I tried a similar 'back to back' comparison between Schwable one's 23C which has been my preferred tyre and Conti GP4000 II's 25C. Initially I was sceptical and liked the narrow tyre look, the bike also felt odd at first on the 25's. I have to say I am now convinced and will be swapping over to 25c's. After a while the bike seemed to be going faster and I am getting used to the wide look so i tried a back to back test and a comparison with my other bikes over the Easter w/c. I have also been looking at the rolling resistance testing in the attached link, the key seems to be run the wide tyres at the same pressure as the narrow. I run all mine at 120psi.
https://www.bicyclerollingresistance.com/
The attached graph comes form my strava account for repeated runs over a 8.4 mile segment of flat straight roads. I hope this makes sense, the w/e runs started with the tagged ride on 19/4, weather conditions good and light wind. The previous ride was also the road bike on 25,s on a different day and the first two were on a fixie on 23c's. So I have three good runs on 25c tyres. The next two runs were the Fixie followed by two on the lo-pro all on 23c's. The PR highlighted in the middle and was on the Lo-Pro and 23c Conti GP TT tyres, the best clinchers I can find on the resistance charts. Can't fit 25c to this bike, not enough room!
After that comes the road bike again on 23c tyres on similar wheels. The first run was ok and gives a good comparison to the 3 25c runs previously, but the second run I trashed the wheel bearings, so had to back out. That’s the lowest point just off the centre of graph. Then come the two single back to back runs on the road bike in a fairly strong wind. First the 23c, and new bearing, then swap wheels and go out on the 25's. I think in both trials you can see a benefit with the 25c's.
So the last four runs are also interesting. I removed the 27" wheels from my Holdsworth Monsoon along with mudguards and front changer and put on the 700x23c wheels I did the tests with. This removed about 1Kg in weight. What a difference, the bike is now 10Kg and runs good folding tyres rather than the 1 1/8" Conti Ultra sport non-folding. Nearly beat the Lo-Pro first run out, but I think that was a fit of enthusiasm due the change I felt as I can't repeat it. The other runs are similar to the road bike, suggesting it would be quicker on 25's which i will try once these tyre are worn out. I did take it on a club run and that confirmed it is not quite as quick as the road bike, but much better than on 27"
My conclusions, for what its worth, is use modern folding tyres and 25c are marginally, but noticeably, better than 23's at the same pressure. They would look better on old bikes too.
