Your overall diameter is your most important aspect. To find this is simple. Remember 25.4 mm in an inch and the "middle" number which is a representation of the sidewall height is proportional to the "first" number, which is the width of the tire in mm. So with your 165/55/14, 165 divided by 25.4 yields ~6.5. So this tire is ~6.5 inches wide. Now take this and multiple by the percentage of the sidewall, in this case 55. So 6.5 * .55 is ~3.57. There are (2) sidewalls when calculating an overall diameter, so 3.57 * 2 is 7.14 PLUS the diameter of the rim, which is 14. So 7.14 + 14 is 21.15 inch diameter.
By the way, some tire websites have overall tire diameters listed which saves you from doing this math. Hope this helped.