a big deal unless you go over 28.
There are a number of factors...weight, rolling resistance and width.
The lightest and best carcasses are AFAIK available only as 23s, which skews things a bit...as 25s and 28s are heavier than they need to be IYKWIM.
Anyway I'd recommend tubs 
Plenty of 25mm quality tyres about, using Schwalbe Ultremo HD's myself. I've converted to 25's on my summer bike but stuck with 23's on the winter bike due to Crud clearance. Tbh I can't really tell much difference but have only tried the 25's 10psi less so could do more experimenting. If you need new tyres and don't have clearance issues to worry about I'd def go 25's over 23's, 28's might be an option but they seem a fair leap in weight, could also run something like 25/28 f/r)
Some really nice wider tyres that are still lightweight and have a high quality casing are the Grand Bois tyres made by Panaracer. Very reasonable price from Planet X as well.
23c Schwalbe Durano Plus running at 85-90 PSI. I found over 100 and the back bounced around (bike is fixed wheel). I wanted the 25c but none in stock. Lovely tyre, supple, good grip in the wet, low wear and puncture free (1000 miles so far). I'd take the puncture proofing weight penalty and narrower width over a 25c without now.
I use 23c in the summer but switchd to 28c 4 seasons over the winter months due to the state of my local roads. Really cant believe the comfort difference for a little loss in speed, imo a much better option than 25c if your frame can take them.
Can I jump in and ask about pressures please ? I run 28 Gatorskins on cx bike when in road mode, and tend to have them at about 100. I am about to take deliverly of a 'proper road bike' 
later this week, which comes with 23c tyres, which is new territory to me. So, weighing about 12.5 st, what should I run them at ?
A little bit more than you have in 28s. It's all down to personal preference, carcass flexibility and whatnot - just experiment until you find what works for you.
(FWIW your weight/width/pressure ratios look to end up on the hard side of things.)
Thanks for the replies. I dont think I fancy 28's, but 25's sound liek they might be the way to go. Or alternatively, I'll try my 23's at lower pressures.
How much difference in weight is there between a 25 and 23 (typically?)
A 25mm Conti 4000S is 25g heavier than a 23mm but there is only 10g difference between a 23 and 25mm 4Season according to the Conti website.
Thank you!
running 23mm 4000s on my bike at 100psi but on a 23mm (wide) rim. really comfy, grippy and fast.
Wide rims for the win 
In fact rolling reistance is a function of tyre pressure, tyre width and rim width along with a bunch of other variables.
ianc the tyre pressures you run are the same that I run and I am 90kg dressed. So try 110 psi and see what it feels like. It's all personal preferance. I tend to run higher pressure has some who have posted here as I possibly heavier than those folk and less pressure results in a flat feeling on the tyre and more drag which I notice.
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