Does anyone know what the largest tires you can put on a 650 without making other modifications?

I would assume you could put 26 inchers on without any problems.

Will it take 27's?

Hi ratman,
I have 27" Blackwaters on mine with no problems.

I have 28x8-12 and 28x10-12 mudillas on my prarrie. It seldom rubs, maybe 4 times so far. I do not feel it needs to be lifted. It only rubs jumping, and doesn't hurt anything, doesn't hit hard.

But, my tires both measure exactly 27" in diameter. Tire dia is a function of pressure and rim width.

So, I say yes to 27" tires and you may hafta lift it on 28" tires.