Wheel & Tire Fitment Tools
Wheel & Tire Fitment Calculators
Compare tire dimensions, wheel offset and backspacing, clearance movement and speedometer effects between a current and a proposed setup. Every figure states what it measures and what it assumes.
Every calculator states what it measures and assumes.
Find the right tool
Start from the problem you have
Changing wheels and tires together?
Wheel & Tire Fitment CalculatorWheel position and tire geometry in one comparison.Open calculator for Wheel & Tire Fitment CalculatorComparing two tire sizes?
Tire Size Comparison CalculatorDiameter, sidewall and circumference differences.Open calculator for Tire Size Comparison CalculatorChanging wheel offset or width?
Wheel Poke & Clearance CalculatorHow far each wheel edge moves.Open calculator for Wheel Poke & Clearance CalculatorAdding spacers?
Wheel Spacer CalculatorEffective offset after the spacer.Open calculator for Wheel Spacer CalculatorChecking speedometer difference?
Speedometer Error CalculatorActual versus indicated speed.Open calculator for Speedometer Error CalculatorConverting a tire size?
Tire Size ConverterNotation conversion with stated approximation.Open calculator for Tire Size Converter
Calculator groups
A focused tool for each dimension
Explore five groups of wheel and tire measurements. Each calculator opens a focused tool.
Complete Fitment
1 tools
The whole setup: wheel position, tire dimensions and speed effects together.
Wheel & Tire Fitment Calculator
Compare the dimensional changes between current and proposed wheel and tire setups.
Wheel Fitment
3 tools
Understand wheel width, mounting position, backspacing, spacers and edge movement.
Wheel Offset & Backspacing Calculator
Convert wheel offset and backspacing with measured overall width, or compare two wheel positions.
Wheel Poke & Clearance Calculator
Compare inner and outer wheel-edge movement relative to a reference setup.
Wheel Spacer Calculator
Calculate effective offset and the dimensional effect of spacer thickness.
Tire Size
2 tools
Read a tire specification or compare the dimensions of two sizes.
Tire Size Calculator
Calculate tire width, diameter, sidewall, circumference and revolutions from one metric size.
Tire Size Comparison Calculator
Compare two tire sizes and their dimensional differences.
Speed & Geometry
1 tools
Estimate speedometer changes from tire diameter; complete tire geometry lives in Tire Size.
Speedometer Error Calculator
Calculate the relative speed change caused by a different tire diameter.
Conversions & Upgrades
2 tools
Translate size notation and explore dimensional alternatives for larger wheels.
Plus Sizing Calculator
Explore nominal tire-size alternatives when changing rim diameter.
Tire Size Converter
Convert metric and flotation tire notation in both directions, with explicit rounding and equivalence limits.
Fitment model
How the measurements relate
Tire dimensions describe the rubber. Width and offset place the wheel. Together they decide how far a setup moves in each direction.
- Tire size
- Width in millimetres, aspect ratio as a percentage of width, and the rim diameter in inches.
- Wheel width
- The nominal distance between bead seats affects wheel-edge positions. Tire-to-rim approval comes from the exact tire manufacturer's specifications, not nominal width alone.
- Offset
- Distance from the mounting face to the wheel centreline. Lower offset moves the wheel outward.
- Backspacing
- Distance from the mounting face to the inner edge of the wheel. It depends on the overall width you measure.
- Inner clearance
- Space between the inner wheel and tire edge and suspension, brakes or body. Measure it on the vehicle.
- Outer poke
- How far the outer edge sits toward the fender relative to a reference setup.
- Overall diameter
- Rim diameter plus two sidewalls. Small changes alter ride height, gearing and speed readings.
- Speedometer change
- Actual speed scales with diameter, assuming the gauge was accurate on the reference tire.
Result experience
Old setup, new setup, change, meaning
Each result is reported in four layers so a number never arrives without its interpretation. Static example computed with the same engine as the calculators.
1
Old setup
The reference dimensions you start from.
2
New setup
The proposed dimensions, entered the same way.
3
Numerical change
Signed differences in millimetres and percent.
4
Plain meaning
What the change does and what it does not tell you.
Swipe to view more, if needed
| Measure | Old setup | New setup | Change | What it means |
|---|---|---|---|---|
| Overall diameter | 634.3 mm | 653.2 mm | +18.9 mm (+2.98%) | The tire is slightly taller. The axle sits about 9.5 mm higher, all else equal. |
| Sidewall height | 101.3 mm | 98.0 mm | −3.3 mm | A shorter sidewall on a larger rim: the extra diameter is in the wheel. |
| Outer wheel edge | 50.3 mm | 72.9 mm | +22.7 mm | The outer bead-seat edge moves toward the fender. Actual fender clearance is not assessed. |
| Inner wheel edge | 140.3 mm | 142.9 mm | +2.7 mm | The inner bead-seat edge moves 2.7 mm toward the suspension. Brake and strut clearance are not assessed. |
| Speed at indicated 100 km/h | 100.0 km/h | 103.0 km/h | +3.0 km/h | Assumes the gauge was accurate with the current tire. |
Why compare dimensions
Numbers narrow the question
A tire that is 3% taller changes speedometer readings, gearing and ride height. A wheel 10 mm further out changes where it sits against the fender. Neither is obvious from the markings on the sidewall.
Comparing a proposed setup against the current one turns a guess into a measured difference. You can then judge it with the right reference: your vehicle, your suspension, your local rules.
Methodology and accuracy
Conventions are written down
Formulas, units and the wheel-position convention are documented, along with how the calculations are tested and where nominal geometry stops being reliable.
Guides
Read the reasoning behind the numbers
Limitations
A calculation is not a fitment check
Next step