- Car Safety & Safe Driving
Tire Sizes : What the numbers mean.
Tire Sizes: The Letters Before The Numbers
The numbers will vary and the exact wording of the question is often different, but the core knowledge being asked for is the same. What do the numbers in a tires size mean? A friend of mine recently placed the following post on her Facebook wall. "Need some automotive help here. Will P225/60R16 tires fit a car that right now is "wearing" LT215/70R16 tires?". The answers she got were as varied as could be.
First, you will notice that your tire size will start with either a P, LT, or possibly ST. This indicates what type of vehicle the tire was meant to go on. The "P" indicates passenger car. The "LT" would be for light duty trucks, and "ST" is for strictly trailers.
Next, lets discuss the "R". The "R" is for radial. Because of its advantages, the radial tire has now become the standard design for essentially all automotive tires. Radial-ply tires, more commonly called radial tires, is an automotive tire where the cord plies are arranged at 90 degrees to the direction of travel. There are two main advantages to radial tires. First, the radial cords in the sidewall of the tire allow it to act like a spring, giving flexibility and ride comfort. Secondly, the rigid steel belts reinforce the tread region, giving high mileage and performance. As a piece of tire history trivia, the radial tire was first patented in 1915 by Arthur Savage, but didn't become the standard until after Michelin developed their Michelin X radial tires in 1946.
Tire : Understanding The Numbers
Tire Sizes And Understanding The Numbers
The first number in a tire size, say 225, is the width of the tire in millimeters. This is pretty straight forward except that in the days of buying tires sized by the inch (remember 31/10.50x15), the first number was the height rather than the width. Back then it was inches, not millimeters.
The second number in the size of a metric sized tire, most often a 60 or 70, is the height of the sidewall expressed as a percentage of the width. This is called the "aspect ratio". People very often get this wrong, thinking it just the height of the sidewall, forgetting that it is a percentage. This small but simple concept is probably the hardest part of understanding tire sizes.
The third number is simply the diameter of the rim ( aka wheel ) that the tire was manufactured to fit on. This number is still given in American inches.
Ok, so lets review this briefly. A P225/60R16 tire is a radial tire that fits a 16 inch rim. It has a width of 225 mm, which equates to being slightly under 9 inches. The sidewall of this tire is 60% of 225 mm, which is 135 mm (approx 5.3 inches). Given this info, how tall is the tire? Many folks would add it quickly up in their heads and say that it is rough 25 inches. This is incorrect because when measuring the height of the tire there is the top and bottom sidewalls. This makes the correct answer for the height of a P225/60R16 tire is roughly 26.6 inches.
Now, we could repeat this process for the P215/70R16 tire size, or we could cheat. If we jump over to http://4lo.com click on the "metric tire size" link, it will do the math for us. We quickly see that the 215/70R16 should be 27.9 inches tall. This means the 215 tire is approximately an inch and a quarter taller than the 225 tire. Many people would never have guessed that because our brains just seem to assume that first number being the largest number by far must be the height, and it isn't.
Note that is your tire size is something like 30x10.50x15 or 34x9x16, you have American sized tires in good ole inches. In these two given examples, the tires would be 30 inches tall and 10 1/2 inches wide for a 15 inch rim or 34 inches tall by 9 inches wide for a 16 inch rim.
Tire Sizes And Some Varibles
So, lets go back to my friend's posted question. Will 225/60R16 tires fit a car that right now is "wearing" 215/70R16 tires? Well, if we now know that 225/60R16 tires are shorter than the current 215/70R16 tires, the answer would seem to be "YES". As always, the safer answer is "Maybe". This is because there are too many variables that we don't know. First, I would want to know the make, model, and year of the vehicle. This information isn't really needed as long as you know the distance of the clearance at the tightest point between the tire and the fender while the tire is turned completely to the left and right. While this spot is usually on the firewall closest to the center of the vehicle, it can also be between the tire and outer fender wall, depending on the styling of the fenders. The 225 tires are 10 mm wider than the current 215 tires.
The other reason to say maybe is that we don't know if the new tires are the same brand as the old tires. Different tire manufacturers seem to measure their tires slightly differently. It is fairly common to see tires that should be 32 inches be as much as an inch taller or shorter than that. If you are comparing between tire from the same manufacturer, this doesn't seem to be such an issue.
Thus far, in this article, we have also assumed we are dealing with brand new tires. Because of the depth of tread, worn tires can in some cases be dramatically smaller than their brand new counterparts.
There is one last thing I feel I should point out here. I have seen cases where two people will have basically identical vehicles, and purchase identical brand, model, and size of tires, yet one person will have problems with tires rubbing and the other won't. The most common cause of this is a difference in their rims or a difference in the offset of their rims to be more exact. Without spending too much time on this, the offset of the rim is sometimes called the depth of the rim. The offset of a rim is the distance from the hub mounting surface to the centerline of the wheel. The offset is measured in millimeters and can be one of three types.
First there is "zero offset". This is when the hub mounting surface is even with the centerline of the wheel. Next, we have positive offset. Positive offset is when the hub mounting surface is toward the front or outside of the rim side. Positive offset rims are generally found on front wheel drive vehicles and newer rear wheel drive vehicles. Negative offset is when the hub mounting surface is toward the back or brake side of the rim's centerline. "Deep dish" rims are typically a negative offset.