if you average 15mph on a cross bike with 32mm cross tyres pumped up to abot 70psi (on road) for t same effort will there be much difference in speed using 23mm road tyres at about 100psi, all other things equal??

Yes, a hell of a lot.

HTH

With mud cross tyres difference is very noticeable. There are some slicker cross versions about though so depends what you are running

landcruiser with slime inner tubes in

Teetosugars – Member
Yes, a hell of a lot.

HTH

What he said.

the difference from 28 slick – 23slick is huge ….. i wouldnt even like to contemplate how much difference to a 32 knobbler