STEP 2: SIZE THE TIRE
When sizing a tire, there are a few important things to know:
1. TIRE WIDTHS ARE USELESS.
I’m referring to the first number in the tire size code such as the “275” in a 275/35-18 tire. This may sound crazy because other than the diameter of the wheel, this is usually the most important aspect of the tire for many people. But hear me out: Have you ever tried a different brand of tire just to realize it does not fit the same as the ones you took off, or not even fit under your fenders at all? The reason for this is the tolerances tire manufactures have to follow when making a given tire size is substantial. Brand X’s 275 can be as wide as Brand Y’s 305! To make matters worse, there is no industry standard to establish how to measure tire tread widths.
Often tire manufacturers will have multiple offerings or bespoke versions of the same size tire (as written on the sidewall). Due to the wide variance in tire construction, an auto manufacturer can specify not only the tread compound, design, and many aspects of the composition of the tire, but the width as well for the specific needs of a certain car. This further illustrates how the written number isn’t all that usefull.
Most people are too focused on the number on their sidewall rather than the true width of the tire, or what will give them the best performance. I get it. Size matters for bragging rights just like the numbering on the side of a muscle car designating the car’s cubic displacement, or the chrome emblems on the side of a “hooptie” designating their wheel diameter. But this article is about optimizing performance; not a “whose is bigger” bragging rights. Instead we need to focus on the tire’s TREAD WIDTH.
TireRack.com has come to the rescue by establishing a common measurement method to address this issue!!! They literally measure every make, model and size of tire with the same test procedure using a 20” long tool with a 30-degree bend in it to measure the tread width of the tire to take account various tire shapes and radiuses. This number, measured in inches, is what I view as the true width of the tread, not what is written on the side wall, since Tire Rack uses the same measurement procedures for all of the different brand tires they sell.
3. SECTION WIDTH:
Is the measurement of the tire’s width from the inner sidewall to outer sidewall with the tire mounted on its industry assigned rim width at proper inflation pressures. While this is typically more consistent and representative of the nominal width, it is not as important to me and is usually a bit wider than the tread width. A good rule of thumb to follow is that: for every 0.5” change in rim width, the tire’s section width will change by 0.2”.
4. MEASURING RIM WIDTH:
Is the industry standard rim width for which the tire must be mounted to meet its dimensional targets. AKA: the tire’s “Design rim width”. Without optimizing a tire’s performance, using this recommendation for rim width will be a quick and easy way to select your tire.
*Billy’s rule of thumb:
“For ideal handling: when sizing a tire for a given wheel, I usually target a tire’s TREAD width to be as wide as the WHEEL width, or 0.5” narrower than the wheel width.”
From my personal experience working on and driving countless track days, time attacks, street cars and race cars, this typically optimizes the tire’s carcass for response and outright grip. It also gives the sidewall a very slight ‘stretch’ or preload, which will improve the tire’s response and break-away characteristics. OEM’s have to size tires to meet a ton of different targets like curb to wheel damage protection, cost, snow chain clearance (almost all cars have to take this into consideration) all which affects the decision on the size of the tire and is usually a compromise at the expense of peak performance.
If a tire is too narrow for a given wheel width, the sidewall becomes overly preloaded and the carcass can distort to have uneven pressure across the surface of the tire. The preloaded sidewall also has greatly reduced flex and compliance, which would improve initial response, but it would ride poorly, be less forgiving over bumps, and the break-away characteristics would likely not be as predictable.
On the other hand, if a tire is too wide for a given wheel width, the sidewall ‘bulges”. This usually results in sloppy handling characteristics, vague steering feel, excessive tread squirm, and the carcass of the tire can also distort and have uneven pressure across the surface of the tire.