I will shortly be purchasing a Fast Foward Carbon disc wheel - Wiggle | Fast Forward Carbon Rear Tubular Disc Wheel | Performance Wheels for a new TT bike.

I am also going to be putting Continental Competition Tubular Tires onto it.

This is the first time I will be venturing into tubular tires and wheels. I see that the rim width of this particular wheel is 20mm. In this case, would the best size for the tubular tire to buy be the 22mm version so that the tire is only slightly larger than the rim width. I run 25mm on my road bike, but would the size difference between 20mm rim width and a 25mm tire width be too much?

I have read all about the benefits of using 25mm tires, but would this be negated by the fact the tire is too big for the rim?

Thanks