Making sense of tire sizes is not as easy as you might assume.
Unlike clothing, shoes, and popcorn buckets at the movie theater, tires don’t come in singular, self-explanatory sizes.
Rather, they’re presented as complex alpha-numeric sequences that most non-car people have no clue how to decipher without the aid of outside resources.
Even when a tire’s size is distilled down to one basic measurement, such as diameter, it’s unlikely that the listed size, or how big the tire is advertised as being, and the true size, or how big it proves to be when put to the ruler test, will be strictly the same.
If you were to wager that the average 35-inch tire measures exactly 35 inches from tread to tread, you would probably lose money.
What Is the Actual Size of a 35-Inch Tire?
35-inch tires aren’t actually a full 35 inches tall, at least not generally—in most cases, they’re about ½-⅘ of an inch short of a full 35 inches. Once you put a whole vehicle on top of them, their diameter decreases even more.
Far from being some throwaway number, the height of your tires has a quantifiable impact on the way your vehicle drives and the accuracy of the feedback it provides you, both computationally and in terms of feel.
Keep reading to learn why it pays to ascertain the ideal proportions of your vehicle’s tires before you start swapping them out haphazardly.
Tire Sizes: Cracking the Code
As I mentioned earlier, tire sizes are anything but straightforward.
To illustrate this point, here’s an example of a full, unabridged tire size code:
Believe it or not, there are at least five separate measurements crammed into that one line of text.
Unless you’re a bona fide gearhead, chances are it just looks like gibberish.
With a little guidance, however, you can begin forming an understanding of what each part of the sequence denotes.
Here’s how that perplexing series of numbers and letters breaks down:
|Wheel Diameter (in Inches)||17|
|Load Index/Speed Rating||115T|
It’s not important to know what every last one of these terms and figures means.
To grasp what we’re talking about in this article, it’s only important to be familiar with one of them.
To make things easier for prospective buyers, the vast majority of whom are simply trying to find out at a glance whether a given set of tires will fit their vehicle, some tire manufacturers and vendors pare down tire size codes to a single dimension.
The idea is that the specified dimension can serve as a kind of shorthand for the tire’s complete specs and therefore its compatibility with various vehicle models.
What is this all-important dimension, you ask? Diameter.
What Is Tire Diameter?
Diameter is the distance from the tread surface on one edge of the tire to the tread surface on the opposite edge.
It’s different from wheel diameter, which is the distance from one side of the wheel to the other.
Tire diameter is calculated by taking the height of a tire’s sidewall, doubling it (since you would pass through it two times if you were to draw an imaginary line across the tire), and adding it to the diameter of the wheel.
As you saw in the table above, diameter is always given in inches, even in the U.K. and other parts of the world that normally observe the metric system.
Anytime you see the word “inch”—or its abbreviation, “in.”—after a number in a tire’s product description, you know right away that the measurement in question is related to the diameter.
When mounted, a standard car tire has a total diameter, or height, of around 27 inches.
That means a 35-inch tire stands a whole 8 inches taller than the kind of tire that goes a typical passenger vehicle.
That’s one (or four, to be precise) big tire!
Just How Big Is a 35-Inch Tire?
Common sense dictates that a 35-inch tire would be an even 35 inches in height, but this isn’t ordinarily the case.
There’s almost always a small but significant margin of difference between a tire’s indicated diameter and its true total diameter.
As such, a brand new 35-inch tire might have a true diameter of only 34.5-34.8 inches; not right on the mark, but close enough to make the number useful for sizing purposes.
Even if you were to find a set with a height of exactly 35 inches, they wouldn’t stay that way for long.
As soon as you installed them on a vehicle, and added thousands of pounds of pressure in the process, they would naturally become compressed.
It’s not uncommon for automobile tires to lose as much as one inch of height due to compression.
A tire with a true total diameter of 34.5 inches might measure out to 33.5-33.7 once it’s saddled with a heavy steel chassis and all the other components that contribute to a vehicle’s weight.
This compression doesn’t hinder the tires’ performance—on the contrary, it makes it better.
Applying top-down pressure on a tire increases the amount of surface area that’s in contact with the road at any given time, thereby improving traction and overall stability.
Of course, increased size and enhanced performance and handling come at a premium.
Based on current market trends, 35-inch tires cost $20-50 more per unit than normal-sized tires.
Does That Little Bit of Variation Matter?
Not to most drivers, no. As long as your vehicle has a wheel diameter of at least 17 inches, there’s essentially no difference between a tire with a diameter of 35 inches and one with a diameter of 34.5 inches.
If you plan on lifting your vehicle or modifying its suspension in some other way, though, you’ll need to account for as many variables as possible, including proper spacing and clearance.
Being off by a fraction of an inch could mean the difference between a successful alteration and an expensive waste of time.
Furthermore, oversized tires can present unexpected complications of their own.
These days, computers are responsible for overseeing many vehicle functions, from monitoring engine performance to implementing life-saving safety measures.
They do this by keeping tabs on different parts of the vehicle and organizing the many individual bits of data they receive to provide helpful feedback to the driver in the form of instrument readings, real-time schematics, and sensor information.
Tire rotation rate is an especially useful piece of data, one on which all sorts of other crucial calculations are based.
For example, newer vehicles use tire rotation rate to gauge the following metrics (and more):
- Speedometer and odometer readings
- Torque measurements
- Gear settings and calibration
- Traction control systems
- Anti-lock brake activation
Driving on tires that are too large can produce a tire rotation rate that doesn’t match the one programmed into the vehicle’s central computer.
As a result, all subsequent calculations it makes using rotation rate as a starting point will also be incorrect, as will much of the feedback you rely on, consciously or unconsciously, to control your conveyance safely and efficiently.
Over time, the mechanical inefficiencies created by too-big tires can even lead to issues like rubbing, grinding, inhibited suspension, and misplaced multidirectional forces that could affect your alignment or cause other types of physical damage
Needless to say, there’s a lot more to choosing tires than just how good they look on your ride.