Tech: How To Measure For Installing Larger Wheels And Tires

 

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Wheels and tires can make or break the look of a car. You can have a clapped-out, rusty Camaro with a nice set of wheels, and it will typically look pretty cool. On the opposite end of the spectrum, a bad wheel choice can make a fully-restored and painted showpiece look less than spectacular.

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Keep in mind, when measuring, the car needs to be at ride height. We have the car on a lift to make it easier for you to see where we take our measurements.

When choosing wheels for your ride, there are two major factors to consider when making your decision — looks and fitment. Since the right “look” is a personal preference, we can’t tell you what wheel you might like. As for fitment, that’s where we can help to make sure you get it right. Rims and tires, even when choosing a less expensive kind, can still be costly, so buying a set that does not fit will definitely put a damper on your day. A disconcerting feeling is an understatement when you order new wheels and tires, only to find out they rub or simply do not fit.

The first measurement we took was to verify our overall width. This will let us know how wide our tire's section width can be.

The first measurement we took was to verify our overall width. This will let us know how wide our tire section width can be. If you decide to trim the inner fender lip to increase tire clearance, that modification can be worth up to 3/4-inch. If you are not interested in cutting the wheel lip on your car, you can roll the lip under.

If your car is close to stock, then you can probably find a lot of opinions about what will fit a certain application. But, what if you have modified things like spring location or widened wheel tubs, then what? What if you want to stuff the largest wheel and tire package possible under your car? Do you know what measurements you need to take? If not, you had better educate yourself on how to measure the car to find the right specs for your new rollers. That is exactly what we plan to do here.

What Brought This On & What You’ll Need

We were hanging out with Aaron Linquist at Ratchet Garage in Lakeland, Florida, when the discussion turned to measuring for oversized wheels and tires. As luck would have it, there was a 1970 Nova on the rack that we could use to show you guys exactly how to measure for upsizing your wheel and tire package on the rear of your ride. What’s more, we are going to show you how to find all these measurements with nothing more than a straight-edge and a tape measure.

You don’t need fancy tools for this, but you do need to have some knowledge of what, and where to measure. According to Aaron, “people ask me all the time, ‘what is the biggest tire that will fit under my car?’ I first tell them if you have a straight edge and a tape measure you can find out. Then I have them bring their car to the shop so I can show them.”

The next measurement we took was to find our total available backspacing. This measurement will not be the amount of backspacing your wheel needs to have, but rather the total room you have to work with.

Stuffing fatter tires with short, stiff sidewalls under the rearend is a little easier than, let’s say, a racing slick. One big reason is that the sidewalls on these lower profile, larger diameter tires tend to stay put — unlike tall, two-ply wrinkle-wall drag tires that flex, stretch, and move around a lot.

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This measurement gives you the total available room from the rim mounting location to the leaf spring. Typically, the tire will rub the inner fender before it hits the spring.

It is always a good idea to measure both sides of the car when fitting tires, because the two sides will typically not possess identical measurements. This is more prevalent with older musclecars. It’s been our experience that the rear axle tends to be offset more to the passenger’s side.

On the inboard side of the wheel opening, the wheel tub is usually the first point of contact, but tire-clearance limitations can also occur at the leaf spring, so we also measure for that distance. There are several aftermarket companies that offer offset-shackle kits for specific cars. For early Camaros and ’68 through ’74 Novas, Detroit Speed offers an offset-shackle kit (PN 040901) that moves the springs inward, so you can gain additional clearance.

…If you have a straight edge and a tape measure you can find out. – Aaron Lindquist, Ratchet Garage

When measuring to find your available area, there are several variables that come into play that will affect tire clearance. The most important one is ride height. Why measure the car for tire clearance, purchase the tires and wheels, and then decide to lower the car? If you are even contemplating lowering the car, do it before you purchase new tires and wheels. Let’s say for example, that you have an early Camaro that is at factory ride height. With this stance, you might be able to accommodate an upsized 255/50R16 tire on a 16×8 wheel. After you install those new wheels and tires, lower the car by 1-inch. Now, the available area you had is decreased, and your combination will likely no longer fit your car.

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The tire on the left is a bias-ply, DOT-approved racing tire, and is very flexible. This type of tire will require more clearance than the radial on the right, as it will flex and move more than the radial.

When measuring rear tire clearance, you must also keep in mind that unless you have a good set of sway bars, or a performance-handling suspension, your car will experience body roll when cornering. If you minimize body roll, this will allow you to run a larger rear tire with less clearance. This is because most rear axles will articulate when they encounter an uneven surface, or when cornering.

body roll

Okay, this is an exaggerated experience, but body roll can and will occur, and needs to be taken into account.

This can occur when you enter a steep driveway at an angle, or a sharp turn at a fairly high speed. The body leaning will cause one tire to dive into the rear wheelwell more than the other.

When it comes to actually measuring for wheel and tire clearance, there are three major factors that come into play:

Rim Diameter

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Rim width is not measured at the outermost edges of the rim, but rather, by the face area where the tire seats into the rim.

The first thing we need to decide is what diameter wheel and tire you would like to use. This is the measurement of the diameter of the wheel from where the tire bead sits if you are viewing the wheel from the front, back or top. Rim diameters are generally stated in inches (15, 16, 17, 18, etc.). Typically, by choosing a wheel with a larger rim diameter, the sidewall of the tire gets smaller.

Rim Width

Rim width is the measurement taken from bead face to bead face, not across the entire lip-to-lip surface of the wheel. In general, the rim lip (flange) will be approximately a 1/2- to 3/4-inch wider than the rim width, so a rim width of 8 inches makes the overall width of the wheel from lip-to-lip 8 1/2 to 9 inches.

Backspacing

You’re reading this article because you are interested in mounting larger tires under your car, which also means larger wheels. Most guys understand wheel diameter and width, let’s face it, these two dimensions are easy to measure. But when it comes to stuffing bigger tires under a classic, it’s all about the backspacing. Backspacing is the measurement between the wheel-to-axle mounting flange, and the inner lip of the rim. Sometimes referred to as ‘offset,’ we’ll try to keep things consistent, and call it backspacing.

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Backspacing is the measurement of the wheel from the hub mounting point, to the tire lip.

For example, let’s say you have a 15×8-inch wheel, and it has 4-inches of backspacing. This places the mounting flange in the middle of the wheel’s width. Even older musclecars — with a relatively narrow wheel opening — still offers room for installing larger wheels and tires, if the wheel is equipped with more backspacing. As backspacing distance increases, the mounting flange moves closer to the outside edge of the wheel, which is called a positive offset. Decrease backspace, and the result is a negative offset.

When measuring for backspacing, the straight edge needs to be on the bead 'face' area, not the outermost edge of the wheel.

The inside diameter of the wheel is also important if your car happens to have a big brake upgrade. Some brake companies are vague about minimum wheel diameter clearance, because each wheel company’s dimension for a given diameter varies with the style of the wheel. What this means is that you can’t assume that all 15-inch wheels will clear a set of Wilwood brakes with 12.19-inch rotors.

Measurements Made Easy

If you’re not sure about taking measurements, Percy’s has engineered a wheel measurement tool that allows anyone to find the correct offset, backspace, diameter, width, and tire profile in a matter of minutes. The Wheelrite tool can accommodate width measurements from 6 to 11 inches, tires between 15  and 30 inches, and backspacing up to 10 1/2 inches. It is designed for use on both 4 or 5-lug-wheel bolt patterns, measuring from 98 to 130 mm.

Tire Section Width

Although impossible to measure clearance for this without a tire mounted on a wheel, it is an important consideration. Tire manufacturers publish section-width information based on a given wheel width, so that can help. Section width is the distance measured across the mounted tire at its widest portion of the sidewall on a given wheel width. The section width measurement will change slightly, depending on the width of the wheel. Again, the tire companies publish dimensions of each size tire based on a given wheel width.

Let’s take a look at Mickey Thompson’s Sportsman S/T radial tire in size 275/60R15. When mounted on a 8-inch wheel, it has a section width of 10.7 inches. As a general rule of thumb, for every 1/2-inch of wheel width change, the section width will vary by roughly 1/4-inch. That means if we mount that same tire on a 9-inch wheel, the section width would be 11.2 inches. As with all tires, there is also an ideal wheel width range for any size tire.

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Keeping it simple, the tire’s section width is the distance of the two sidewalls of the tire after being mounted and inflated. Section width is almost always wider than the actual tread and rim width.

Whatever you do, do not assume that all same-size tires have identical section widths. While tire diameter is closely maintained, section width has a tendency to vary between manufacturers. We have heard of the same size BFG, Firestone, and Goodyear tires that are mounted on the same wheel, yet having major differences in section width.

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Installing larger, racing-style tires does involve a little more room consideration. This is because the taller-sidewall on these tires do exhibit much more lateral sidewall movement. This will require much more clearance than short-sidewall tires. With DOT-legal drag tires, you need to be careful when researching rear tire dimensions. Remember to always check the clearances at ride height, not with the car sitting on jack stands under the frame and the suspension in full extension.

A good rule of thumb is that you want to have at a minimum, 1/4- to 1/2-inch of space between the tire’s sidewall, and anything in the wheelwell. If the dimensions of your new wheel and tire cross into that space, you might have clearance issues.

Hopefully, we have answered your questions, and with this basic guideline of how to measure for tire and wheel clearance, the only task left for you to do is to go out to your garage and find out how big of a tire will fit under your ride.

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Our Nova has a set of 275/60R15 radials mounted in a set of 15×8 wheels with 3 1/4-inch backspacing. The rearend has also been narrowed 1-inch on each side.

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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