I would think 26" tires would be fine on stock rims? They are not that big and I have seen guys with bigger(taller tires)on stock rims. I think the most important thing is to make sure you don't run anything bigger than say a 26-10-12 on the front. Most aftermarket rims are off-set which is an added bonus for more stability, they are usually lighter and stronger if you go with the aluminum...........Irv (nice salmon by the way! About a 30 to 40lbs?)
I just put a set of 26" mudlite radials on my wolverine with stock rims. No problems here. But I have been told that my wolverine rims will not fit my grizzly due to the independent suspension and the offset of the rims. Kodiak and Grizzly are basically the same frame. I don't know if this helps any with your problem.
We install 26 tires on stock rims all the time. I also own a 450 Kodiak (2005). Got it for the wife (LadyLovesthe Grizzly660). Its a little over a year old now with app. 3500 miles on it. She likes it but still loves the Grizzly. Tells me there is only one thing wrong with the Kodiak.....its just not a Grizzly. Will be replacing stock rims on the 450 soon. Will be adding MotorSport Alloy rims. We took the stock tires off....just as soon as the stock wore out....in this territory it doesn't take long to wear them out. Replaced them with Pirelli "Rut Busters". This tire is rated as a 4 ply and does give a smooth ride. Only thing I dislike about them is that they do not do well in snow. LLG wants to replace the tires when we do the new rims. Leaning toward putting on the "Wings" by BKT ( available thru Parts Unlimited). When going up to a larger tire....the main thing you may find is that a larger tire especially on the front will take away from your steering. It will make it harder to steer, just do not go larger than a 26x10x12 on front. Mudlite does make a 26x9x12 for the fronts.
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