32x17 would have you spinning like mad at Thetford, even in the Winter slop. You could ride a 36x17 easily there. For a 3hr race, maybe a 38 would be better
Depends on how you ride really. There's a lot of stuff at Thetford where it helps to ride out of the saddle, so the higher gear would help. 40 x 17 would be a bit tough in places.
26" wheels, I take it?
32x17 would have you spinning like mad at Thetford, even in the Winter slop. You could ride a 36x17 easily there. For a 3hr race, maybe a 38 would be better