Radial Versus Bias-ply Tires and What Those Tire Numbers Really Mean
by Les Roth
Today’s radial tires are a wonder of modern engineering. In the late 1800’s bias-ply tires were used on bicycles. They were used on automobiles from the 1900’s to the late 1960’s. Then in 1969, radial-ply tires were introduced. The radial tire was a major improvement over the old bias–ply construction. They rolled easier and improved handling and gas mileage. The old bias-ply tires were more flexible and seemed to follow ruts in the roadway. Bias-ply tires have belts that criss-cross the tire while radial belts loop across the tire.
If you own a classic car with wheels designed for bias-ply tires, mount radial tires on those rims at your own risk. Those rims were designed for bias-ply tires only. Too many classic owners have found they lost wheel covers and even worse, had major blow-out and control issues with their car when they used radials on bias rims. Remember, radial tires are designed to be mounted only on wheels designed for radial tires.
Ever wonder what all those confusing numbers on the sidewall of your tires really mean? What you are looking at is an alphanumeric system that describes your tire and its performance characteristics. Let’s use tire size P225/70R16 100S as our example:
In the above tire size the first letter indicates the type of tire and its intended use. The “P” indicates the tire is a metric size used primarily on passenger vehicles. You can also find tires with other letter designations. They include LT (Light Truck Metric), C (Commercial), ST (Special Trailer Service) and T (Temporary Spare),
Then the numbers 225 following the first letter indicate the section width of the tire. The section width is defined as the widest point on the tire measured from sidewall-to-sidewall. For example, this tire has a 225-millimeter width. The rule here is the larger this number, the wider the tire. And remember, this number is the width of the tire in millimetres. If you are using tires larger than those specified for your car, be extra careful in measuring the available space. Raised lettering on a tire’s sidewall can make the difference between a scraped sidewall and a tight fit.
Following the slash, the number 70 shows the height of the tire as a percentage of its section width. The rule is...the lower the number, the lower the profile of the tire. Our number is 70 and that tells you the tire’s height is 70% of its section width.
In our example, the next letter is “R” which tells you the construction of the tire, which in this case is radial. Other designation types may include “D” for bias ply construction and “B” for belted tires, but most tires today are of radial design.
Next the number 16 that indicates the size of the wheel that the tire will fit. This number is in inches and in this case, this tire would be designed to fit a 16-inch wheel. Nowadays, you’ll find tire sizes starting at 13-inches going up to 18-inches. If you buy aftermarket tires wheel sizes can run up to 22-inches or larger.
Then comes the number 100 which tire manufacturers call load rating. This number indicates the approved load rating of that tire in the Load Index. The Load Index starts at 71 (761 pounds) and goes up to 110 which would be 2,337 pounds for passenger vehicles.
And finally comes the speed rating. Our tire speed rating is “S.” Speed ratings start at M (81-mph) and go up to Y (186-mph). In this case the “S” rating shows our tire would be rated for 112-mph.
