Not sure about the Grizzly 450s, but the Kodiaks 450s will do right around 60 or better. (Depends a little bit on the rider, lighter=faster) They might be a little lighter though considering they have a solid rear axle. Hope that helps a little bit.
I was riding on a straight path got my 2009 grizzly 450 to go around 47 my dad got his to around 55. Don't think they have been tuned. I do know though my dads grizzly has a chinese carburetor. But other than that they are identical. I think it juat matters how they are ridden and taken care of.
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