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Chilton Auto Answers: Fitting Bigger Custom Wheels

Fat drag slicks barely fit under fender

Fitting new wheels is a source of joy for many automotive enthusiasts seeking to personalize their rides. There is no universal guide to style; that choice is still on you. However, there are guidelines for selecting wheels that fit your car properly, and we can help with that.

There is, unfortunately, much confusion in wheel and tire shopping because some values are measured in millimeters (offset and tire width), and some in inches (wheel width and diameter). Just remember that there are 25.4mm to an inch, and change from one value to another if needed.

Bolt Patterns

Wheel_Lug_Pattern_Chart 1500 x 750

The first part of wheel selection is to determine your bolt pattern. The bolt pattern is simply the number of studs (or bolts) that hold on the wheel and how far apart they are, but it isn’t quite that simple.

On a four lug car, measure the distance from the center of one stud (or hole) to the center of the lug directly across from it. On a five lug car, measure from the center of one lug or hole, skip one, then to the far edge of the second hole. The most common sizing for vehicles sold in the US is 5 x 114.3 mm, formerly referred to as 5 x 4.5 inches, or 5 lugs on a 114.3 mm circle. This sizing dictates what wheels bolt to your car. (On some vehicles, the size of the center hole on the wheel is also critical, but that is less common.)

Wheel Sizing

Custom Alloy Wheels with Gold Center

The next aspect of the wheel for the overall fit is the diameter. It’s essential to know the stock size for your vehicle before you begin. Sizing up from stock (for example, from a 15” to a 16” wheel) needs to be done carefully to keep the overall tire diameter the same and the speedometer readout accurate.

A larger or smaller overall diameter can also effectively changes the final gearing, which is especially crucial with taller truck tires. Downsizing wheels is challenging because of brake clearance issues but is often done in autocross for better acceleration. It’s important to measure the overall diameter of your brakes (from the edge of the caliper to the opposite edge of the rotor) to ensure that downsizing will work. Keep in mind the dropped center inner diameter of the rim, plus possible wheel balancing weights, and include a significant margin for error.

Offset and Backspacing

Blue import with custom wheels

An often overlooked part of wheel selection is the width and offset of the rims. Every wheel is sized “diameter in inches x width in inches, (+/-) offset in mm,” or 17” x 9” +22mm, for instance. Where the wheel mounts can be offset toward the inside or outside of the true center of the wheel. A “0” offset is centered inside the rim. A “+30” wheel will mount 30 millimeters toward the outside edge of the rim, while a “-30” mounts 30 millimeters away from the center of the wheel, towards the inside.

Similar to offset is backspacing, though more commonly used with classic car and truck wheels. Backspacing refers to the amount of the wheel behind the hub, typically in inches. An 8-inch wide wheel with a 0 offset has 4 inches of backspacing. This is a less common concept in modern wheel fitment, but some still refer to backspacing depth, rather than an offset value.

wheel offset diagram

The wheel’s width is important too, especially relative to the stock size; a wider wheel may require less positive offset to avoid rubbing. Even if you keep the same width as stock, the offset measurement still needs to be considered, as it determines the wheel’s place in the fender (set in, or protruding out vs. stock). This diagram is exaggerated, but much simpler to understand.

Changing offset significantly from stock values can cause issues if not carefully measured and planned. A higher positive offset can cause the inside of the tire to rub the inner suspension or fender, and a significantly higher negative offset can cause the wheel to hit the fender lip on suspension compression, causing damage to both tires and bodywork. The proper way to choose a wheel offset is to measure stock clearance from the inside of the tire to the strut or suspension, and the clearance from the outside of the tire to the fender lip, to determine how much space is available before rubbing. Be sure to measure with the steering turned fully as well, to check for interference with the inner fender liner.

It can be tricky thinking about both offset and a wider wheel together when planning a wheel change. If you transition from a 6 inch wide, +0 wheel, to an 8-inch wide, +25 offset wheel, the edge of the new wheel in relation to the outer fender is the same, but the inner side is two inches closer to the strut and the fender liner. Remember, an inch is about 25 mm.

In general, it’s good to keep in mind that most trucks and older rear wheel drive cars use close to 0 offset wheels, and more modern vehicles and most front wheel drive cars use a high positive offset stock setup.

Tire Sizing

narrow vs. wider tires

Once a wheel has been selected, you need to choose tires. Modern tire sizing is a number that looks like “235/55 R 17”. The final number, after the R for “radial,” is just the diameter of the wheel the tire will fit in inches, which is a simple choice – just select the diameter of your wheels.

The first number is the width of the tire from sidewall to sidewall, in millimeters – most tires recommend a range of wheel widths considered safe for a given tire width. A wider tire will have a wider contact patch where the rubber hits the road, for better grip in cornering. Choosing a narrower tire may give you less contact patch, but typically better fuel mileage.

stock verses upsized wheels/tires

The middle number, after the slash, can be confusing – that’s the sidewall height of the tire, as expressed as a ratio to the overall width. The larger this number, the taller the sidewall is. This is complicated in the abstract, but simple by example: 255/40r17 – 255 mm wide, on a 17-inch wheel, with a 40 profile sidewall. The sidewall is 40% of the width, which means a 102 mm sidewall, and a tire about 25 inches in overall diameter. 255/60r17, a popular truck size, ends up having an overall diameter of close to 29 inches, a big difference.

In the Real World

Supra on stock Wheels

Now, let’s demonstrate with a real-world case study in wheel selection. I have a 1988 Toyota Supra, which has stock wheel sizing of 16″ x 7″ +37, with 225/50 R16 tires. This gives the OEM wheel and tire combination a 25″ outer diameter and means my wheels have roughly 2″ from the hub to the face, and 5″ from the hub to the inside of the car.

For more grip and a better-looking package, I changed my stock wheels to a set of Yokohama Super Advan SA3 wheels. The new sizing is 17″ x 9″ +22, with 255/40 R17 tires. The overall tire diameter remains identical despite stepping up an inch in wheel size, so my speedometer does not need calibration, and the ride height remains the same as well.

This setup moved my wheel edges about 1.63 inches (41.5 mm) further outward, and .47 inches (12 mm) further inward, both of which I had room to clear (albeit with mild front fender modification to fit while turning).

My car now has a better contact patch, and over 3 inches of added track width, which met my goals. When paired with a sticky tire compound, cornering and handling performance improved significantly. As a bonus, the car now looks more aggressive, with the tires filling the wheel wells, and even sticking out slightly.

Of course, if you are fitting tires to your 4 x 4 truck that isn’t what you are going for, but the wheel sizing information should be helpful no matter what you drive.

Larger custom wheels on a Supra
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