If a cross bike with 700c wheels weighing 28lbs was compared with a mountain bike with 26 wheels also weighing 28lbs both used the same tyre (tread and width). Which would require less effort to be pedalled on a smooth road? Would these both be the same (but require the rider to use different gearing) or would the bigger wheels be easier? Why?

Legs and lungs would make the biggest difference…

If a cross bike weighed 28lbs, it would basically be a 29er with drops!

But which bike would need the most legs and lungs?

On Tarmac, in you look at rolling resistance, wheel size doesn’t matter – Alex Moulton proved that decades ago, what matters is tyre pressure. Because of other factors – air resistance mostly – small wheeled bikes are generally faster.

I’ve just punched a kitten..

The wheel size makes an insignificant difference compared to the tread pattern and compound, tyre carcass characteristics, inner tube characteristics and inflation pressure. Of my bikes my 16″ wheeled Brompton is quickest on tarmac, then my 26″ hardtail with 2.1″ XC tyres, then my 20″ BMX, then my 26″ hardtail with big tubeless tyres, then my 27.5″ full-sus. A proper road bike would be quicker than the Brompton but mostly because of aero benefits.

Great thanks a lot.

I reckon handle bars/ridding position make a bigger difference! I’ve got KOM on a 5k segment before on my cx bike, then tried it again on my 29er with wide bars and upright position and been way off. Different tyres admittedly but I think the riding position had the biggest affect on it.

I’d very much like to know where this smooth road is. I bet it’s a long way from Edinburgh

On the road, bigger wheels are less aerodynamic than smaller ones, all other things being equal, but they roll over bumps better. (Moultons have suspension because their tiny wheels mean they need it to get the same ride as a normal bike.) Which one is faster? Depends on your speed and the state of the road.