Reading offset, backspacing and poke
Offset and backspacing describe the same thing — where the mounting face sits across the width of the wheel — in two different units. Offset (ET) is the millimeter distance from the mounting face to the wheel's centerline, and it is the number aftermarket wheels are sold on. Backspacing is the inch distance from the mounting face to the back lip, the number you measure with a straightedge and a ruler in your driveway. Quote a wheel either way and this calculator gives you the other, so two wheels described in different units become directly comparable.
The reason a fitment change is worth modeling is that width and offset move the two edges of the wheel independently. Drop the offset and the whole wheel slides outward toward the fender; add width and each lip moves out by half the extra width. Put those together and the outer lip — the one people mean by poke — and the inner lip near the strut can move by very different amounts. A wheel that is wider but higher-offset can poke out at the face while barely changing its inner clearance, or the reverse. The two big figures above separate those so you are not guessing which edge is the problem.
A spacer is the third way to move the wheel, and the calculator treats it as exactly that: it sits between the hub face and the wheel and pushes the whole assembly outward by its own thickness, so the effective offset is the wheel's ET minus the spacer. A 15 mm spacer on an ET45 wheel puts the rim where an ET30 wheel would sit — same poke, same gain in inner clearance — which is why a spacer is the usual way to fix a wheel that tucks too far in. What it does not change is the wheel's own backspacing, and what it costs is thread engagement: the studs or bolts lose the same millimeters the wheel gains, so a slip-on spacer needs longer fasteners and a bolt-on one carries its own. Use hub-centric spacers, and check the rules where you drive before fitting them.
The tire matters as much as the rim. A wider tire carries more section width, roughly half of it hanging over each side of the wheel, so a plus-size can add poke and eat inner clearance on its own before the wheel numbers are even counted. The calculator folds a nominal estimate of that into the readout, but tire width is genuinely approximate: the same tire measures wider on a wider rim, and manufacturers' section widths are quoted on one reference rim. Treat the tire contribution as a close guide, not a micrometer. And remember the whole model is geometry — it shows where metal and rubber move, not whether they clear a caliper, a control arm or a fender lip at full lock. It is the fast way to shortlist a setup and catch an obvious mistake, not a substitute for a test fit.
FAQ
What is wheel offset (ET), and how is it different from backspacing?
Offset (ET, from the German Einpresstiefe) is the distance in millimeters between the wheel's mounting face and its true centerline. Positive offset moves the mounting face toward the street, tucking the wheel inward; lower or negative offset pushes the wheel outward toward the fender. Backspacing measures the same thing in inches — the distance from the mounting face to the back lip of the wheel. This tool converts between them, so you can compare a wheel quoted in offset against one quoted in backspacing.
Why does the backspacing formula add one inch?
Backspacing is measured to the back lip of the rim, not to the edge of the barrel, so the two bead flanges add roughly half an inch of width on each side beyond the nominal rim width. The industry convention is to add one inch to the nominal width before halving it: backspacing equals the nominal width plus one, divided by two, plus the offset converted to inches. That extra inch is why an 8-inch-wide wheel measures about 9 inches lip to lip.
How much does a change in offset actually move the wheel?
Millimeter for millimeter. Every 1 mm of offset change moves the wheel 1 mm in or out at the hub face, and the effect splits between the outer lip and the inner barrel as you also change width. A 25 mm drop in offset pushes the whole wheel 25 mm outward — enough to turn a flush setup into a poke, or to move a tire from clearing the strut to rubbing it. That is why even a 10 to 15 mm change is visible in the arch.
How do wheel spacers change offset?
A spacer sits between the hub face and the wheel, so it moves the whole wheel outward by its own thickness. The effective offset is the wheel's ET minus the spacer: a 15 mm spacer on an ET45 wheel puts the wheel exactly where an ET30 wheel would sit. It also takes that same 15 mm of thread engagement away, so slip-on spacers need longer studs or bolts, and bolt-on spacers need the original studs to sit below the spacer face and carry their own. Use hub-centric spacers so the wheel is centered on the hub rather than on the fasteners. This calculator takes a spacer for each setup and shows the effective offset, the added poke and what to measure on the car.
Does a wider tire change the fitment too?
Yes. A wider tire adds section width that sits roughly half the difference on each side of the wheel, so a tire 20 mm wider than stock adds about 10 mm of poke on the outer edge and 10 mm toward the suspension on the inner edge, on top of whatever the wheel change does. This tool folds that estimate into the readout, but it is nominal: the measured section width of a tire grows with rim width, so treat it as a close approximation rather than a guarantee.
Is the calculator enough to know a wheel will fit?
No. The figures here are pure geometry — they tell you where the lip and barrel move, not whether they clear your brakes, struts, fender liner or suspension at full lock and full droop. Ride height, camber, spring and tire choice all change real clearance. Use the numbers to shortlist setups and rule out obvious problems, then confirm the specific wheel, offset, width and brake clearance with the seller or manufacturer before you order.
Wheel fitment by vehicle
These are the factory setups the presets above are built from, and the starting point for any change you model here. Each vehicle page carries the full wheel specs, bolt pattern and center bore — and lets you try real wheels on that car. On the staggered cars (BMW M3, GR Supra, Mustang GT) the figure is the front wheel.
- Ford F-150 wheel fitment — 18×8.5 ET44 · 275/65R18 · 6x135
- Toyota 4Runner wheel fitment — 17×7.5 ET15 · 265/70R17 · 6x139.7
- Jeep Wrangler wheel fitment — 17×7.5 ET44.45 · 245/75R17 · 5x127
- Chevrolet Silverado 1500 wheel fitment — 18×8.5 ET26 · 265/65R18 · 6x139.7
- Toyota Tacoma wheel fitment — 17×7.5 ET30 · 265/65R17 · 6x139.7
- Ram 1500 wheel fitment — 18×8 ET19 · 275/65R18 · 6x139.7
- Toyota RAV4 wheel fitment — 17×7 ET35 · 225/65R17 · 5x114.3
- Tesla Model 3 wheel fitment — 18×8.5 ET40 · 235/45R18 · 5x114.3
- Tesla Model Y wheel fitment — 19×9.5 ET45 · 255/45R19 · 5x114.3
- BMW M3 wheel fitment — 19×9.5 ET20 · 275/35R19 · 5x112
- Honda Civic Si wheel fitment — 18×8 ET50 · 235/40R18 · 5x114.3
- Toyota GR Supra wheel fitment — 19×9 ET32 · 255/35R19 · 5x112
- Subaru WRX wheel fitment — 18×8.5 ET55 · 245/40R18 · 5x114.3
- Ford Mustang GT wheel fitment — 19×9 ET52.5 (approx.) · 255/40R19 · 5x114.3