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I guess it depends on what you use it for and what your budget is , you can go to a 13.5 and still use pump gas but i would recommend Premium and Non Ethanol only (at minimum) or even mixing it with Av Gas , which is non Ethanol and about 110 octane and about $4.00 a gallon so its not outrageous , you could mix 1 gallon to 4 of Premium and be good for just about any piston bore combination , you just go to your local small plane airport and pull up to the pump and fill it up as they take Credit Cards right at the pump , you could also stick with a 12.0-5 and get a HotCams , depending on the type of riding you do , would be the type cam you use , , for Single Track riding i would recommend a HotCams Stage 1 , it will give you loads of bottom end and mid range torque , but it will not be much (slightly more than stock but barely) top end , a Stage 2 will give you plenty of Mid Range , some bottom and more top end torque/power , again , everything depends on what your looking for as far as main use of the bike , this determines what to change and why ......................Hope that helps some ...........
On a side note: . i run a HotCams Stage 1 in my 06 and the stock piston , with the airbox opened up , the carb rejetted and a 450R muffler with a Pro Moto Billet Endcap , my bike does excellent in single track to mildly open desert riding , i am geared 13/52 and have a top speed of about 75 mph , though i don't get anywhere near the rev-limiter , so i could most likely go faster , but prefer not to rev out the engine , so that will give you some idea of what to expect