Just spotted that Michelin are offering a Pro4 700 x 20c.

I'm hoping it might be faster than the 23 in the same way as the 23 is faster than the 25?

I think my rims are 19mm.

I can't personally figure out how the energy is saved? It it only weight / aero or is there something about the sidewall flexing that changes?
Do they handle well?
Any thoughts?

There's the flaw in your thinking because it's not actually true. In fact, I've found the reverse is true (same tyre, same route, ridden multiple times). But, I think, in reality, it depends on the quality of the road surface and the speed that you're going. 25s roll better over less smooth surfaces and 23s have a slightly narrower profile from a wind resistance point of view. I'd have thought that 20s roll considerably less well, will need to be inflated to absurd pressures so will ride like rocks, and will present only a very marginal aero gain. If you were riding on a velodrome, then I might be tempted to say yes but generally I'd have thought it was a mistake.

I think science will show that the 25 is faster than the 23 and that is faster than the 19mm in as far as rolling resistance is concerned. But as the profile of the tyre gets wider the drag increases.

When Michelin came out with their Super Comp HD tyres in the late 80s they were a revelation in clincher tyre technology. They came in 19mm and 23mm. 19mm was all the rage especially for the alpaca and skinsuit brigade but they soon went out of favour as a few magazine rolling resistance tests showed the 19mm to be poor, add the fact that you will get more punctures (as the tyre deforms less).

I have just switched to 25mm tyres with my latest set of Pro Race 4's and find them more comfortable. Can's say i notice a speed difference as i am in no great shape at the moment.

The latest in thing with tyres it matching them to rims to get a perfect wind cutting profile. MAvic are working hard on this.

Thanks gents, that's useful info.

The 25s I have are heavier so it must be a weight issue.

Are you able to get 25s to fit under your Crud mudguards John?

Crudguards on my summer bike

They would probably fit but I will stick with Pro Race endurance for the wet weather bike and I have a stock of 23mm to get through that will last a year or two.

Sorry John - I can only imagine it is the heat making my brain play up a bit.

Just noticed the free Lithion deal is back on at Ribble.

So buy a pair of Michelin Pro4 SC / Endurance and get a free pair of Lithions to kill on your winter bike.

Works out about £11 per tyre.

I have a 20mm Supersonic on the H3C because 23's walk-off the rim, however I like a narrower tyre at the front anyway (19mm Podium/Tempo tubular on the DS and the 4 spoke) with 22/23 on the rear (Disc and 4 spoke).
Honestly, I think the combination track better as well as offering marginal aero gains.

That's a good point.
The wear / puncture rate on the front is much lower in my experience.
So maybe a 20 front 23 rear might be the right thing?

I swapped Schwalbe Ultremos in 23c @~145psi for the Pro 4's in 20c @~115psi and I haven't noticed much difference. The main difference is I can now see both sides of the wheel rim which is a bit weird.

I got a refund for the 3 year-old Ultremos after they developed cracking on the sidewalls and so ended up with an extra tenner after the swap - bonus!

Did you get some tiny tubes for them?
Is 115 the upper limit on the Pro4s?
I've been putting about 125 in mine.

My tubes are 18-23c and I'm pretty certain the upper limit is 115psi.