I want the biggest tires I can get without changing my rims out. Is there a difference in the width on some rims? I've seen bigger tires marked as 12.50 or something as the width rather than in MM?

 

Well your title doesn't really match your question but yes 17 inch wheels are the smallest you can go on a 3rd gen Ram. As for the width I think in 17s the widest width I've seen was 9 inches which is what you want if you're going for 12.50 inch wide tire but I believe you can get away with a 8 inch wheel too

 

Generally you can go with a tire that is about 4" wider than your wheel width max. For factory 17x7 that would mean 11" wide tires, for factory 17x8 this would mean a 12" wide tire max.

For a 12.5" wide tire you should be looking at an aftermarket 17x9 or a 17x10 wheel. Then you need to start thinking about offset and backspacing to make sure it'll clear front end components...

 

They make a 17x10!? The widest I've seen is a 17x9. That's crazy wide for a small wheel but I can see it for like drag racers

 

Hell, I've had 15x10 wheels...

 

Go to tirerack and look at the specs it will give you the rim width range. So like a 265 on a 7" or 305 on a 8". Personally I would think keeping it near the measured rim width is ideal.

 

I run 285/70/17 on a 9 inch wide rim. I used tire rack to make sure I got the correct offset and back spacing as well. Post pics when you get um