Originally Posted by
rpenmanparker
Why would you run 23 mm tires on 25 mm tubular rims? There is no affect on the tire width by the width of a tubular rim like there is with clinchers. You might as well just save the weight and get narrower rims. The aero drag coefficient would be better. Or run 25 mm tires on the 25 mm rims if you want another approach to improved aero effect that provide wider tires. The one thing you don't want is to run smaller tires on wider tubular rims. They would be sitting in the middle of the rim, with some edge exposed. Besides aero I don't know what is wrong with that, but it must be something.
Current rims are 40mm deep (27mm wide) and ~405 grams. I have seen 25mm
deep (~22mm wide) rims advertised at 260-280 grams. As you say, I would not use narrow tubulars on wide rims aside from briefly mounting a skinny spare to get home.