Wheels And Tires

[ Wheel Rims | Tire Sizes | Overall Diameter | The Plus Rule | Pinion Gears ]


On this page I will attempt to uncover the mysteries behind wheel and tires sizes to help you decide what you want to do with yours.  Most of this information will apply to any vehicle.

Wheel Rim Sizes

Wheel rims are measured by a few dimensions.  The most important are the diameter, width, and offset.  Most low-end Chrysler FWD vehicles came with 14.0 inch diameter rims with a 5.5 inch width.  Some low-end models got 13.0 inch by 5.0 inch rims.  Performance versions of the vehicles often came with 15.0 inch rims with a 6.0 inch or 6.5 inch width.  All Chrysler FWD vehicles with 6.0 and 6.5 inch wide rims have a positive wheel offset of 40mm.  The offset is the distance from the hub mounting surface to the center of the rim and a positive offset means the mounting surface is closer to the outer rim.  The proper offset ensures that the wheel is properly centered across the bearing and the designed axis-point of the front suspension.

Tire Sizes

Tire sizing tends to be a bit confusing to most.  That is probably because the numbers aren't really accurate.  A P205/50 VR15 tire size is translated as follows:

The "P" designates it as a passenger car tire.  Many tires assuming this and do not show the letter.  A "T" here would mean it is a temporary-use tire (your spare tire).  This tire has a 205mm nominal sidewall-to-sidewall width, with a profile (or aspect ratio) of 50%, which is the height of the sidewall measured as a percentage of the tread width (102.5mm in this case).  The "nominal" width means that this is an estimated number.  The actual width will vary somewhat.  The "V" is the speed rating of the tire, which is the maximum constant speed that the tire can handle.  By exceeding this speed, you are at risk for tire failure due to heat build up and centripetal force.  Below is a chart describing the speed ratings and their meanings.

Rating Maximum Speed
Q 99 mph (160 km/hr)
S 112 mph (180 km/hr)
T 118 mph (190 km/hr)
U 124 mph (200 km/hr)
H 130 mph (210 km/hr)
V 149 mph (240 km/hr)
W 168 mph (270 km/hr)
Y 186 mph (300 km/hr)
Z 149mph - 186 mph
(240 km/hr - 300 km/hr)

The "Z" rating is sort of a large category describing a high speed tire.  The actual rating would be "V", "W", or "Y".  The "R" in the above example indicates that this is a radial ply tire, which means that the cords are applied around the diameter of the tire instead of perpendicular to it (bias-ply).  Radial tires will not distort at higher speeds and all modern street tires are radials.  Your space-saver spare tire is probably a bias-ply tire, which would have a "D" there.

It's important to match your tire size to your rim size.  The nominal tread width is not really an exact number and will be wider than the rim width.  For instance, a 6.0 inch wide rim will take a 205mm (about 8.1 inch) wide tire and a 6.5 inch rim width will take a 225mm (about 8.9 inch) wide tire.

Calculating Overall Wheel Diameter

You can take your wheel rim and tire sizes and calculate your overall wheel diameter.  So using the previous example, a P205/50 VR15 has a wheel diameter of about 23.1 inches.  Here is how I calculated this:

Take the nominal sidewall-to-sidewall width and multiply it by the profile to get the sidewall height (205mm x 50% = 102.5mm or about 4 inches).  Add the sidewall height to the wheel rim diameter twice (15in + 4in + 4in = 23in).  I rounded the sidewall height, which is where the 0.1 inches went, but there you have it.  Using this method, I calculated most of the typical sizes used on our cars.  The wheel rim size is shown in the tires size, but I have left off the irrelevant speed ratings.  They are sorted by increasing wheel diameter, then by increasing nominal width, then by increasing profile.

Tire Size Overall Diameter
P165/80 R13 23.4in (594mm)
P175/80 R13 24.0in (610mm)
P185/70 R14 24.2in (615mm)
P185/75 R14 25.0in (633mm)
P195/70 R14 24.7in (629mm)
P205/50 R15 23.1in (586mm)
P205/60 R15 24.7in (627mm)
P205/70 R15 26.3in (668mm)
P225/50 R15 23.9in (606mm)
P225/65 R15 25.6in (651mm)
P205/55 R16 24.9in (632mm)

The Plus Rule

To change the size of your rims without effecting the overall diameter of your wheel, your have to follow the "plus rule".  The reason for going with a larger wheel rim is to reduce the profile (sidewall height) of the tire, which results in better handling.  This is because a shorter sidewall will not flex as much while cornering.  A flexing sidewall distorts the thread that is in contact with the ground, reducing traction.  You generally want to try to maintain the same wheel diameter so that the wheel still fits in the wheel well, and to maintain the accuracy of your speedometer.  If you can fit a larger overall wheel diameter, or want to go with a smaller one, you can change your speedometer pinion gear to maintain accuracy.  See the Speedometer Pinion Gear Reference page for details on which gear to choose.

You start out by deciding what size rims you would like to use.  Then you calculate the sidewall height that you need to maintain the same overall wheel diameter.  As an example, I will show how to calculate the right tire size for a set of 15x6.0 inch rims to match a typical stock 14x5.5 inch wheel diameter (P185/70 R14).  Typically, you would go with a 205mm nominal width tire for this.  The reason is that a wider tire will very likely rub the fenders when you turn and may even rub the strut or shock absorber.  This size tire seems to fit almost all cars quite well, without a need to change the steering column stops.  It is easier to calculate this with trial and error, but I will show you how to directly calculate the right tire profile, also.

For the trial and error method, you can just view the chart in the previous section.  You can see that a P185/70 R14 has an overall wheel diameter of 24.2 inches.  Since tires are only available in fixed sizes, you will probably have to choose one of the 15 inch sizes from that list.  The P205/60 R15 with an overall diameter of 24.7 inches appears to be a close match, with an error of about 2%.  This error is small enough so that a pinion gear change is not necessary (yellow 19 tooth, in this case).  If you wanted to try a 16x6.0 inch rim, you can see that the P205/55 R16 is also a close choice (24.9 inches or about 3% error).

To calculate the ideal profile for your conversion, use this method.  Take your original overall wheel diameter and subtract your new wheel rim diameter to get the overall sidewall height (24.2in - 15in = 9.2in).  Now divide the overall sidewall height by 2 to get the tire sidewall height (9.2in / 2 = 4.6in).  To calculate the profile from this, you need to convert the sidewall height to millimeters using the rule that 1 inch equals 25.4 mm.  So, our sidewall height in millimeters is the height in inches multiplied by 25.4 (4.6in x 25.4 = 117mm).  Now you need to divide the sidewall height by the nominal width of the desired tire, 205mm in this case, to get the aspect ratio of the tire (117mm / 205mm = 0.57).  Multiply the aspect ratio by 100 to get the profile (0.57 x 100 = 57).  So, you need a P205/57 R15 tire to keep the exact same wheel diameter.  Well, they don't make tires in those sizes.  :)  So, you need to find the closest one.  Again, if you look at the chart, you'll see that the P205/60 R15 is the closest match.

You can experiment to find the size that works best for you.  Just remember that you have to keep wheel clearance in mind.  make sure that there is enough room in the wheel wells for the tires you want.  Otherwise, you will be greeted with annoying sounds and short-lived tires, possibly even blow-outs.

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Updated 11/12/2003.

Copyright © 1996-2003 Russ W. Knize.