There are fewer questions other than perhaps, “what oil should I use?” that have been asked more times in more venues than, “Will these wheels fit?”  I have often wondered if Thor, “the self-proclaimed inventor of the wheel” was constantly bugged by new wheel owners about wheel fitment.  In this article, I will attempt to break down the wheel specifications that will fit on a Genesis Coupe, the terminology, and hopefully, shed some light on the great mystery that is wheels.

Although with enough money any wheel can be made to fit any car, it is the desire of most car owners not to have to modify their new purchase to fit.  As a result, understanding what fits becomes important knowledge when there are so many wheel choices available.

Wheel terminology

• Backspacing – The distance between the mounting pad and the back of the wheel.
• Bolt Pattern – The diameter of an imaginary circle formed by the centers of the wheel lugs. Bolt patterns can be 4, 5, 6, or 8-lug holes. A bolt circle of 5×114.3 would indicate a 5-lug pattern on a circle with a diameter of 114.3 mm.
• Camber Angle – Camber angle is the angle made by the wheels of a vehicle.  Specifically, it is the angle between the vertical axis of the wheels used for steering and the vertical axis of the vehicle when viewed from the front or rear.
• Cast – A wheel manufacturing technique that requires molten (liquid) metal be poured into a mold that will yield the final shape of a wheel.
• Diameter – The physical diameter of a wheel expressed in inches. Whole increments (i.e. 16, 17 or 18) are the most common.
• Plus sizing – The practice of altering the wheel diameter in conjunction with tire aspect ratio to improve both the look and performance of a vehicle without changing the stock tire footprint.  Plus sizing should not change the overall tire diameter of the original equipment tires too drastically.  This is important since larger variances can cause problems with transmission shift points and other important systems
• Pressure cast – A wheel manufacturing process incorporating a pressure chamber and compressor to eliminate bubbles from the molten metal cast to ensure that castings are bubble free.
• Forged – A wheel manufacturing technique that uses intense heat and pressure to transform a solid “block” of alloy material into the final shape of a wheel.
• Hubcentric – Wheel with a center bore that matches a vehicle’s hub diameter and provides a snug, centered fit. Adapters can help achieve this.
• Hub bore – The center bore of a wheel is the machined opening on the back of the wheel that centers the wheel properly on the hub of a vehicle.
• Offset – The distance between the plane of the hub-mounting surface of a wheel to the centerline of the wheel and tire assembly.  Almost always measured in millimeters, a wheel offset can be positive, zero or negative.
• Staggered fitment – The wheels on the back of the vehicle are a different size than those on the front.  Wider wheels are usually run on the back, for example, a 19×8 on the front and 19×9.5 on the rear.
• Wheel/Caliper clearance – The distance from the outside edge of the brake caliper to the inside of the wheel drum.
• Width – The distance between the inner and outer flanges of a wheel.

So what does this all mean to me?

The non-negotiable parts of the wheel spec are the bore size and wheel bolt pattern.  The hub of the Genesis Coupe is 67.1mm and the wheel bolt pattern is 5×114.3.  The wheel you get should have a bore size of at least 67.1mm and match the hub bolt pattern.  All other specifications can be modified/adjusted for fitment.

The one thing you need to understand about the Genesis Coupe is that it is fitted with a staggered wheel setup from the factory.  What this means to you is that the front wheels spin faster than the rear wheels on the factory setup.  The stability control system on the Genesis Coupe is set up to recognize this.  If there is a significant difference in wheel speed from back to front with a different wheel setup, it is possible for the stability control system to activate while you are driving.  This is due to the fact that the stability control system uses wheel speed as a factor in determining if the wheels are slipping.  Inadvertent activation of the stability control system can result in significant performance and handling issues, cause premature wear on the brake pads and rotors, and potentially overheat your brakes.

Although this is a wheel thread and not a tire article, it is important to understand the effect width has on the profile of a tire.  The height of the sidewall is related to the width of the tire, which is related to the width of the wheel.  The profile spec 245/45R18 (in red) is a percentage (45%) of the tire width.  In the case of a 245 mm wide tire, the sidewall would be 110.25 mm tall (45% of 245mm).  The overall height of the tire would be 18 inches (for the wheel) + 4.34 inches (bottom sidewall) + 4.34 inches (top sidewall)… or a total of 26.68 inches in diameter.  The front tires are 225/45R18 (225 x .45 = 101.25 mm) or 4 inches for a total diameter of 24 inches.  As you can see, the width of the wheel plays into how tall the wheel is, and ultimately how fast the tire spins at a given speed.

Everything else is negotiable.  What that means is that any wheel can be made to fit using spacers, adapters, potentially rolling or cutting your fenders, or increasing your negative camber (tilting the wheel in from the top) to make them fit.  Just because you can make a set of wheels fit doesn’t mean that you should.  Wheel size and weight can have a dramatic effect on performance and handling.

As implied above, you really want to try to keep the overall diameter of the wheel and tire combination as close to the OEM spec as possible.  While there is a bit of a fudge factor with the stability control, staying close means not having to worry about potential issues.

What this also means is that the larger the wheel (diameter and width), the smaller the profile of the tire you’ll have to run to meet the OEM spec.  A smaller sidewall means less tire deflection when the tires are laterally stressed (as in a turn), which also means less warning before the traction breaks.  Additionally, less sidewall flex also equates to a much rougher ride as your tires can no longer absorb road impact as well.

So, how do I measure all of this?

The short answer is, you drop a string.  An ancient technique used by the ancient Babylonians is to take a string and tie something heavy at one end… then take the other end and hold it against what you are trying to measure and let the string hang.  This gives you a vertical line to measure from.  Crazy, right?

What do you need to know?  You need to know the distance from the hub face and the outermost point in/on the fender that you want your wheel to sit and the distance from the hub face to any obstruction (like the strut).  Take your measurements in millimeters and not inches… this is going to pay off later.

Okay, so now I know what my car specs are, now what?

When shopping for wheels, keep in mind that width and offset work together to determine where the wheel will sit in your wheel well (or outside of your wheel well).

If you have an 11″ wheel and an offset of 0 (zero), you will have exactly 5.5″ of  the wheel protruding out towards the fender and 5.5″ of the wheel towards the strut.  When the offset changes, you’ve got a little math to do.

The offset is measured in millimeters.  That thing that was invented by Gabriel Mouton because he had 10 fingers and 10 toes and figured, “Why not?”  What this means is that you need to convert inches to millimeters so that you have a common unit of measure.  So… using our 11″ wheel from above and converting it to millimeters, we find that 11″ is 279.4 millimeters.  Now, remember that number because we’ll be coming back to it.

Let us say that we have 110 mm of clearance from the center of the hub face to the outer edge of the fender and 180 mm of clearance from the center of the hub face to the strut in our perpendicular measurement in the rear of the car.  If we drew an imaginary line down the center of our wheel, we would have exactly 139.7 mm of the wheel from the centerline to the outer edge of the wheel.  This would be how the wheel would sit with a zero offset… in other words, the wheel would stick out of the wheel well by 29.7 mm (1.12 inches).

For most people, this would be unacceptable and in some countries, this would be illegal.  So to bring the wheel in, we need to subtract about 29.6 mm from the centerline.  If you account for about a 1mm of negative camber in the wheel (.5mm in the front) ~for a 20-inch wheel, you need 30.6 mm of offset.  In other words, you are looking for about a +31 offset.  A +31 offset would allow the wheel to sit 109.7 mm from the hub towards the fender and 169.7 mm in towards the strut.  This would be a flush fit.  So the offset spec you would be looking for in a 20″x11” wheel to sit flush inside of the fender is +31.

Cool huh?  This will work for any car to determine what width and offset you need in order to get a proper fit.  If you are interested in just Genesis Coupe specs and are not interested in doing the math, you can use the following chart to guide you.

Now, tire fitment is altogether a different thing, although related.  Even if your wheels fit the way you want them to, the tires you put on them may force you to make negative camber alignment adjustments to keep the tire from rubbing on your strut!  Keep that in mind if you are going for a very aggressive fitment and your wheel is very close to the strut.  If you have your tires mounted and you cannot align the car, you could find yourself removing the new tires and remounting the old ones so that you can drive.  Also, if you lower the car, remeasure your clearances.  They will change a small amount because of the angle of the control arm.