See exactly how a new wheel will sit compared with your current one. Enter the width and offset of both wheels, and the calculator shows the backspacing, how far the outer lip moves out toward the fender (often called poke) and how far the inner barrel moves toward or away from the suspension, in inches and millimeters.
Current wheel
New wheel
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Backspacing assumes the usual extra 1 inch of total wheel width beyond the nominal bead seat width (about 0.5″ per flange). Measure your own wheel to be certain. Clearance also depends on the tire, the brakes, the suspension and the steering angle; test-fit before you drive.
Offset, backspacing and front spacing explained
- Offset is the distance in millimeters from the wheel’s centerline to the surface that bolts against the hub. A positive offset puts the mounting face toward the outside of the wheel, so the wheel sits further in. A negative offset pushes the wheel out.
- Backspacing is the distance from the mounting face to the inner edge of the wheel. More backspacing means the wheel reaches further in toward the suspension and the brakes.
- Front spacing is the distance from the mounting face to the outer edge. More front spacing means the wheel and tire stick out further toward the fender.
The formulas behind the calculator
A wheel sold as 7.5 inches wide is about 8.5 inches across its outer edges, because the bead flanges add roughly half an inch on each side. The calculator uses that usual allowance:
- Backspacing = (width + 1) ÷ 2 + offset ÷ 25.4
- Front spacing = (width + 1) − backspacing
For a 7.5-inch wheel with a +44 mm offset: (7.5 + 1) ÷ 2 = 4.25 inches, plus 44 ÷ 25.4 = 1.73 inches, gives 5.98 inches of backspacing and 2.52 inches of front spacing.
Worked example: a stock 7.5×17 +44 wheel to a 9×17 0 offset wheel
This is a typical change when fitting wider all-terrain tires. The new wheel’s backspacing drops from 5.98 to 5.00 inches, so the inner barrel moves 0.98 inches (25 mm) away from the suspension. Its front spacing grows from 2.52 to 5.00 inches, so the outer lip moves 2.48 inches (63 mm) out toward the fender. That gives more room for a wider tire on the inside, but the tire will sit much further out, which can mean rubbing at full steering lock and a tire that sticks out past the fender.
What else decides whether a wheel fits
- Bolt pattern and center bore must match your hub, or the wheel needs the correct hub-centric rings.
- Brake clearance: large calipers need enough room inside the wheel’s spokes and barrel.
- Load rating: the wheel’s rated load must suit your vehicle, especially on trucks that tow.
- Tire size: a wider or taller tire changes the clearance as much as the wheel does; compare sizes with our tire size calculator.
Frequently asked questions
Is a higher or lower offset better?
Neither is better in general. Stay close to the vehicle’s original offset for the most predictable fit and steering feel. A lower offset widens the stance and can clear larger tires on the inside, but it moves the tire out and adds load to wheel bearings.
Why does a wider wheel with the same offset still rub on the inside?
Because the extra width is split between both sides. With the same offset, a wheel one inch wider sits half an inch further in and half an inch further out.
Can I use spacers instead of a lower offset wheel?
A spacer lowers the effective offset by its thickness, for example a 25 mm spacer turns a +44 mm offset into +19 mm. Use quality hub-centric spacers torqued to specification, and check that enough wheel stud thread engages.
How accurate is the backspacing figure?
It assumes the common 1-inch flange allowance. Some wheels differ slightly, so for a close fit measure your wheel: lay it face down, put a straight edge across the back and measure from the mounting face to the straight edge.