Wheel offset calculator: tire size, backspacing, and poke

Compare wheel offset before buying wheels

Start with the wheel diameter, width, offset, and tire size that are actually on the car. Then enter the wheel package you are considering. The useful comparison is not the offset number by itself; it is how both wheel edges move when width and offset change together.

Outer wheel position is the change toward the fender, often called poke. Inner wheel position is the change toward the strut, spring perch, and other suspension hardware. Lower positive offset moves the wheel outward. A wider wheel adds material to both sides unless the offset changes to compensate.

Wheel offset, backspacing, and poke explained

An 18x8.5 +55 wheel is 18 inches in diameter, 8.5 inches wide, and has a positive 55 mm offset. Offset is the distance between the wheel centerline and the hub mounting face. +38 on a 9.5-inch wheel does not sit in the same place as +38 on an 8-inch wheel.

Backspacing measures from the hub mounting face to the inboard wheel lip. The calculator shows estimated backspacing in inches using the common lip-to-lip convention, which treats overall wheel width as nominal width plus about one inch. It is a useful comparison number, but measure the actual wheel when clearance is tight.

Poke is the opposite side of the same fitment question: how much farther the outer edge sits toward the fender. More poke can give the flush look some builds are after, but it uses fender clearance. More backspacing uses inner clearance. Neither number can stand in for a brake template or a full suspension-travel check.

How tire size changes the result

A 245/40R18 tire has a nominal 245 mm section width, a sidewall height equal to 40 percent of that width, and fits an 18-inch wheel. The calculator compares overall tire diameter, sidewall, circumference, and indicated-speed difference in millimeters, inches, mph, and km/h.

Tire width is not a fixed physical measurement across every model. A 245 from one manufacturer can mount differently than a 245 from another, and the wheel width changes the tire's shoulder shape. Use the tire diameter result for gearing and speedometer planning, then measure the actual tire and wheel combination when fender or spring-perch clearance is close.

Using spacers with a wheel offset calculator

A spacer moves the wheel outward by its thickness. To compare a wheel with a spacer here, subtract the spacer thickness from the wheel's offset before entering it. An 18x9 +45 wheel with a 5 mm spacer behaves like +40 for wheel position. That creates more outer poke and more inner clearance, but it also adds separate hub, stud-engagement, and hardware checks.

Using the Subaru WRX presets

The WRX menu gives you a quick start for common factory packages from the GD, GR/GV, VA, and early VB generations. Trim, market, and model year can change the factory wheel or tire, so use the wheel stamping and door-jamb tire placard to confirm the starting point. Switch to Custom for aftermarket wheels, a different factory trim, or a non-US specification. Our WRX wheel guide covers the separate brake-clearance and chassis-fitment questions.

What to check before ordering wheels and tires

A wheel can have the right offset and still fail elsewhere. Confirm the bolt pattern, hub bore, wheel load rating, lug-seat type, and tire load and speed rating. For brake clearance, use the wheel maker's vehicle-specific fitment listing or brake template; spoke shape and barrel design matter as much as diameter.

Test a new package at full steering lock and through suspension travel. A tire that clears while the car is parked can touch the liner, fender, or spring perch during a bump or with passengers aboard. If the new tire is taller or shorter, use the speedometer result here as a reference, then keep the final tire choice appropriate for the vehicle's drivetrain and intended use.