If I got 23mm would they fit on Fulcrum racing 7 rims which i think are 24mm. Everyone I ride with has narrower tyres - currently I have michaelin 25mm (?) which look v big. What do you think about thickness of tyre?
These should fit on the Fulcrum 7's with no problems.
As far as thickness of tyres, it depends what you mean. If by thickness you mean puncture protection, the tyre isn't especially thick, though I have found the protection to be very reliable.
If by thickness you mean tyre width, there is a lot of debate about what is 'best'. I have always ridden on 23's, though you will find that pro riders are increasingly using 25's and the famous Continental GP4000s 23s are actually 23.8mm. The logic/debate is over the fact that a wider tyre actually has less rolling resistance due to a better contact area, meaning it's actually faster. It also absorbs more road impact, so you're not taking as much abuse for the reward.