'Upgrading' to skinny tyres pointless for London commuting

I was thinking, 10% wider isn't much really, but I forgot that you'll actually have about 20% more air in a 25mm tyre (Pi squared R).

I'd like to try like-for-like, so maybe my 23mm GP4000S tyres versus the wider equivalent.

Remeber the volume of air is also affected by the rim width. These new rims in pro racing are as wider than open pros and plenty wider than a 19mm rim.