How the wheel & tire fitment calculator works
This calculator compares two wheel-and-tire setups and shows exactly how the new one changes your car's stance, clearance and speedometer reading. Every number updates live from standard fitment geometry — no guesswork. Here are the formulas it uses (lengths in millimeters; 1 inch = 25.4 mm). Outside North America a tire is spelled tyre, but the math is identical.
Poke and inset — how far the wheel sits out or in
Poke is how far the outer lip sits past the hub mounting face — more poke means a more aggressive stance and a higher chance of catching the fender. Inset (also called backspacing) is the inner equivalent: the room the wheel needs before it fouls the strut, spring or control arm. A lower offset (ET) or a wider rim both increase poke.
Rolling diameter, rolling radius and circumference
A tire's profile (aspect ratio) is its sidewall height as a percentage of its section width, so a 225/45 tire has a sidewall of 225 × 0.45 = 101.25 mm.
This calculator uses the standard rolling-diameter formula (rim + twice the sidewall). It does not apply a tire-stretch adjustment, so overall-diameter figures can differ by a few millimeters from a heavily stretched fitment. Poke, inset, offset, ride-height and speedometer figures are exact.
Ride-height change
A taller tire raises the axle by half the increase in diameter — and closes the gap to the fender by the same amount. A shorter tire lowers it. Rim size alone doesn't change ride height: go up an inch in wheel but drop the tire profile to keep the same rolling diameter ("plus sizing") and the car sits at exactly the same height.
Speedometer error
Your speedometer is calibrated to the original tire's circumference. Fit a larger-diameter tire and the car travels farther per revolution than the speedometer assumes, so you are actually going faster than it shows (a negative error). A smaller-diameter tire does the opposite and reads high.
Wheel & tire fitment — frequently asked questions
Will these wheels fit my car?
This calculator estimates how a new wheel and tire setup changes poke, inner clearance, rolling diameter and speedometer accuracy versus your current setup — the fastest way to compare two fitments side by side. It can't guarantee a fit on its own, though: real-world clearance also depends on your bolt pattern, center bore, suspension, camber, fender rolling and ride height. Always confirm those, plus the tire's load rating, before buying.
What is wheel poke?
Poke is how far the outer face of the wheel and tire sticks out past the hub mounting face. More poke gives a more aggressive stance but raises the chance of the tire catching the fender. It's calculated as poke = (rim width in inches × 25.4 ÷ 2) − offset (ET).
What does offset (ET) mean?
Offset, stamped "ET" on a wheel, is the distance in millimeters between the wheel's mounting face and its true centerline. A higher ET tucks the wheel further under the car; a lower or negative ET pushes it outward for more poke. Offset is the single biggest factor in how a wheel sits in the arch.
What's the difference between offset and backspacing (inset)?
Offset is measured from the wheel's centerline, while inset — also called backspacing — is measured from the inner lip and tells you how much room the wheel needs before it hits the strut, spring or control arm. This calculator shows both at once so you can check outer stance and inner clearance together.
How does tire size affect my speedometer?
Your speedometer is calibrated to the rolling circumference of the original tire. A larger-diameter tire covers more ground per revolution, so the car goes faster than the speedometer reads; a smaller tire makes it read high. The calculator shows the exact speedometer error percentage and your true speed at any indicated speed.
What is tire stretch, and does this calculator account for it?
Tire stretch is mounting a narrower tire on a wider rim so the sidewall pulls inward at an angle. This calculator uses the standard rolling-diameter formula (rim + twice the sidewall) and does not model stretch, so overall-diameter numbers can differ by a few millimeters on a heavily stretched setup. Poke, inset, offset, ride-height and speedometer figures are unaffected and exact.
How do I calculate rolling diameter and rolling radius?
Rolling diameter = (rim diameter in inches × 25.4) + 2 × (tire width × profile ÷ 100), and rolling radius is half of that. For example, a 225/45 R17 tire measures 17 × 25.4 + 2 × (225 × 0.45) = 431.8 + 202.5 = 634.3 mm in diameter.
Will bigger wheels change my ride height?
Only overall rolling diameter changes ride height, not rim size on its own. If you go up an inch in wheel but drop the tire profile to keep the same rolling diameter ("plus sizing"), ride height stays the same. This calculator reports the ride-height change as half the difference in rolling diameter between your two setups.
Is this wheel fitment calculator free?
Yes. This is a free, browser-based wheel and tire fitment calculator with no sign-up and no app to install. It works on desktop and mobile, in metric or imperial units, and nothing you enter ever leaves your browser.
Does it work in inches and metric?
Yes — switch between metric (millimeters, km/h) and imperial (inches, mph) at the top of the page. Enter wheel widths and offsets in the units printed on the wheel and every result updates instantly. (Outside North America, "tire" is spelled "tyre" and this tyre size calculator works exactly the same.)