I have a atc 400 auto whant to know what the bigest tire I can put on whith out changing rim or puting a big foot kit. The best tire I can get to ride on the mud that I can put on the rims I have 12X6.5 front and a 12X8 back

 

Larger diameter tires kill the performance of the 400 real bad. Major loss of acceleration and top speed, mainly because the motor has so few cc's. Not a real good idea to go bigger in diameter than the stock 25's. Wider will not hurt.

 

i just gat a set of 27x12x12 for my 06 400 4x4 they are itp mud lite xxl's they work great in the mud looking forward to the snow

 

I had a problem running 26 inch tires on mine (06 400 auto) I don't know how you can run 27's

 

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i put on a set of high-lifter lift springs that gave me an inch of clearance and i put on a high-lifter lift kit that gave me another inch of clearance