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Personally, I think 18" wheels are a mistake. The ride will be terrible due to the high pressures you'll need to run, the wheels will bend easily, and the handling will not be improved enough to be worth it. Also, spacers are not highly recommended for performance applications (but some guys do use them). My opinion is that it's strictly an appearance change with all sorts of downside. Although I have nothing against significant auto-bling, I think 18" wheels go too far into poser territory. As they say, my humble opinion. You're entitled to yours!
But, if you're convinved you want to do it: The first question is what the new wheel width is. The Viggen has a 42mm negative offset and a 7.5" wheel. If you go to 55mm, you will be 13mm (1/2") more inbound with the wheel. So, strictly mathematically, you need spacers that thick. But, if you have wider wheels, or wider tires (actual tire width varies even for a stated width) you might need to recalculate based on the change in width and existing inner vs. outer clearance (where you can put the extra width). You will also need 1/2" longer wheels bolts.
The next question is "what tires". The Viggen with the 42mm has a 215-45-17. It looks like 215-40-18 is the best match on diameter in an 18" tire. That will give you the same rough OD and width as the stock 17" option and you can minimize any clearance issues.
If you get wider tires (or wheels) you will have to make up a template or get under the car and check clearances while it's on the ground. Remember that the car body shifts relative to the wheels in turns. Also, tires distort in hard maneuvers. That part of the estimating involves some guesswork. Be sure to check the actual specs on the tire width of the tires you are buying - Tirerack has detailed specs per actual tire size. Also, one known rub point on the 9~3 is on the plastic gas filler tube at the right rear tire. I've seen a little of that with just minor changes in tires and wheels.
Here are two links that might help you with the calculations for tires and wheels if you are not familiar with the manual calcs:
http://www.1010tires.com/WheelOffsetCalculator.asp (offset)
http://www.miata.net/garage/tirecalc.html (tire size calcs)
posted by 141.154.250...
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