I would ask over on the UK time trialling forum.

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´Get the most aero and light bike you can get. With the aero advantage you can be saving minutes and with the weight advantage you can be saving seconds. In a race against the clock both matter.´

BMANX

If you don't get any others answers, I would guess the tyre should be the same width as the rim?

First. Does your cassette fit?
My old corima was a 7 speed cassette. Changing freehub was a pain. Running 7 speed was fine for flat TT's.
It's a narrow rim but if your frame fairs the rear wheel go for rolling resistance over aero on the rear.

Pretty much the skinniest you can get. I'd go with a 19mm.

That being said, the penalty for a wider tire is quite small for a rear disc.

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ECMGN Therapy Silicon Valley:
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People, that is amazing thanks. It's been modified to take 11 speed and I have a cassette on order. Just put it in the frame to check clearance (good shout!) and it looks like there is plenty of space.

I'm going to track down that site of the dude who does the rolling resistance tests and see what 19mm I'll be best off with.

Thanks for the help! :)

Google blather about bikes for Tom A's rolling resistance tests

Ah bugger, Blather... and the other site I found (http://www.bicyclerollingresistance.com) only review 22mm + in tubulars. Perhaps this is the price I'll pay for a older wheel :(

The general rule of thumb is that the tire should be a bit smaller than the rim to avoid the "lightbulb" profile. My 28mm rims recommend 23. Zips recommended a 21 for the original 808 tubbies.

My feeling is that you can run a 23 on the rear for improved rolling resistance and better cornering if you want. If not, 21 or less.

Generally tubs don't "lightbulb" unless they're massively oversized for the rim.

I suspect the Corima disc has changed very little over the years, so should be basically the same as mine which is about 3y.o
If that's the case, corima says 23mm, as any narrower won't protect the rim if it flats.

I'm running 22mm schwalbe ultremo TT's and I don't think I'd go any narrower. Have also run 23mm corsa's.
24mm is probably the widest I'd go the other way, at which point you might start having clearance issues anyway with your frame.