Yup. I'm gonna join the voices of the others here, saying: They make a difference

The bigger the tyre, the heavier it is - therefore the more weight you have to push/drag along with you.

The wider or knobblier the tyre, the more friction you have to fight, the more drag resistence on the road you have to work against.

Yes, tyre width and slickness definately will make a difference to your speed.


NB: I think the articles around spending money on aero bars and bike helmets are written with the assumption that you are beginnning with a road bike - not a mountain bike or a hybrid/comfort bike. Therefore those articles would see aero bars and helmets and wheels as the next step, because they will be assuming the bike already has slicks on it.