Looking for new wheels for your new civic? Don't know what size to get? There is a nice easy to use calculator online to help you size your wheel and tire correctly to make sure you're speedo isn't off or to at least let you know how far off it will be.
http://www.miata.net/garage/tirecalc.html
Stock sizing:
DX/LX
15 x 6.0 in. wheels
P195/65R15
EX
16 x 6.5 in. wheels
P205/55R16
SI
(2012 & 2013)
17 x 7.0 in. wheels
P215/45R17
(2014+)
18" wheels
P225/40R/18
Depending on what model you have punch in your stock tire and wheel size. For example with an EX you would put the 205/55/16 in the first set of boxes. On the new tire side put in the third number first, the wheel size you are going to. Say you wanted to go to a 17" wheel. Assuming the new wheel is also a 6.5" wide, you would just have to adjust the tire width and "ratio" which is the middle number until they match or are at least pretty close.
With this example a stock 205/55/16 and going to a 17" you would need a 205/50/17. It is off by 0.8%. At 60mph you would actually be going 60.5 mph. Pretty close. Again choose your wheel first and then decide how wide and play with the tire numbers that fit.
Say your new wheel was a 17x7 or 17x7.5. Pretty common sizes. Best fit tire size is going to be a 225/45/17. It's only off by 0.4% meaning when your speedo reads 60 you are going 60.2 mph. Nearly a perfect fit. Also the wider tire provides a better contact patch and better traction.
Offset is a whole 'nother issue of personal taste. Anything between +35 to +45 should fit fine with no rubbing in a reasonable size. *17's & 18's.
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