by chance, it was the GP5000 in 25mm that caused me trouble. Mine measured 22.5mm wide on the Mavic MA-2.





The frustrating part was that I had some Conti Grand Prix Classics in 25mm that measured about 26mm on the same MA-2 rims.
I also have some Conti touring tires that measure 3 or 4mm smaller than the labeled size, so I knew that Conti's sizing was sketchy. However, I assumed that there would be some consistency in the Grand Prix family of tires.
I was wrong.
Efforts to contact Conti and get info on their sizing was unsuccessful, despite full cooperation from the US distributors. They all said that Conti was known to be unresponsive to questions.

Steve in Peoria