I ran a 42 rear on my MC21 for the track, livens up the acceleration and I don't think I ever found full top speed anywhere.

Saying that, I've never ridden it at Donnington...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball