Will this tire size fit my car? What the math can tell you
A size comparison can tell you how the nominal dimensions change. It cannot tell you whether a particular tire and wheel will fit your car. Start with the original specification, calculate the differences, then verify the physical setup.
Start with the vehicle specification
Use the tire size on the vehicle placard or in the owner's manual as your reference. A tire already mounted on a used car may be a previous owner's choice. Enter your original size to open a comparison from that baseline.
What does the diameter percentage mean?
For a metric tire marking, the nominal diameter is:
diameter_mm = rim_in × 25.4 + 2 × width_mm × aspect_ratio / 100
difference_% = (new_diameter / original_diameter - 1) × 100A negative result means the replacement is smaller; a positive result means it is larger. The comparison tool shows that change and its theoretical effect on rotation rate.
Why a 3% filter is not installation approval
The plus-sizing calculator uses ±3% to limit its list of nominal candidates. That is a search filter, not a universal safety limit. Even an identical size marking does not establish tire-model dimensions, wheel compatibility, load capacity, or vehicle clearance.
Vehicle requirements can be more restrictive, including requirements for matching tires across an axle or drivetrain. Follow the vehicle manufacturer's guidance for the actual setup.
What remains unknown after the calculation?
| Check | What to verify |
|---|---|
| Tire specification | Exact model, actual dimensions, load index, speed rating, and required inflation. |
| Wheel specification | Approved rim width, diameter, offset, bolt pattern, hub fit, and load capacity. |
| Physical clearance | Brake, suspension, and body clearance through full steering and suspension travel. |
| Vehicle systems | Manufacturer requirements for the drivetrain, speedometer, ABS, stability control, and TPMS. |
Use the comparison as a worksheet
- Record the original and proposed tire markings.
- Compare nominal diameter, section width, and sidewall height.
- Read the tire manufacturer's specifications for the actual model.
- Have a qualified tire professional verify the wheel and vehicle installation.
NHTSA's tire guidance recommends the original size or another size recommended by the vehicle manufacturer.
Related vehicle information
caroutlay.com is a related vehicle information resource. Use the manufacturer specifications for the final fitment decision.
Sources and calculation limits
Source note updated
June 2026
Calculation scope
Nominal dimensions from the size markings supplied. Vehicle installation is not verified.
Method and inputs
- Nominal geometry calculated from tire-size markings
- Original-equipment size references supplied by users or vehicle placards
- Diameter differences and theoretical rotation ratios; no installation approval
Calculated dimensions do not verify the tire model, load or speed rating, approved rim width, wheel offset, physical clearance, or vehicle systems. Confirm those against manufacturer specifications before installation.