Wheel Fitment Calculator

How the wheel fitment check works

A wheel and tire package has two lateral edges that matter: the outer one, which has to stay inside the fender lip, and the inner one, which has to clear whatever sits inboard of the tire, typically the strut, a spring or a control arm. This calculator places both edges of your current package and of the new one, measured from the hub mounting face, the one surface both wheels share. The difference between them is how far each edge moves.

Each edge is whichever sticks out further: the wheel lip or the tire sidewall. The tire's width comes from its size code, the section width in millimeters for a metric tire or in inches for a flotation tire. Most tires bulge a little past the lips of the wheel they sit on, so the sidewall usually sets the edge, but a wide wheel with a narrow tire can put the lip outside the rubber.

Moves alone don't answer whether a package fits, so the first tab takes the two clearances you measure on the car today and subtracts the moves. What is left is the estimated clearance with the new package. A negative number means the new package would reach past the fender lip or into the part inside, by that much.

What to enter and where to find it

• Wheel width and offset. From the wheel size (18x9.5) and the offset, usually cast on the back of a spoke as ET or as a signed number in millimeters. Have backspacing instead? Convert it with the wheel offset calculator.
• Tire size. Exactly as printed on the sidewall. Both metric (245/40R18) and flotation (33x12.50R15) sizes work.
• Measuring rim (optional). Tire makers publish a measuring rim width for each size: the rim the listed section width is measured on. Enter it and the calculator adjusts the tire's width for the wheel it is mounted on. Leave it empty and the width in the size code is used.
• Inner clearance today. With the car on the ground and the wheels pointing straight ahead, find the tightest point between the inner sidewall (or the inner lip) and the closest part inboard of it, and measure the gap.
• If the tightest point inside sits next to the rim. The inner clearance assumes the closest part inside faces the inner edge of the package, usually the sidewall. When it sits at rim height instead, such as a control arm or a steering knuckle close to the wheel, use Inner wheel lip moves in rather than the inner edge move: the lip and the sidewall can move by different amounts.
• Fender lip clearance today. Hang a plumb line from the edge of the fender lip, or hold a straightedge vertically against it, and measure horizontally to the outermost point of the tire. If the tire already sits outside the lip, enter the amount as a negative number.
• Flange allowance. How much wider the wheel is across its lips than its nominal width. Conversion charts use 1 in; real wheels run about 0.5 to 1.0 in. It only matters when a lip, not a sidewall, is the outermost point, and for the inner lip move.

Will these wheels fit? Reading the result

The geometry check reads positive on both sides when neither estimated clearance goes below zero, and names the side that goes negative otherwise. Treat a small positive number as a reason to look closer, not as a green light. The estimate is static: the car standing still, wheels straight. On the road the tire also moves up into the arch over bumps and leans with the body in corners; on the front axle it also sweeps forward and back at steering lock. The fender lip clearance you measured at ride height shrinks in all of those cases.

Diameter matters as well. When the new tire is taller, the tread rises toward the wheel arch by half the difference in diameter, shown as Tread moves up toward the arch. A larger tire can clear the lip at its widest point and still touch the arch above it. For the full diameter comparison and revolutions per mile, use the tire size calculator.

Nothing here accounts for clearance behind the spokes, which depends on each wheel's design. Wheel makers publish templates for checking that; the calculator only deals with the lateral position of the lips and sidewalls.

Poke and inner clearance

Poke is how far the new package's outer edge sits outboard of the old one. It comes from three things: half of any extra width, any reduction in offset, and any change in the tire's width. The inner edge works the same way in reverse: half of the extra width moves it in, and a lower offset pulls it back out. That is why a wider wheel with less offset can keep the inner clearance almost unchanged and put all the extra width outside, as the first worked example shows.

Tire width depends on the rim. Tire data books give the rule that a tire's section width changes by about 0.2 in, or 5 mm, for every half inch of rim width away from its measuring rim. On a wheel an inch wider than its measuring rim, a tire is about 10 mm wider than on its measuring rim (and the width printed in the size code is itself a nominal figure, a few millimeters off the measured one). The Poke & inner clearance tab shows those moves without asking for measurements, which is handy for comparing several packages before you get under the car.

The wheel fitment formulas

Every position is measured from the hub mounting face, with outboard positive. All of it is geometry except the tire width on a non-measuring rim, which uses the data-book rule of thumb.

1. Wheel centerline = −offset (mm) ÷ 25.4
2. Wheel lips = centerline ± (width + flange allowance) ÷ 2
3. Tire section width = width in the size code + 0.4 × (rim width − measuring rim width)
4. Tire sidewalls = centerline ± section width ÷ 2
5. Outer edge = the larger of the outer lip and the outer sidewall; inner edge = the smaller of the inner lip and the inner sidewall
6. New clearance = clearance measured today − how far that edge moves toward the obstacle
7. Tread moves up = (new tire diameter − current tire diameter) ÷ 2

The section width is measured unloaded, so a loaded tire's sidewall bulges slightly more near the ground, but the fender lip and the parts inboard of the tire sit higher up, where the nominal width is a fair guide. Raised lettering and rim protector ribs can add a few millimeters beyond the section width; if your tires have them, measure the clearance today to their outer face.

Worked example

Two common packages, worked out with the same functions that run the calculator.

Typical change of tire width with rim width

Using the data-book rule of 0.4 in of section width per inch of rim width, the table shows how much a tire grows or shrinks when it goes on a rim narrower or wider than its measuring rim, and how far each sidewall moves.

The rule holds within the tire's approved rim width range, which the tire maker lists next to the measuring rim. Outside that range the sidewall shape changes more than a straight line predicts, and the tire is not meant to be mounted there anyway.

Common mistakes

• Measuring to the wheel instead of the tire. The sidewall usually sticks out past the lip. Measure today's clearances to whichever part of the package is closest to the fender lip or the part inside.
• Measuring with the car in the air. With the wheels hanging, the tire sits lower and often tilted. Measure with the car on the ground at its normal ride height and the wheels straight.
• Reading the size code as the real width. The first number of a 275/35R18 tire is its width on the measuring rim. On a wider or narrower wheel the tire is wider or narrower; enter the measuring rim for a closer estimate.
• Changing width without the offset. A wheel an inch wider at the same offset moves both edges half an inch, out and in. Compare offsets for different widths with the wheel offset calculator before you shop.
• Treating a small positive clearance as a guarantee. It is a static estimate. Bumps and body roll all use up clearance (and steering lock, on the front axle), and so do tolerances in the parts themselves.