Lower overall rolling resistance overall in a bigger tyre on a road that’s anything but perfect tarmac comes from being able to use less pressure so the tyre can deform to the ground. If every ~3mm bump in the road, every few cm, has to lift the bike against your weight because your 23C tyre is at a rock-hard 120PSI, that’s momentum lost. A 28C tyre at lower pressure will allow the tyre to deform more, bumping you around less and maintaining momentum better. It’s a tiny difference per bump but it adds up fast. Called ‘suspension losses’ apparently and Bicycle Quarterly have written some interesting stuff on it. I think I agree with it from personal experience. Comfort advantage really adds up after a number of hours too.

This chimes with my experience. Less skittering around means more confidence when cornering too.

To the OP’s question – I went from 23 to 25c in Michelin Pros, the bigger tyres are better in every way IMO.

I just don’t get why wider tyres feel so much slower yet the tests say they’re faster.

because how you feel isn’t an objective assessment of performance. They feel fast because they feel harsh.

25mm tubeless is the way to go (28s on the commuter).

http://velonews.competitor.com/2012/03/bikes-and-tech/technical-faq/tech-faq-again-bigger-tires-roll-faster_209888

I just don’t get why wider tyres feel so much slower yet the tests say they’re faster.

My old 998cc Mini felt like I was about to win a WRC rally stage just doing 20mph round the ring road. 😉

http://cyclingtips.com.au/2009/03/what-is-the-optimal-tyre-pressure/

so having read most of that, for a 12 and bit stone rider on GP4000S2’s in 25c guise, I should be at about 90 PSI both ends…i think ?

I go about 80psi on Michelin Pro 4s.

Similar weight.

There appears to be much more margin for error anyway, I’ve had no flats at all since I went 25c.

again, people are confusing rolling resistance with speed – from Macavity’s link

Dear Lennard,
Before I go out and buy myself some fatter tires, what testing have you done with aerodynamics? Does the increased wind surface area with the larger tires cancel the positive effects of lower rolling resistance?
— Brad

Dear Brad,
Very good question. As you’re probably aware, the aerodynamic data put forth by Hed and Zipp in support of wider rims for improved aerodynamics shows that it definitely depends on the rim. In general, if you ride very fast, the aerodynamic drag of the fatter tires will cost you more in speed than the rolling resistance benefit will gain you

Read more at http://velonews.competitor.com/2012/03/bikes-and-tech/technical-faq/tech-faq-again-bigger-tires-roll-faster_209888#QusaH6UV4ZEmtMwA.99

If you’re fast you’ll be faster on narrow tyres, if not then go for the a fatter tyre

hilldodger – Member
again, people are confusing rolling resistance with speed – from Macavity’s link

Not all of us. Few of us ride fast enough for aero to be everything. Comfort (and suspension losses, what do Zipp say about those?) and resulting fatigue are relevant, if psychosomatic.

23mm Vittoria Tubs – 140psi
25mm Schwalbe Ones – 100psi

not sure i can tell the difference

I used to find anything below about 100psi was too puncture prone for riding on normal roads – commuting say.

80psi seemed low even for a race in the wet a couple of weeks ago

Noooo, fat tyres and low pressures is the way to go – a mountain biker told me on the internet 😆