Wheel And Tire Sizes - Measuring Up

How To Select The Correct Wheel And Tire Size The First Time

Damon Lee Jun 1, 2000 0 Comment(s)
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Most enthusiasts know that wheels and tires can make or break a street machine. Whether you're putting big billet hoops and low-profile radials on a custom Impala or rally wheels and resto-style Wide Ovals on a restored Chevelle, the size and fit of the rolling stock has to be correct for the finished car to look, sit, and drive right.

While finding the right tire and wheel size for a stock-style restoration or mild street machine is typically pretty easy, squeezing large-diameter, extra-wide rims and rubber into the confines of a stock Chevy wheel opening can be a challenge. Heck, even putting moderately sized rolling stock on a lowered Bow-Tie can be tricky. And unfortunately, too many enthusiasts rely on guesstimates and second-hand statistics when ordering wheels and tires for their cars, only to find that they don't fit right when mounted in place.

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We at Super Chevy know firsthand what a hassle it can be to end up with improperly sized wheels or tires for that prized project car. So we've put together this guide to help you get the right dimensions the first time. The following photos, diagrams, definitions, and suggestions should arm you with the knowledge you need to make an informed purchase. But keep in mind that we can't make the final selection for you. It's up to you to take measurements, ask questions, and make the final determination on which wheels and tires are right for your ride.

Wheel Words
When preparing to purchase a new set of custom wheels, it's extremely important to take accurate measurements to determine which size you need. The following diagram and definitions show the dimensions that wheel manufacturers need to know. Of these, bolt pattern, rim width, and rim diameter are the most obvious and important. In addition, backspacing (or rear spacing) is a vital dimension to know.

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Bolt Pattern: The pattern determined by the number of bolt holes (or lug holes) and the diameter of the bolt circle.

Hub Diameter: Also called the center bore, this is the measurement of the hole at the center of the wheel.

Offset: The distance from the centerline of the wheel to the mounting surface of the wheel. Measured in millimeters.

Negative Offset: When the back of the mounting pad is closer to the inside of the wheel; when the mounting surface is inboard of the rim centerline.

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Positive Offset: When the back of the mounting pad is closer to the street side of the wheel; when the mounting surface is outboard of the rim center line.

Rear Spacing: Also called backspacing. The distance from the backside of the wheel mounting pad to the outside of the rim flange.

Tire Terms
It's also important to consider the dimensions of your tires when selecting a wheel-and-tire package. To do this, you'll want to know some basic specifications on the tires. The following measurements should help you out, with the most important considerations being aspect ratio, section width, and overall diameter.

Aspect Ratio: The ratio of the tire's section height to its section width. Typically calculated by dividing the section height by the section width and multiplying that number by 100.

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So where do you begin when measuring for new wheels and tires? A logical starting point is with the existing (presumably stock-dimension) wheels and tires. A straightedge and tape measure can help you determine backspacing.

Deflection: The measured difference between the tire's free radius and loaded radius when mounted on the measuring rim, inflated to the test pressure and placed under a prescribed load.

Free Radius: The distance from the wheel axle centerline to the outer tread surface of the unloaded, properly inflated tire.

Loaded Radius: The distance from the wheel axle centerline to the tread contact surface. Measured after the tire has been mounted on its measuring rim, inflated to the test pressure, and placed under a prescribed load.

Overall Diameter: The linear distance between the tire's tread surfaces measured at the widest point. This measurement is taken with the tire mounted on the measuring rim and no load applied.

Overall Width: The exterior measurement of a tire's width from inner sidewall to outer sidewall (including protective ribs, lettering, etc.) when properly mounted and inflated.

Section Height: The measurement of the vertical distance between the tire's bead seat and outer tread surface when properly mounted and inflated, but with no load placed on the tire.

Section Width: The measurement of a tire's width from sidewall to sidewall (excluding protective ribs or decorations) when properly mounted and inflated, but with no load placed upon the tire.

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Then compare your figures to the room available in your wheelwell. Estimate how tall your wheel and tire package will be, then measure the distance to the fender lip and any inner side obstructions (frame, suspension components, etc.). Take these measurements with the suspension compressed to approximate ride height. Make sure to take measurements at several different points on the wheel radius, and try it with the steering turned to full lock in both directions.

Tread Width: The distance from the outer edge to the inner edge of the tread.

The Metric System
We often take for grated that everyone knows how to "read" the sizing information on radial tire sidewalls. That's not necessarily the case. Here's a refresher on P-Metric sizing.

Example: P215/70R14
P = Passenger car tire
215 = Section width in millimeters
70 = Aspect ratio
R = Radial construction
14 = Rim diameter

Add It Up
"Plus-sizing" is a common concept with most enthusiasts, but is still worthy of discussion. This practice allows you to achieve a higher level of performance and handling by mounting tires with wider section widths and lower aspect ratios to rims that are larger in diameter than stock wheels. The goal is to maintain the same approximate overall tire diameter while gaining a wider tire footprint and stiffer sidewall. Just keep in mind that, from a visual standpoint, a lot of '50s and '60s cars need a relatively tall sidewall to make them look right.

*When increasing section width, rim width may have to be widened to accept a wider tire.

**Adjust according to your needs. Tires with too low an aspect ratio may look out of place on vintage cars. Try finding tires that "fill the wheelwell" of your particular car.

Rules Of Thumb
Plus 1:
* Increase section width by 10mm*
* Decrease aspect ratio by 10 points**
* Increase rim diameter by 1 inch

Plus 2:
* Increase section width by 20mm*
* Decrease aspect ratio by 20 points**
* Increase rim diameters by 2 inches

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Repeat the process on the rear of the car. Again, remember to compress the suspension to approximate ride height, account for suspension travel, and watch for obstacles on the inside.

Size Does Matter
* When measuring for wheel sizes, remember to account for the tire's height and width. Tire dimensions like overall height and section width are usually available from the tire manufacturer.

* Just because a particular wheel-and-tire combination fits your buddy's Chevelle street machine does not mean it will automatically fit your Chevelle, too. Never assume that it will. Instead, take measurements for yourself or trial-fit the wheels and tires on your car.

* In addition to checking clearance between the rolling stock and outer sheetmetal, check the inner side of the wheel and tire. Make sure they aren't hitting suspension components, the inner wheel housing, or the frame, even when the steering is turned to full lock.

* Remember that tires will flex and expand during cornering and other normal driving conditions. Don't cut clearances so close that the tires will begin rubbing the first time you roll over a pebble in the road.

* Disc brake conversion kits, dropped spindles, tubular control arms, and other aftermarket suspension components can alter the track width of your front suspension, which will affect tire and wheel clearance. Keep this in mind when planning modifications or selecting wheels and tires.

* On a similar note, if you're planning to lower your car or perform other suspension modifications, we suggest making those changes before ordering new wheels and tires.

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This handy tool from Percy's High Performance can take a lot of the guesswork out of measuring for wheels and tires. It lets you select a diameter and width (with varying backspacing), then bolt the fixture to your brakes and see if it's going to hit anything.

* If you're using extra-large aftermarket brake rotors and performance calipers, make sure they will work with the wheel size and design you choose.

* Always take wheel and tire measurements with the suspension compressed to approximate ride height.

* When in doubt, it's best to err on the side of caution. You can always use a set of wheels and tires that are a little narrower than what you wanted, but it can be difficult or impossible to drive a car with rolling stock that's too wide.

* Trial-fit any new set of wheels you get before having the tires mounted. Wheel manufacturers and retailers will often let you return or exchange wheels that haven't been mounted yet.

* When using wheels with different offsets, diameters, or widths for front and rear, you may encounter difficulty should a flat occur. It's wise to choose a spare that will fit both the front and rear.

* Big openings in large-diameter wheels will leave puny (or dirty) brakes exposed. One way to conceal such unsightly items is to use wheel inserts like those available from Air Ride Technologies.

* Unique (and especially large) tires may prove hard to locate- especially in smaller towns or while on the road. Take this into consideration before deciding on those 20-inch rims and 395/25ZR20 tires.

Sources

Yokohama Tire Corporation
Fullerton, CA 92831
800-722-9888
http://www.yokohamatire.com
Percy's High Performance
Camdento, MO
573-346-4409
http://www.percyshp.com
Weld Wheel Industries
Kansas City, MO 64101
800-788-9353
www.weldracing.com
Michelin North America
Greenville, SC 29602
866-866-6605
http://www.michelinman.com
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