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Asking What Size Wheel Do I Need? Here’s the Answer

Shopping for aftermarket wheels? Finding the wheels you like is usually the easy part. Then you’ve got to figure out if those awesome wheels will actually fit your car or truck. It’s confusing. Wheel fitment is more than just about knowing the answer to “what size wheel do I need”. You’ve got to consider the bolt pattern, offset and backspacing in addition to wheel size. If you’ve confirmed that the bolt pattern is a match and the offset and backspacing are good to go, then it’s time to decide what size wheel you need.

Matching Wheels to Tires

Are you just buying wheels and want to keep the tires you have? That makes the answering that “what size wheel do I need” question way easy. You need the same size wheel as you have now. Just pay particular attention to the width of the wheel you want to buy. Wheel width is measured from bead seat to bead seat, straight across the barrel.  Need a refresher on the parts of a wheel? Just refer to Wheefire’s Complete Guide to Wheel Anatomy and Definitions. In order for your existing tires to fit, the diameter and width must be the same as the wheels you are replacing. So, for example if you are swapping out the 18” wheels on your BMW 3 series, you’ll stick with an 18 x 8 5×120 with a positive offset of 35 mm because that’s what you have now. Except those new Niche Sport Verona’s will look much better with your tires.

Plus Sizing for Better Performance

Gear Alloy Wheels
Gear Alloy Wheels

If you aren’t married to those tires and are shopping for new wheels and tires, you’ve got so many more options. The answer to the question “what size wheel do I need” becomes more like “what size wheels do I want”.  That’s because you can safely change the size of your wheels and tires to increase the performance of your car or truck as long as you stay within just a few parameters (and keep those bolt pattern, offset and backspacing requirements in mind.)

The general rule of thumb is that as long as you keep within 3% of the original OEM wheels, you can upsize your wheels for better performance and a much better look. But you must keep in mind one critical factor. Your vehicle was engineered to cover a defined distance of road with every wheel rotation. From the transmission gearing to the accuracy of your speedometer and odometer, that geometry is one crucial factor that cannot be altered without degrading performance. And the goal is better performance not worse. So, the new total tire and wheel combination must not exceed the original OEM diameter. Confused? Don’t be; it’s easy. For every inch you go up in wheel size, just go down one inch in the standing size of the tire. In other words, as the wheel diameter increases, the sidewall of the tire gets shorter. The overall diameter stays the same. That keeps you in good graces with your car’s engineering. Tuners and wheel and tire pros refer to this as upsizing or plus sizing the wheels.  When your buddy says he has “plus 2” wheels, it means he increased the wheel size by 2 inches, decreased the standing height of the tire 2” and maintained the OEM geometry. You’ll say “sweet” because this wheel and tire combo will look better because it fills the wheel well better. Pure eye candy.

Plus sizing gives you better cornering and stability. You get less tire sway and roll as well. The tire tread will ride more squarely on the road, improving response. Smaller tires may save you money as well. Often they cost less.

Sizing Wheels for Staggered Fitment

If you want a really aggressive look, then ditch that square fitment and go staggered. A square fitment places 4 wheels of the same size on a vehicle. A staggered fitment puts larger wheels on the back and smaller wheels on the front.

Say what?

Let’s go back to the BMW 3 Series where we started this discussion. Instead of sticking with the stock 18” wheels, you plus 1 and go with 19” wheels. Now, those Niche Sport Verona’s on the back will be 9.5” wide but the wheels on the front will be 8.5” wide. The rear wheels on the 3 Series are the drive wheels. By placing wider wheels back there you increase grip during acceleration. That gets you off the line faster. This is true for any rear-wheel drive vehicle with lots of horsepower (sorry Prius – this leaves you out). Just use bigger rear tires and transfer all that power to the pavement.

The front wheels are the wheels that steer to the right or left. No matter the quality of the tire, there is always a chance of hydroplaning. You don’t want those “steering wheels” hydroplaning, so staggered fitments reduce that risk.

A staggered fitment is also a way to control unsprung weight. That’s the weight not attached to the chassis, and you want to keep it minimized for best performance. Bigger wheels equals more mass and more mass means more weight. Reducing the width of the front wheels reduces the amount of mass added.

Still Asking What Size Wheel Do I Need?

Now that you understand bolt patterns, offset, backspacing, plus sizing and staggered fitments are you still asking “what size wheel do I need”?

Don’t worry. Wheelfire’s proprietary fitment program will make sure the wheels you buy are the wheels that fit. Have a question? Give us a call at 1-800-866-450-3473.

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