Current vs proposed setup
Enter two tyre or tire sizes plus wheel width and offset. The calculator shows diameter, speedometer impact, and nominal movement toward the suspension and fender.
Setup inputs 205/55R16 → 225/45R17 Edit
Results are dimensional estimates, not a vehicle-specific fitment approval. Verify brake, hub, suspension, fender, load and tire-manufacturer requirements physically before purchase.
How the wheel and tire fitment calculator works
This calculator compares an existing wheel-and-tire setup with a proposed setup. It combines tire-size geometry with wheel width and offset so you can review overall diameter, circumference, ride-height change, nominal inner clearance, outer wheel position and estimated speedometer effect in one result. It is a planning tool, not a vehicle-specific approval.
Inputs explained
- Tire width: the first number in a size such as 225/45R17, expressed in millimetres.
- Aspect ratio: sidewall height as a percentage of tire width. A 45 profile is 45% of the stated width.
- Rim diameter: the final number in the tire size and the wheel diameter, expressed in inches.
- Wheel width: nominal width between the bead seats, not the complete outside width of the wheel.
- Offset (ET): the signed distance in millimetres from the wheel centreline to its hub-mounting face.
- Spacer thickness: optional material between the hub and wheel. A spacer reduces effective positive offset by its thickness.
Formulas and assumptions
Theoretical sidewall height equals tire width multiplied by aspect ratio divided by 100. Overall tire diameter equals rim diameter multiplied by 25.4, plus two sidewalls. Circumference equals diameter multiplied by pi. Effective offset equals stamped offset minus spacer thickness. Nominal inner and outer wheel-edge positions are calculated from half the wheel width and the effective offset.
These formulas use nominal dimensions. A mounted tire may measure differently because of its brand, model, approved measuring rim, pressure, load and wear. The calculator also cannot reproduce the three-dimensional shape of a brake caliper, suspension arm or body panel.
Worked example
Suppose the current setup is 205/55R16 on a 16×8 ET35 wheel and the proposed setup is 225/45R17 on a 17×9 ET45 wheel. Enter the current values in the first column and the proposed values in the second. The result will show that the two tire diameters are close, while the wider wheel and different offset move the nominal inner and outer edges by different amounts. That distinction matters: a small tire-diameter change does not guarantee adequate suspension or fender clearance.
How to interpret the results
- Diameter difference: shows how much taller or shorter the proposed tire is.
- Ground-clearance change: half of the diameter difference because the axle sits at the tire radius.
- Inner rim movement: movement toward or away from suspension-side components.
- Outer rim movement: additional poke or retraction toward the wheel arch.
- Speedometer estimate: compares theoretical travel per revolution; it does not recalibrate the vehicle.
- Measured-clearance estimate: subtracts calculated movement from a gap you measured while the vehicle was static.
Common uses
Use the comparison when planning a plus-size tire, changing wheel width or offset, considering a spacer, checking why a replacement setup may sit farther inward or outward, or preparing measurements to discuss with a tire shop. Save or share the complete result so the other person sees the exact inputs and assumptions.
Limitations and safety checks
Before purchase, confirm bolt pattern, centre bore, fastener type and engagement, brake clearance, approved tire-to-rim range, load and speed ratings, and local legal requirements. Check clearance at full steering lock and throughout suspension compression. Static clearance is not dynamic clearance. If any requirement is uncertain, consult the vehicle, tire and wheel manufacturers or a qualified professional.
Frequently asked questions
Does a small diameter difference mean the new setup will fit?
No. Similar diameter helps preserve ride height and gearing, but wheel width, offset, tire section shape, brakes, suspension and body clearance are separate fitment constraints.
Does a spacer increase or decrease wheel offset?
A spacer decreases effective positive offset by the spacer thickness and moves the wheel outward by the same nominal amount.
Why can the real tire measurement differ?
The size code is nominal. Construction, model, measuring rim, inflation pressure, load and tread wear all affect the mounted measurement.
Sources and further checks
- NHTSA tire-safety guidance
- Your vehicle owner manual and tire-information placard
- The wheel and tire manufacturers’ current technical specifications