About

Changing your car's wheels is one of the most impactful modifications you can make, but getting the fitment right requires precise math. The "offset" (ET) determines how far the wheel's mounting surface is from the centerline. A wrong calculation can lead to the wheel rubbing against the suspension strut (inner clearance issues) or poking out past the fender (illegal or damaging to tires).

This tool visualizes the cross-section of your current versus new setup. It calculates the exact changes in millimeters, helping you determine if you need spacers, if the new rims will fit, or if you will achieve that flush look you are aiming for.

Formulas

The calculator uses the rim width and offset to determine the position of the inner and outer beads relative to the hub mounting surface.

Inner Position = (Width2 × 25.4) + ET

Outer Position = (Width2 × 25.4) − ET

All width inputs are converted from inches to millimeters (1 inch = 25.4mm) before applying the offset.

Reference Data

Car Model Example	OEM Width	OEM Offset (ET)	Fitment Goal	Typical Upgrade
VW Golf GTI (Mk7)	7.5J	ET49	Flush	8.5J ET45
BMW 3 Series (F30)	8.0J	ET34	Aggressive	9.0J ET30
Subaru WRX (VA)	8.5J	ET55	Flush	9.5J ET38
Honda Civic Type R	8.5J	ET60	Track Width	9.5J ET45
Ford Mustang GT	9.0J	ET45	Stance	10.0J ET35
Mazda MX-5 (ND)	7.0J	ET45	Sport	8.0J ET35
Toyota GR86	7.5J	ET48	Flush	8.5J ET40
Audi A4 (B9)	7.5J	ET38	OEM+	8.5J ET35

Frequently Asked Questions

Positive offset (e.g., ET45) means the mounting surface is closer to the street side (face) of the wheel, tucking the wheel into the body. Negative offset means the mounting surface is closer to the brake side, pushing the wheel outwards. Most modern passenger cars use positive offset.

If the inner clearance decreases significantly (the wheel moves closer to the car's center), the rim or tire may rub against the suspension strut, shock absorber, or brake caliper. This is dangerous and requires either lower offset wheels or spacers.

Measure the distance from your current tire's sidewall to the inner lip of the fender. If you have 15mm of gap, you can choose a wheel setup that extends an extra 10-15mm outwards (Poke increase).

If your new wheels have too high of an offset (e.g., changing from ET30 to ET50), they might hit the suspension. Spacers reduce the effective offset (e.g., ET50 wheel + 20mm spacer = ET30 setup), pushing the wheel back out.