I need some help with wheel size, mainly offset. I have a 2002 f250 4x4 crew cab with the factory 16" x 7" wheels on 265 tires. I'm going to be ordering my new wheels very soon and need some help. I'm going to be going to a 18" x 8 1/2" wheel on 305/70r18 tire. What offset do I need? I know the back space is actually the best number to go by but most all wheels I'm looking at give only offset. Either one will work though with doing a little math. I hope one of you have the set up I'm looking for already. I don't want my wheels/tires sticking out any farther than needed. In other words I want my tires to just clear my leafsprings at full trun........ I've tried two ford dealers and no one can tell me what my offset is on my factory wheels, don't understand why but they can't!?!? I did try a tire store and they told my their computer says they are 0 to +15mm. Reason being that it depended on which ford factory the truck came from, again I'm not real sure of that answer either? Somewhere along the line I thought I was told that my factory wheels were -15mm? Does anyone really know! I don't want to drop a grand on the incorrect wheels!!!!! Anyway and as always, I really appreciate ya'lls help. Remember, I'm going to a 8 1/2" x 18" wheel on a 305/70r18......

 

This will give you a pretty good idea of how to convert for any wheels you may be looking at. Pretty easy to go back and forth between the two once you know how it works. I'm not sure what the factory offset is. Measure the backspacing and convert it.

Mickey Thompson Performance Tires & Wheels

FWIW my 17x8.5 Welds have a 4.5" backspacing.

 

So your 35's clear your leaf springs at full turn with a 4.5" backspacing? So with me running 305's I should be able to get by with a 3.5" to 4" backspacing? I sure appreciate you helping me 306!!!!! Just want to get it right the first time...........

 

I would not go any less than 4 inch backspace if you are concerned with the tires"sticking out". IMO, I think you would be ok with 4 3/8" Backspacing.I'm not sure what the offset refers to, but 15mm is only about 5/8th's of an inch...I'm running 285's and there is NO clearance issues on stock wheels..Again ,the difference between 285's and 305's is 20mm or less than 1 inch.Hope this helps

 

Here's another good wheel reference sight Wheel Technical Information.

More info then just Backspacing & Offset.