Urban's Friendly Tire Guide
I figure, since my best friend & his dad are in the tire business and a lot of their information has rubbed off on me, I'd create a nice thread to let everybody know exactly what you're getting yourself into when buying new tires.
The first thing you're obviously going to need to look at is the tire sizing. This isn't very difficult, but some people overlook bits & pieces, and some people just don't know what the numbers mean. So let's start with the stock tire size for a Redline; 215-45-ZR-17 (the numbers after that are load ratings, treadwear, traction, and temperature. We'll get to those in a bit.)
Here's the breakdown:
215 - The width of the tire, in millimeters.
45 - The "profile" of the tire. This measurement is actually a percentage of the width; i.e., with a 215-45-17, the sidewall will be as tall as 45% of 215, or 96.75 mm of rubber.
17 - The rim diameter that the tire will fit.
To determine how tall your stock wheel & tire package is, all you've got to do is take your width, multiply it by the width by the profile, which with the 215-45-17 we have been talking about is 96.75 mm. That means that on either side of the wheel there is going to be 48.4 mm of rubber, or 1.88". Then, you will take your wheel diameter (17"), and multiply it by 25.4, ending up with 431.8 cm. Add the two up, and you have your total rolling diameter, which ends up at 528.55 mm, or 20.6 inches.
The conversions used are as follows:
Inches to Millimeters: 25.4
Millimeters to Inches: .039
So, now, let's say that you want to step up in width to a 225-mm or even a 235-mm tire, and are trying to figure out if you will maintain about the same rolling diameter. We'll go with 3 tire sizes, a 225-45-17, a 225-40-17, and a 235-40-17. You come up with the following TOTAL diameters (sidewall+wheel diameter), and the percentage off of the stock size.
Width, Total Diameter, Sidewall Width, MM +/- from stock, +/- % size
225-45-17 - 538.05 mm, 101.25 mm, +4.5 mm, +1.79%
225-40-17 - 521.80 mm, 90 mm, -6.75 mm, -1.29%
235-40-17 - 525.80 mm, 94 mm, -2.75 mm, -0.50%
So as you can see, there are many variations, and you're honestly going to have a VERY hard time stepping up in diameter and getting the perfect match. Within 3% of the stock diameter is not going to create a problem. If you go up in size, your speedometer is going to that much over or under your actual speed. For example, if we were to go with the 225-45-17's, our speedo would read -1.79%, or at 100 mph, our speedo would tell us we're going 98 mph or so. So as you can see, it's not a massive difference.
Another thing to take into account before choosing a tire is aesthetically, how it's going to look. Adding 10 mm of rubber on the same size wheel is going to mean that the tire is not going to have to stretch as much as to fit on the wheel. Subtracting that 10 mm is goin to mean that you'll have to stretch the rubber moreso to fit over the wheel. Therefore, going up to a 225-45-17, it will not only look that extra 4.5 mm taller, but it will also be slightly more rounded at the edges.
What other things can you expect with increasing or decreasing tire size? There will be a difference in handling, which may be noticeable. When I put 205-50-17's on the front of my car, it definitely felt more "pillowy" in all aspects; acceleration, handling, and braking. A taller tire is going to change the gearing of your car, but only by that percentage of height you added or subtracted.
Something to consider if buying all-season or snow tires is, what kind of winters does your area have? If it is frequently very icy, you may want to step up in width, as a wider tire will distribute weight more easily over ice. If there is a lot of snow accumulation, going to a thinner tire may be the answer as you will get a "plow" effect with a wider tire, as instead of cutting through the snow, it just builds up underneath & around it.
Now for the other numbers on a tire. Aside from your dimensions, there are also traction, temperature, and treadwear ratings, as well as load & speed indexes. The descriptions for these are as follows, and should be heeded as a guide, not an absolute fact, as manufacturers may give their own tires different measurements; a 200-treadwear Nitto may wear much faster than a 200-treadwear Michelin.
Traction: This denotes the amount of traction you can expect out of this tire. An AA rated tire would deliver much more traction than a B rated tire.
Temperature: The temperature rating denotes whether these tires can function well at higher operating temperatures. For example, you wouldn't want to use a snow tire with a very low temperature rating in the summer, as it could disintigrate and cause a whole world of problems. A tire with an AA temperature rating will not fare well in the winter, as it is designed to operate at a higher temperature than it will ever manage to get to in the dead of winter.
Treadwear: The treadwear is a rough estimate of how rapidly the tires will wear. For example, if you bought a set of super-sticky gumball DOT-legal competition tires like a Michelin Pilot Sport Cup with a treadwear rating of 140, expect them to last about 5000 regular miles, or maybe 10 track days. On the other hand, a set of Cooper tires designed for economy with a rating of 340 could easily go for 50,000 gentle miles before replacement.
Load Index: This number tells you how much weight you can safely load on THAT TIRE without any risk. For example, a load index of 89 will support 1279 pounds. You obviously wouldn't want to use that tire on an Excursion, where there could easily have 3000+ pounds placed on it in a cornering maneuver.
You may also notice that some cars, mostly higher performance, have different load indexes for the front & back. For example, a new Z06 with Eagle F1 Supercars has a 98 load index on the front and a 101 on the back. This is because not only does the load change dramatically during acceleration, braking, and handling, and the weight bias is different front & rear, it also has to accomodate for downforce. A Ferrari Enzo that generates 1750 pounds of downforce also has to accomodate for that 1750 pounds of load in the tires.
Speed Index: This is a guideline for how fast you can safely sustain a speed on that tire before it risks disintigration. For example, a Z-rated tire you'd find on a performance car allows for travel at 189+ miles per hour, but an H-rated tire is only rated up to 130 miles per hour. If you never expect to travel any faster than 70 miles per hour, you really don't have to be concerned, but for the performance enthusiasts here, we should be aware of what our tires are capable of.
That's all I've got for right now. I may add more later regarding the tire's compound, tread pattern, and other information. I'm tired at the moment though.
If you're asking yourself why I wrote this, it's to help everybody here out. Your tires are the only thing that connect your vehicle to the road. Making the best, most informed decision can be the difference between life and death.
Happy tire buying.
Red Redline club member #002
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