So ...I think, what your asking is if your mower had a 5" engine pulley and a 10" "Rear" pulley...could you put a 10" on the engine and a 5" on the rear and still use same belt and it all"work"...
The short answers are
1. Yes
2. No
the Long answer...
Yes you could do that and the belt would still be the right length, but most assuredly the clutch would no longer work correctly, if at all.. And since most mowers are from 3-4" on the engine and 8" or bigger on the rear....if you swapped it directly like that your stock engine....even most built engines would never pull the thing!!
Now, if your intent on using your stock belt what you would need to do is figure how much wrap each pulley has...SO say a 4" on front has a circumference of 12.56 and if your wrapped 35% around it that is approx 4 1/2" of belt which means....were doing wayyyyyy to much math!!
The answer you really need is this...
Building a racing Mower is alot more complicated than swapping pulleys around, you need to do a whole lot of reading more about it.
Start here -
http://www.heymow.com/index.php?topic=2590.0Good luck with your build!
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