If you buy cycling tyres with a width 25 mm this does not automatically mean they are exactly 25 mm or 25,4 mm (1 inch).
I have collected some data from my road bike.
| Tyre | Size | Rim (inside) | Pressure | Width front tyre | Width back tyre |
|---|---|---|---|---|---|
| Continental Grand Prix 4000S ii (folding) | 25-622 | 17 mm | 7.0 bar | 27.1 mm | 27.3 mm |
I will update this table when more data is available.