I spent a lot of time researching wheel fitment, and offset can be a confusing way to measure the clearance that a given wheel needs to fit.  For a complete explanation, go here: http://www.rsracing.com/tech-wheel.html

AFAIK all this stuff is 2g only, although it could easily be adapted to 1g’s. Disclaimer: every car is different, every wheel is different, and every tire is different.  If you buy a package based on what I say and it doesn’t fit, don’t hold me responsible, variances apply everywhere, and this is only to be used as a rough guide, not a rule book. 

Now that that's out of the way, what you care about is how much clearance there is from the hub mounting surface to the inside edge of the wheel.  This measurement is called backspacing, and will always tell you the same thing: how far your wheel sticks into the inner wheel well.  Fender clearance is less important, partially because you can roll the fender, but also because if you select the right backspacing, fender clearance will come naturally.  Some manufacturers actually provide wheel backspacing as a spec, you can substitute that for my backspacing equation for better accuracy. 

To calculate backspacing you use this formula:
(rim width ) / 2 + 0.5 + (offset /25.4) = backspacking

However, since we never install wheels without tires, I mocked up a formula to include the tire size. 
(offset /25.4) + 0.5 + ( Tire width in mm/25.4) / 2 = “Check Spacing” (yep I made up that term, We'll abbreviate that as CS from now on). 

The higher the CS, the farther inboard wheel/tire will sit, thus the more likely it will rub on the inside control arm.  The lower the CS, the more likely the wheel will rub on the outside fender.  It seems like a CS number of around  6.8 is the limit on a 2g dsm, and will have a little rubbing.  6.8 is a good target for maximum outer fender clearance, while not rubbing inside.  Some manufacturers provide actual tire widths that will help make this formula more accurate, since a 255mm tire is different things to different companies. 

This formula makes a few assumptions:
1)  Your overall tire _diameter_ is equal to or smaller than stock. 
2)  You haven’t changed your static camber significantly (camber changes due to lowering are ok); positive camber will mess up fender clearance.  Negative camber might mess up inner arm clearance. 
3)  Your tire is wider than your wheel

So plugging in a few wheel setups for examples:
My 1995 GSX: 17x9 +35 offset 255/40/17 kumho MX = 6.898" CS (Barely ok.. lots of fender rolling required)
  - This rubs noticeably, although not prohibitively, on the rear inside control arm: see picture at right ---------------->
My 1995 GSX: 17x9 +25 offset 255/40/17 kumho MX = 6.48" CS OK (sticks out)
My 1995 GSX: 17x7.5 + 44 offset 235/45/17 Kumho 712 = 6.85 CS barely OK
- This rubbed noticeably, although not prohibitively, on the rear inside link
Stock setup: 16x6 +45 offset 205 mm tire – 6.31 CS Plenty of room here
RX7 rims: 16x8 +50 offset 235mm tire - 7.09 CS definite rubbing (I’m guessing a 10mm spacer would fix it)
18x7.5 +45 offset 225mm tire: 6.7 CS – possible rubbing
18x7.5 +48 offset 225mm tire: 6.82 CS – possible rubbing
17x8 +40 offset 245/40/17 tire:  6.898 CS - no rubbing because tire is shorter than stock (24.7" as opposed to 25.3")
17x8 +44 offset 235/45/17 Bridgestone S03 tire: 6.86 CS - According to Dave M: Clearance on the inside of the rears is about 2mm, more in front

Dennis Grant has a 1997 talon with 9.5” +12 offset wheels with 275mm tires, we can compare my formula to his setup:
( 6.5 – (275/25.4) / 9.5) -.5 -4.875 ) * 25.4 = offset = 11.7 mm
Looks right!

When you are processing these numbers, note that late 2g's have been said to have a different suspension design that provides more clearance.  I haven't personally confirmed this, but make sure to send the year of your car with your datapoints.  The mitsubishi does list two different part numbers for early and late 2g's, FWD with the June 1996 build date (new body style?) should have the revised control arms, although the change seems to have happened on AWD cars a little earlier. 

9401.1 - 9605.3 Fwd-T: MB864921/2 w/abs (MB864923/4 or MR333865/6 w/o abs)
9606.1 - 9806.1 (present?): MR223651/2 w/abs (MR223653/4 w/o abs)

9401.1 - 9506.3 MB864931/2 w/abs   (MB864933/4 w/o ABS)
9507.1 - 9806.1 (present?): MR223657/8 (MR223655/6 w/o ABS) 

Here is a Check Spacing calculator, use it to see if your desired wheel tire combo will fit, or send me a datapoint.
What Wheel/Tire Combo's will fit my 2g dsm?

Check Spacing Calculator

Offset (mm)

Tire Width (mm)

Result = 

255/40/17 17x8 +38
275/40/17 17x9.5 +12
275/40/17 17x10 +12
255/40/17 17x9 +25 offset
rear inside link clearance - damn
255/40/17 17x9 +35 offset