Need help from the backspace and offset guru's

I have a 92 mazda ex cab, gonna be flat rocker bd'd on stockfloor frame. I have found a set of wheels I want to use but I'm not sure if they will work or not. Fronts are 20x8.5 6.16 bs and +36mm os.  Rears are 20x10 7.27 bs and +45mm os. I am running and early Toyota rearend that is 5" narrower than the stock Mazda rearend and I'm running the factory redrilled hubs up front with factory uppers and lowers and spindles. I just need to know if they will clear the fenders,

the front wont fit , you need a 7" bs on a 8.5 wheel , rear should fit

With the backspace and the offset combined its 7.57732 total backspaceing. the +36mm offset gives me another 1.41732 inches of backspace. Right?

no backspacing measures the center of the wheel to the rear edge of the wheel hoop,  offset measures from the wheel center top the front edge of the wheel hoop

---------------------------------------------Originally posted by MazberryDelightno backspacing measures the center of the wheel to the rear edge of the wheel hoop,  offset measures from the wheel center top the front edge of the wheel hoop --------------------------------------------- backspacing measures from wheel mounting surface to back edge of wheel hoop.offset measures from wheel mounting surface to centerline of wheel.

ooops i was wrong , i always just use backspacing

Word, backspacing just seems more straightforward.

What I was told by the shop selling these wheels is that the backspace is the distance from the back hoop to the mounting surface ( I knew that) then the offset is the amount in mm's it is offset, sounds messed up but if the wheel is 8.5 wide your true center is 4.25 and would be normal backspacing without any offset. If the wheel has a backspacing of 6.16 and also an offset of +36mm (36mm = 1.41732 in) then your true backspacing is 7.57732. Right? Thats what the guy told me.

I was looking at some other wheels that an offset of +12, there wasnt a number for backspacing, I asked them what the backspace was and they went into this whole thing about how wheel offset is measured from the centerline of the wheel so the backspace was 4.25 on a 8.5" wheel and if it was +12mm offset it gave you more backspace and if it was -12 it gave you less backspace in relation to the center line. I cant drive to Tx just try them out on a truck thats in pieces. HELP!!

No, backspace and offset are two different ways of locating the wheel mounting surface.The guy was correct, you take half the wheel width, and add a positive offset, or subtract a negative offset to find out the backspace, but it's not adding the backspace to the offset.Also, on a 8.5" wheel the true centerline is at 4.75", not 4.25".  You have to add 1/2" to the stated wheel width to account for the rear mounting lip that the tire bead seals to.It's a lot of confusing stuff that can mess you up if you're not paying attention.So on a 8" wide wheel with an offset of +25, take 8 divided by 2 to get 4, then add the 1/2" to get 4.5".  Then add the offset of 25mm, which is 1" to get 5.5".  That would be it's backspace.

ok, so what is a decent priced wheel i can use? I cant afford billets. hard to find wheels with that drastic of a backspace that arent billet.