Just brought home this 400 in good condition but needs tires. It has the elec/auto transmission and I want to use 25" tires where it currently uses 24" tires. I would have to replace the rear rim as it is an 11" rim and I would use 12" rims all around. The front is already a 12" rim. My question is can I swap out the taller tire (25" vrs 24")without serious repercussions? Looks like about 5% difference

 

You can fit 25 outside dia tyres with no problems, but why are you changing to 12" rims?

 

Changing to 12" rims because I bought a CanAm and want to use the OEM tires off the CanAm on her ATV and buy more aggressive tires for my 570

 

Go for it. I put rear 26x11 and front 26"x9 Zilla's on a Mid chassis Yamaha 350 Bruin with out coming close to anything rubbing.
I always thought it was odd that Honda put little 24" on the ranchers. Cost saving I would guess.

Is two new rims cheaper then buying two 25x10-11 tires?

 

As long as you keep the same size tires all the way around, going to 25" tires is no problem. Honda puts 24" tires on the new Pioneer 500 side by side. Most people change those out pretty soon after they get it. Its geared so low that it actually improves performance by going with a bigger tire. As far as using 12" rims, I'm sure you can find some older Foreman wheels which I think should fit and you'd have your 12" wheels.

 

Make sure the off set of the rim is the same. Here is a good rim chart to look up off set and bolt pattern. ATV Tire and Wheel Application Chart : ATV Tires Free Shipping : ATV Wheels : Mud-Throwers.com