Tire size calculator

How do you compare two tire sizes?

Enter your current and new tire size. TireFitLab instantly shows the difference in overall diameter, sidewall and width, the speedometer error, and whether the swap stays within the selected tolerance.

Looks valid
Looks valid
Units
Editorial preset

These are TireFitLab editorial diameter-screening presets. Regional names choose a preset, not a national legal limit, ETRTO rule or vehicle approval. The AWD preset does not establish an allowable drivetrain mismatch. Use the exact vehicle and tyre manufacturer instructions; nominal geometry alone cannot approve fitment.

Requires verificationWithin tolerance · Diameter +1.7% (+0.43 in) · US preset (±3%)
  • Requires verification · This rating covers tire geometry only, not vehicle compatibility.
  • Diameter +1.7% · Ride-height change +0.21 in.
  • Speedometer reads low · 60 mph → 61.0 mph.
  • Sidewall -0.29 in · Width +0.39 in.
Diameter+1.7%+0.43 in
Speedo @ 60+1.7%60 mph → 61.0 mph
Sidewall-0.29 inWidth +0.39 in
Ride-height change+0.21 inDiameter ÷ 2
How this result is calculated

Computed from nominal tire-size geometry: diameter, circumference and speedometer error. It does not check vehicle placard approval, load/speed rating, rim-width range, wheel-arch, suspension or brake clearance.

Methodology

Before fitting

  • Check the vehicle placard and owner’s manual first.
  • Match or exceed the required load index and speed rating.
  • Confirm rim-width range, bolt pattern, hub bore, offset and fasteners.
  • Inspect wheel-arch, suspension and brake clearance under real vehicle load.
  • Check TPMS, cold pressure and local roadworthiness rules.
Diameter
24.97 in 25.40 in+0.43 in
Sidewall
3.99 in 3.70 in-0.29 in
Width
8.86 in 9.25 in+0.39 in
Circumference
78.45 in 79.80 in+1.7%
Revs / mile
808 794
Speedo @ 60
60 mph 61.0 mph+1.7%
Ride-height change
0.00 in +0.21 in
True distance per 100 indicated
100 mi 101.7 mi+1.7%
Δ⌀ +0.43 inΔ↔ +0.39 in
  • Current — 225/45R17
  • New — 235/40R18
  • Grounded to the road: the axle rises +0.21 in.

Tires and wheels in one comparison

Result report

225/45 R17 235/40 R18 · Requires verification · Within tolerance

Plan budget for this size: 235/40 R18

Copy/print uses this page state only; TireFitLab does not create an account or store a server-side report.

Plan budget for this size
Embed or cite this result
Open embed preview
Estimated budgetBudget$85–$120Per tireMid-range$110–$160Per tirePremium$160–$225Per tire

Geometric estimate. Always verify physical clearance (arches, suspension, brakes) before fitting.

In-tolerance alternative sizes

In-tolerance alternative sizes
SizeDiameterSpeedo @ 100Use this
−1″ 225/50R16-0.5%-0.5%
OE 225/45R17+0.0%+0.0%
+1″ 225/40R18+0.5%+0.5%
+2″ 225/35R19+0.9%+0.9%

Worked examples

These rows use the same geometry engine as the calculator, so the numbers match the interactive result above.

Worked examples
Current sizeCandidate sizeDiameter deltaSpeedometer at 60 mphTolerance
225/45R17235/40R18+1.7% (+10.9 mm)60 mph → 61.0 mphwithin
205/55R16225/45R17+0.4% (+2.4 mm)60 mph → 60.2 mphwithin
215/65R16225/60R17+2.3% (+15.9 mm)60 mph → 61.4 mphwithin

What to do with it: Use the exact vehicle placard size before buying.

Quick comparison

Start with the vehicle placard

Enter the listed tire size and the replacement size you want to compare.

Size comparison only. Vehicle fitment and clearance need separate checks.

Seasonal check

Winter tire rules coming up?

Check tread depth, markings and local winter requirements before the first cold spell instead of waiting for the appointment rush.

Open winter tire guide

Compare sizes

Popular sizes

More tools