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If you want to get maximum grip out of 140 tyre, you must use 4.0" wheel. Otherwise tyre-profile will be pinched and more rounded than front. Meaning when you've reached edge of front, there's still unused rubber at edge of back. You're only using 130 of 140mm tyre and only getting 130mm worth of grip anyway.
Pinched rear tyre causes nervous , yet slower turn-in with unpredictable handling. This is why many, many 400 racers go down from stock 150 to 140 rear tyre. Better handling and more grip. Kawi dropped ball and didn't spec 4.5" rear rim which is better for 150 tyre. Using 4.0" rim just doesn't work well at track for performance with 150 tyre.
In both these 300 & 400 examples, it's best to use optimum tyre-size for rims you have. In our AFM races, fastest 250s use 90/120 tyres for factory wheels.
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