Offset and backspacing can get freaky if you focus to much on it, offset is just a measure of where the bolt pad is in relation to the middle of the wheel. Back spacing is what you have to worry about when you buy. if your stock wheel has a 5" back space and you buy a wheel that has a 4.5" backspace the wheel will have moved out of the fender well a half inch.

 

I believe the stock backspacing is 5.5"

 

I got 35x12.5x18 nitto mud and a motto metal 18x9 do i need -12mm offset or +18mm offset??? I got a 05 f 150 fx4.

 

You need the +18mm offset which is very close to the stock 5.5 backspacing.

 

okay i hope i say this right... if i dont i am sure someone will correct me.. and that is cool. offset is needed to know about a wheel just as much as back space is.. back space tell you how much space between the hub mount and the point back but offset tells you where the hub spacing is compared to the rim itself.. Just like a 2008 has a very HIGH offset... meaning that most of the wheel is tucked inside the fender... so most of the wheel width is behind the hub mount point.. a lower offset means that most of the wheel width is outside the hub point which in going with a wider wheel could make it stick outside the fender well... hope this helps a little.

 

ok I've read this hole thread and wxmcpo you seem to answer my question the closest to what I will be up against this spring I have my new bfg's (LT305/65/18's) on my stock rims I want to get new rims (18x9) but I want to keep the tires flush with the fender like it is now. Would I go with a higher backspacing (example 6") or smaller (4.5") this has me a bit confused

 

This will help you out a little...

https://www.rsracing.com/tech-wheel.html#backspace

 

Thanks sin that helped

 

I've got a 2005 f150 that I have a 2 inch front strut spacer blocks to level it. I have stock rims 18x7.5 right now but I'm looking at xd778 monster black 18x9. Would I better off with 18 offset with 5.71 backspace Or 35 offset with 6.38 backspace. I want it to be aligned with my fender or slightly out but not much at all maybe half an inch if it is past the fender. I don't want any rubbing issues or anything either of course.

 

From my experience most wheel manufacturers make 2 versions of each wheel for each truck, save for different finishes. One with less offset for stock and leveled trucks and one with higher offset for lifted trucks.

Iirc later model 150s have a stock offset of 44mm. Also keep in mind 25.4mm = 1".