Wheel Fitment Calculator
Enter your current setup and the wheel you are considering. This calculator shows how the outer edge, inner edge, backspacing and overall diameter change.
- Compare wheel width, offset and tire size in one place.
- See outer and inner edge movement in millimetres.
- Check overall diameter and speedometer error.
Compare two setups
Use the same units on both sides. Wheel width is measured in inches; offset and tire width are in millimetres.
What changes
These are dimensional changes, not a fitment guarantee. Check brakes, suspension, fenders and tire load with a specialist.
How to use the fitment calculator
Start with the wheel that is on the car now. Enter its diameter, width, offset and tire size. Then enter the wheel and tire you want to compare. The calculator keeps both sides in the same units.
Width is the wheel width between the flanges, not the tire width. Offset is the ET number. Tire width and aspect ratio come from the sidewall, for example 225/45R17.
How to read the result
A positive outer edge number means the new wheel sits further out. A positive inner edge number means it sits closer to the suspension. Backspacing tells you how far the mounting face is from the inner lip. Overall diameter and speedometer error come from the tire size, not the wheel width alone.
Small changes can still matter. A few millimetres at the inner barrel can be the difference between clearing a brake caliper and touching it. Treat the result as a measurement, not a approval.
What the calculator cannot see
It does not know the shape of your fender, the size of your brakes, the suspension travel or the condition of the hub. It also cannot confirm tire load rating, lug hardware or whether a specific tire model runs wide.
A fitment calculator is useful for narrowing a choice and spotting a setup that is obviously too aggressive. It does not replace a physical check by someone who knows the car.
Check the look and the fit
If the numbers are close, measure the car before ordering. If they are clearly outside the original range, change the width, offset or tire size before you spend money.
Use a photo preview for the visual decision and a specialist for the mechanical one. The two checks answer different questions.
Read the wheel fitment guideCheck rim width against tire widthPreview the wheel on your car
Frequently asked questions
Will they fit my car?
The calculator can show how the dimensions change, but it cannot answer that on its own. You still need to check brake clearance, suspension clearance, fender clearance, PCD, center bore and tire load rating for the exact car.
What does a positive outer edge mean?
It means the outer edge of the new wheel sits further out than the current wheel. A negative number means it sits further in.
How is overall diameter calculated?
Overall diameter is wheel diameter multiplied by 25.4, plus two times the tire width multiplied by the aspect ratio divided by 100. The calculator uses the tire sizes you enter.
Does offset change the speedometer?
Offset changes the wheel position, but the speedometer is driven by overall tire diameter. A larger or smaller overall diameter changes the reading; a pure offset change does not.
Can I use this for any wheel and tire?
It works for conventional passenger-car wheel and tire dimensions. It does not account for unusual rim profiles, tire stretch, brake packages or suspension geometry, so confirm the final setup with a specialist.
Standards and sources
This calculator uses the wheel and rim vocabulary from ISO 3911 and the passenger-car tyre dimensions in ISO 4000-1. Fitment guidance also draws on ETRTO references.