Narrowing a Ford Ranger axle for 20x9" wheels

How much should I narrow my axle to fit 20x9" wheels on the rear of my ranger? I do have a mazda bed which has a mild step side to it. I'm thinking 4" on each side should be more than enough.. Any thoughts on this?

you can do 2 inches by using a short side axle from another housing.

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bagged85 said:

you can do 2 inches by using a short side axle from another housing.

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That's what I was going to do originally but with the offset and the backspace of the wheels, I would need more than 2". +12 offset with a 5" backspace. A couple people told me to go 3" for 20s. I have to go back and measure again but I think 4" should be plenty in case I decide to run 22s in the rear