I've got a dumb question for you guys, I have a 2008 Ram 1500 Big Horn w/20" OEM wheels (not sure about width, eyeballing I would guess 8" or 9"), I want to get the OEM 17" wheels. I found a set from an 09' are they the same? 5 on 5 1/2, 7" wide, 25.4mm offset.

 

Yeah they will fit. 5x5.5 bolt pattern will always fit..

 

Yep, 17" is the minimum size necessary to fit calipers, you'll be fine...

 

Stock 20's are 9" wide and have a 19mm offset(5.75" backspacing) A 7" is a pretty skinny wheel! 25mm is equal to a 5" backspacing, but now it's 2" skinnier so that would be equivilant to your 20's setting inside the fender almost an entire inch. Are these for snow tires? 245 is the widest tire you should run on those.
It'll kinda look like a v6 sports car look running those rims/tire combo between being further inset and a super skinny tire. A bit ridiculous IMO.
Sounds like they are jeeep wheels or something. What's the weight rating on them?

 

Oh crap I cut / pasted and missed the 7" part, so sounds like the 17" overall size and bolt pattern are good though. Gonna run 35x12.50's on it, so to keep a somewhat stock look I think I'll need at least a 4.75" backspaced 8" wheel, that sound about right?

 

Well, you might have "volunteered" you were intending to run 35x12.5s in your original post. Not just "will the wheel fit"?

8" wide wheel is a bit thin for a 12.5" tire, and I think you are gonna have a bit of a rubbing issue running a wheel with a positive offset, it'll be close.

You really need at least a 9" wheel to run a 12.5" tire. Pavement Queen?, you'll prolly be ok. Guys run 12.5s on 8s, the bead will seal but you better not run 'em too low on air, forget airing down for off-road. I've known a shop or two who won't mount 12.5s on 8s as it is technically "out of spec", so you may want to check with your local shop...

 

edit: re tire size, ya my bad, psoting while looking at wheels on ebay and rolling through different tire/wheel/lift ideas

edit2: AHHHH that offset vs backspace finally clicked, stare at the pictures long enough and it makes sense.

Oh the offset vs backspacing part has forever confused me, in spite of looking at the diagrams. Thanks for the faq post btw, the offset part made more sense after looking at it, as for backspace, the smaller the number, the more tucked in the wheel right?

As for the 12.5"s on 8", I've been running 35x12.5's on 8" wheels on my crawler for about 13 years now, blown a bead here or there, but I generally run 12lbs off road. The truck however might not work with that due to weight though, so good point, I might need to stick with a 9 or 10" wheel.

 

9" wide wheel will accept up to a 12.5" tire and with with a minimum of 5.25" backspacing 0m offset to run those 12.5's at stock height. Most guys run 4.5" BS with that wide of a tire if they are 35's.