Tyre fractionally narrower than the outside width of the rim seems to be the most likely to give low drag and good aerodynamic stability, and its perfectly fine in terms of ride and handling.
Is 28 gp5000 on this a good bet then? Assuming it measures very slightly larger than a 25 gp4000 according to other places. It's an AR56 rim so 23mm internal, 30mm external. Front is ar46 so 21/28.
At the moment I'm thinking 28 front and back as the roads are shit round here.
Tyre fractionally narrower than the outside width of the rim seems to be the most likely to give low drag and good aerodynamic stability, and its perfectly fine in terms of ride and handling.