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Understanding Tire Size

When you look at the side of any tire, you will see a series of numbers that are used to determine the tire size, such as "195/60R15" or "205/40ZR17." Using these numbers to determine a tire’s overall diameter is easy if you understand what the numbers mean and how they relate mathematically to the overall size of the tire. The best aprt is that this will allow you to choose a correct tire size by yourself!

Tire width:
The first number in the series is the tire width in millimeters. So a "195/60R15" tire is simply 195mm wide.

Aspect ratio: The second number is the height of the tire sidewall expressed as a percentage of the width. So a "195/60R15" tire uses a sidewall height that is 60% of 195mm, or 0.60 x 195 = 117mm.

Rim diameter: The third number is the rim diameter in inches. So a "195/60R15" tire has an inside diameter of 15 inches and will fit on a 15 inch wheel.

Calculating overall diameter (OD):
Looking at the equations above makes it easy to understand how to find the overall diameter (OD) of any tire. Adding from top to bottom (or bottom to top,) you add: sidewall height + rim diameter + sidewall height. The sum of these numbers gives you the overall diameter or total height of the tire from top to bottom. The only trick is converting millimeters to inches.

To make things really simple, just use this formula:

[(tire width x aspect ratio) x 2]/25.4 + wheel diameter = overall tire diameter in inches

or in the 195/60R15 example…

[(195 x 0.60) x 2]/25.4 +15 = 24.21 inches


One final note, not all tire manufacturers rate their tire sizes identically. It is likely that two "195mm" tires from two different manufacturers may have different widths – even though they are both labeled as "195." Be prepared to adjust for slight deviations if you are comparing tires from different manufacturers.

Fitting Tires to a Saturn


Following a few simple guidelines will make it easy to get tires that correctly fit your Saturn. For starters, use the table below to determine the tire size – hence overall diameter – that was originally used on your vehicle.

YearModels
Original Tire Size

Original Overall Diameter

91-currentSL, SL1, SW1, SC1
175/70/14
23.65 in.
91-95
SL2, SW2
195/60/15
24.21 in.
93-95
SC1 optional tire
195/60/15
24.21 in.
96-current
SL2, SW2
185/65/15
24.47 in.
96-current
SC1 optional tire
185/65/15
24.47 in.
91-current
SC, SC2

195/60/15
24.21 in.
  1. If you are upgrading to a larger tire, it is highly unlikely that you will be able to find a tire that will perfectly match the diameter of the OEM tire. This is not necessarily cause for concern. However, the tire that you select should have an outside diameter within 1 inch of the original tire. (More deviation than 1 inch will cause significant speedometer errors and/or potential fit problems.)
  2. It is also generally best to deviate toward the small side of the original diameter rather than the large side. This offers two clear advantages: a) It helps avoid fit and clearance concerns. b) It creates a smaller "effective radius" which slightly improves acceleration, holding all else constant.
  3. A wider tire offers greater traction thanks to a larger contact patch. However, it is generally wise to limit the maximum tire width for street use to 205mm. Most tires wider than 205mm are likely to rub against struts and fender linings unless further steps are taken (as on our ITA race car). At the same time, using a tire width smaller than 205mm fails to take advantage of potential performance gains.

Putting It all Together

As you read through the rest of the tech information on our site you may find that there is a common thread – whenever you are trying to improve the dynamic performance of your ride, the first place you should look is at the four palm-sized patches of rubber which connect your car to the road. Until you enable this interface to generate higher forces, your performance gains may not be a great as you would hope or expect.