UNDERSTANDINGTIRE SIZES
by Victor A. Seely

INTRODUCTION

     Many V6 owners want to upgrade the wheels and tires on their Mustang.  It has been obvious by the questions asked on the message board that there is a misconception about tire sizes when upgrading to "BIGGER" tires.

STOCK TIRE AND WHEEL SIZE
 

     The stock tire size on the 94-00 V6 Mustang is 205/65/R15 mounted on a 15" X 7" rim.   But what does this mean?

RIM HEIGHT (15")

     The diameter of the rim, which is also the diameter of the hole opening of the tire.  You MUST match tire inter hole diameter to rim height.   Which means you cannot put a 16" tire on a 17" rim.

RIM WIDTH (7")

     The width between the flanges of the wheel. (the part the bead of the tire touches).

SECTION WIDTH (205 millimeters)

     The linear distance between the outside of the sidewalls of an inflated tire without any load on it (this does not include any sidewall raised lettering).

SECTION HEIGHT (65% of Section Width)

     This is also known as the Aspect Ratio.  This is figured on a percentage of the section width.  A tire size of 205/65R15 would have a section width of 205 millimeters and a section height of 65 percent of 205 millimeters or 133.25mm.

R:  The "R" means Radial.

UPGRADING TO "BIGGER" TIRES

       It is important to maintain approximately the SAME overall stock tire height when upgrading to 16" or 17" or even 18" rims and tires.  In reality, you are not putting on "BIGGER" tires.  You are putting on wider tires with a lower profile.  This is known as Plus Sizing.

PLUS SIZING

     Plus sizing is one of the easiest ways to achieve enhanced performance and improve the appearance of your vehicle.  In the example above, a 15X7 wheel with a 205/65/R15 tire is considered the O.E. (original equipment) size.  Converting to a plus 1 size would mean increasing the wheel diameter by 1" (16X7 (or 7.5 or 8)) and selecting an appropriate tire to fit (225/55/R16).  Likewise, moving to a plus 2 fitment would result in a 17X8 (or 9) wheel and a 245/45/R17 tire.  It is important to note that although the wheel diameter is increasing, the overall diameter, or height, of the tire remains consistent.  As a matter of fact, this method typically maintains the overall diameter of the tire within +/- .25 inches.  This is crucial to preserving the speedometer reading and gear ratio.

     A couple of things happen with the tire to increase performance when we begin to move into plus sizes. First, the tire is wider (section width) and therefore gives a larger footprint.  Second, the sidewall becomes shorter (aspect ratio) and more rigid.  The two added together give better lateral stability and increased steering response.

DETERMINING TIRE HEIGHT

     Tire Height = Rim Height + 2 (Section Height).  You will notice that tire sizes are measured in TWO units, millimeters and inches.  To keep things simple, it is best to convert to ONE unit.  I will use inches.  To convert millimeters to inches divide by 25.4 (Example: 205mm/25.4=8.07" or 8").  So, the stock tire is 8.07" wide.  To determine the section height, take the Aspect Ratio times the tire width.  On the stock tire, the Aspect Ratio is 65 or 65 percent of the width.  Therefore, the section height on the stock tire is 8.07" X .65 = 5.2455" or rounded off, 5.25 inches.  Remember, there are two section heights when determining the overall height of the tire.  The rim size is 15 inches.  Plugging these figures into the tire size formula, we have:

Stock Tire height = 15" + 2 (5.25") which equals 25.5 inches.

EXAMPLES OF SOME TIRE SIZES THAT YOU CAN UPGRADE TO
 

Tire Size Width Height
205 65 15 8.07 25.49
225 60 15 8.86 25.63
225 55 16 8.86 25.74
245 50 16 9.65 25.65
245 45 17 9.65 25.68
255 40 17 10.04 25.72
275 40 17 10.83 25.66
275 35 18 10.83 25.58

    You will notice that the overall heights of the examples are within a quarter inch of each other even though the rim size varies from 15 to 18 inches.  The table above is not all inclusive.  There are other sizes that can be utilized.  These are the more popular sizes when upgrading.

HOW "BIG" CAN I GO?

     I personally think the "ideal" size to upgrade to is 245/45/R17 on the V6 Mustang.  However, you can easily go "BIGGER."   Some people have upgraded to 255/40/R17 in front and 275/40/R17 in the rear without any problems of rubbing on the fender well.  Remember, that with two different sizes on a car, the tires CANNOT be rotated.  Also, remember that "BIGGER" also means more $$$$ for the rims and tires.  A 18' or 20" tire can easily cost in excess of $300 each, plus the cost of the rim.