i know that my wheels are 15x 7.5 with the 5x139 or 5x5.5 lug pattern but what is the stock back spacing?

thanks Nick

 

All stock Ford wheels have 0 offset which means the hub mounting surface is in the middle of the wheel, so the backspacing is just 1/2 of the total wheel width or 3.75" in your case.

 

Thanks for that information. But I'm still confused!
I have an offset coversion table from the back of a Wheel Pros catalog, that shows 7.5" rims at 0 offset, 4.25"

8.0" rims at 0 offset, 4.5"
10.0" 0 offset, 5.5"

These measurements all seem to be off by 1/2", or not match how you have just divided by 2.

According to this chart, 7.5" @ 3.75" is -12

I think, like many people, I want to buy aftermarket wheels, that match the factory offset. But these offset/backspace numbers seem like Greek to me!

If I am going to replace my 1990 Ford 15x8's, with aftermarket wheels and want them to fit like stock, what backspace/offset numbers should I be looking for?

 

The advertised wheel width is measured from the inside of the bead while the actual width is about 1" more, so a wheel that measures 7.5" outside would actually be considered a 6.5" wheel in the chart.
With these Ford trucks you don't want any more than stock backspacing or you start to get tire rub on the trailing arms and sway bar, so if you wanted to use an 8" aftermarket wheel you want it with -19 backspacing.

 

I'm pretty sure the mag rims have abit of offset, say for example what was originally on my '94 F150, it had those hurricane steel mags, and I replaced those with regular steelies with dog dish hubcaps, and the wheels are completely inside the wheel wells instead of sticking out slightly like they did with the hurricane wheels.

Same tire size on both rims.

 

So in other words, my stock 15x8's, have an inner size of 7.5", so that offset would be 3.75"
Then on the chart 8" at 3.75" = -19

And depending on how different manufactures list, I'm looking for 8" with 3.75" offset or -19mm

And to change to a 10" wheel, 4,5" offset, or -25