When I was Technical Editor of Cycling Plus magazine in the mid 90s I conducted two lots of roll down tyre tests - one for 700C and tubulars and another for road 26in 559 tyres. In the 700C/tubular test the fastest tyre were Clement Criterium tubulars which significantly outrolled all the others except for the Veloflex tubulars and wired-ons. But we did test three sizes of a Michelin tyre and I remember clearly that the 18mm was significntly slower than the 20 and 23mm tyres of the same model with the 23mm tyre just edging out the 20mm. We did include in the test some Vittoria TT tubulars - 19mm and these did prove to be pretty fast but not as fast as the Clement Crits (22mm) or the Veloflexs. One fundamental of any testing is that the results should be repeatable - and this we showed - we were very disappointed with the Continental Grand Prixs we had on test so a couple of days after the initial testing we went back and retested the Contis against the Veloflexs. We got the same results. Our test method was:
We had two bikes, two riders. One bike and rider was the control. Neither bike had a chain. We started simultaneously from 0mph, there was enough of a slope to get started and coasted approx 300-400 metres - at no point did our speed go over 12 mph. And we measured the difference between the two roll-out spots. In the test we repeated the coast-down three times and averaged the results. The road surface was fine granite chips rolled in tarmac so not especially smooth and very typical of the road surfaces found in the UK.
Tyre presssures did certainly affect the results and generally higher tyre pressures were faster but this was not the case with all the 18 and 20mm tyres... A bigger difference was the tyre construction - high TPI certainly helped but we also discovered that Nokia tyres (in both the 559mm road tyre test and the 700C/tubular test) rolled significantly better than others of equally low tpi - we thought that this possibly due to other actors in the tyre construction which we could not point our fingers too with any certainty.