Hey I'm wondering if 17x9 +26 will fit on the back of my sw20

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I run Enkei RPF01 17x9 +22 fitted with a 235 40 17 on the rear of my sw20 with no issues.  I sits just inside the guard - almost flush.
You will have rub issues though if your guard isn't rolled & if you are running on a super low spring.
 

Carmikey
I run Enkei RPF01 17x9 +22 fitted with a 235 40 17 on the rear of my sw20 with no issues.  I sits just inside the guard - almost flush.
You will have rub issues though if your guard isn't rolled & if you are running on a super low spring.
 

I'll be running coilovers with them on. Will camber help them not rub. Not excessive amounts but enough to hold it behind the guard

And my guards are rolled

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Ok, u are running coil overs... that changes a lot from my experience, depending what size fronts you run, you often have to run a 5mm or so spacer to avoid inside of rim rubbing against the coilover. 
 
Coilovers
This is where the fun starts

where as with normal struts the spring seats are above the wheel/tyre, with coilovers the spring, although a lot thinner than normal, runs down past the wheel/tyre giving less room for the wheel/tyre.

Common setup's that work with coilovers without spacers or arch work (coilovers vary depending on model so something that works on one type may not work on another)

17x7 ET35 215/40/17 front
17x8 ET35 245/35/17 or 245/40/17 back

17x8 ET32-ET33 215/40/17 front
17x9 ET38 245/35/17 or 245/40/17 back

The other fun aspect is how low you run your mr2. Carmikey shows a modestly low mr2, the lower you go the more accurate you need to be with offset/width/size/tyre.
In saying that rims can be shaved a few mm, guards can be rolled, ride height can be raised on coilovers.
You didn't mention the fronts, and that's the usual challenge with mr2s in getting rims with the right offset and also importantly is the rim width staggered, so 9s on the rear, then 8s on the front (similarity offset as per examples listed). The wheel tracking is based on offset and width impacts your mr2s handling.