Classic Mustangs List Archive

This read-only message was archived from a public mail list.
Mail From: David Spangler (email redacted)


In case anyone's interested... here's what the metric tire numbers mean -
from another list I'm on... Sure seems like it was easier when it was
E7014...

---------- Forwarded message ----------
Date: Sat, 19 Apr 1997 11:03:49 -0400
From: (email redacted)
To: (email redacted)
Subject: Truck Tire Sizes

I chose the following numbers for illustration purposes only. Substitute
your numbers as applicable.

Example 1: P195/65XR15
P = Passenger vehicle, or car tire, with Metric sizing
195 = Section Width (overall width, not tread width, in millimeters)
65 = Aspect Ratio (height from bead to tread, percentage of width in
millimeters)
X = Speed Rating (lowest to highest: none, S, T, H, V, Z)
R = Tire Configuration (R for radial, B? for bias-ply)
15 = Wheel Diameter (in inches)

Example 2: LT265/75R15
LT = Light Truck tire
The remaining numbers are the same as Passenger vehicle tires, with the
exception of the speed rating which isn't applicable in truck tires.

Example 2 would be the "metric equivalent" of Example 3 below. This tire
would be approximately 10.50 inches wide. This is found by dividing the
width in millimeters by the number of millimeters per inch: 265/25.4=10.43.
The overall height of the tire would be approximately 31 inches. This
process is slightly more complex. First, multiply the section width by the
aspect ratio: 265*.75=198.75. Divide the product by the number of
millimeters per inch to find the height (from bead to tread) in inches:
198.75/25.4=7.82. Next, multiply this number by 2, because the tire has a
top and bottom, and add the product to the wheel diameter to find the
overall tire height: 7.82*2+15=30.65.

Example 3: 31X10.50R15
Flotation Truck Tire (no, they will not make your truck float)
31 = Height (overall tire diameter, in inches, after mounting)
10.50 = Section Width (overall tire width, in inches, after mounting)
R = Tire Configuration
15 = Wheel Diameter (in inches)

How will all of this affect your speedometer? By a ratio of the old tire
circumference to new tire circumference. Your tire is a 255/70R16, which is
equivalent to 30X10.00R16. This tire doesn't actually exist in a flotation
size. They do however, make a 30X9.50R15 and a 31X10.50R15. The wheel
diameter is irrelevant because a flotation tire states the overall diameter
(a 33X12.50R15 and a 33X12.50R16 are both 33 inches tall). Circumference of
a circle is calculated by the following equation: 2*3.14*radius. For your
tire this would be 94.2 inches. A 33X12.50R16 tire would have a
circumference of 103.62 inches. The resulting ratio would be: 1.1:1
(103.62/94.2). Now, if you did install the 33's, simply multiply your
speedometer reading by 1.1. At 55 m.p.h. you would actually be traveling
almost 61 m.p.h. This is just an example, I do not think 33's would fit.

Dave Love
Powell, Ohio