Start with your current setup.
Read the tire marking and wheel specifications. Include an existing spacer in your current effective offset.
Wheel and tire comparison calculator
Where do I find these specs?
Read the tire size on its sidewall. For a wheel marked 7.5J × 17 ET40, enter a width of 7.5 in and offset of 40 mm; 17 is the rim diameter in the tire size. Check wheel markings or product documentation. Include any existing spacer in the current effective offset.
What changes
INSET · INNER EDGE7.7 mm closer to the suspension.
POKE · OUTER EDGE17.7 mm farther outward, toward the fender.
Relative to your existing wheel’s bead-seat edges. Remaining clearance is not measured.Overall diameter+0.38%
+2.4 mm taller
Nominal radius+1.2 mmHalf the diameter change
60.23 mphEstimated actual speed, assuming the original setup read accurately.
ⓘ
Position changes are calculated; fitment is not confirmed. Check actual tire dimensions and clearance through steering and suspension movement. Also verify bolt pattern, center bore, hardware, brake clearance and load requirements. Calculation assumptions ↗
THE BIG PICTURE
Wheel-position comparison
ExistingRim width 165.1 mmEffective ET 45.0 mmTire Ø 631.9 mm
NewRim width 190.5 mmEffective ET 40.0 mmTire Ø 634.3 mm
Existing NewNominal geometry
Red is existing; yellow is new. Both share the same axle and vehicle mounting reference. Edge changes are measured at the bead seats, excluding rim lips. Tire casing, rim profile and suspension are illustrative; the diagram does not predict tire stretch or remaining clearance.
EXISTING205/55R166.5″ wide · ET45
→NEW225/45R177.5″ wide · effective ET40
Explore your next setup
Use these specs when shopping. Confirm vehicle compatibility with the retailer.
Detailed measurements
| Measurement | Existing | New | Change |
|---|---|---|---|
| Overall diameter | 631.9 mm | 634.3 mm | +2.4 mm |
| Section width | 205.0 mm | 225.0 mm | +20.0 mm |
| Sidewall height | 112.8 mm | 101.3 mm | -11.5 mm |
| Circumference | 1985.2 mm | 1992.7 mm | +7.5 mm |
| Revolutions / mile | 811 | 808 | -3 |
| Nominal backspacing | 127.5 mm | 135.3 mm | +7.7 mm |
| Axle centerline track change | — | — | +10.0 mm |
ONE QUESTION AT A TIME
All the math, explained ↗The right tool for the job.
UNDER THE SURFACEUseful numbers.
Useful numbers.
Clear boundaries.
Compare the dimensions you know. Understand what moves. Then check the things a calculator can’t measure.
Read our calculation methodology ↗Compare a proposed setup.
Use Position to compare inset and poke, Diameter for tire height, and optional 3D for proportions. The measurements update as you edit. Red is existing; yellow is new. All changes are proposed minus current.
Confirm the physical fit.
Brake clearance, actual tire dimensions, bolt pattern, load and speed ratings, and suspension movement need separate verification.
A few good questions.
Does this tell me whether wheels will fit my car?+
It compares nominal dimensions and wheel positions. It cannot confirm brake, body or suspension clearance, bolt pattern, hub compatibility, or load and speed ratings. Those require vehicle and product specifications and physical checks.
Why do actual tires differ from these numbers?+
A size marking describes nominal dimensions. The actual diameter, section width and rolling circumference vary with tire model, measuring rim, load, pressure and wear. Use manufacturer measurements for close tolerances.
What does a positive diameter change mean?+
The proposed tire has a larger nominal diameter. Assuming the original speedometer was accurate and calibration is unchanged, estimated actual speed is higher than indicated.
Can I share a comparison?+
Yes. Share comparison copies a link with your inputs. Anyone opening it can see and adjust the same setup; no account is required.