Tire size difference in inches · Plus-one wheel size
225/45R17 vs 245/40R18
245/40R18 is 0.74 in (18.9 mm, 2.98%) taller than 225/45R17. Section width 0.79 in wider. Sidewall 0.13 in shorter. At an indicated 60 mph you'd be doing about 61.79 mph.
When replacing 225/45R17 with 245/40R18
- Overall diameter
- 24.97 → 25.72 in
- +0.74 in (+18.9 mm), +2.98%
- Section width
- 8.86 → 9.65 in
- +0.79 in (+20.0 mm)
- Sidewall height
- 3.99 → 3.86 in
- -0.13 in (-3.3 mm), -3.21%
- Ride height
- +0.37 in
- +9.5 mm, half the diameter change
- Dash reads 60 mph
- 61.79 mph
- +1.79 mph actual vs indicated
- Revolutions per mile
- 808 → 784
- -2.89%, nominal
Each value is rounded on its own, so a change can differ by 0.01 in from the difference between the rounded sizes.
Open this comparison in the interactive calculatorVehicle compatibility not checked. Diameter difference within 3%. These results compare nominal dimensions. They do not confirm vehicle fitment. Actual clearance varies by tire model, approved rim width, wheel construction, suspension, alignment, body tolerances, steering angle, load, and brake geometry. Verify load and speed ratings and physically test-fit the complete setup before driving.
Tire dimensions only. Wheel width and offset are not assumed on this page, so fender-side and suspension-side position changes are not calculated. Enter your wheel specifications in the calculator for those estimates. How we calculate
What changes from 225/45R17 to 245/40R18
- Speedometer: at an indicated 60 mph you're actually doing about 61.79 mph, and about 72.09 mph at an indicated 70 mph.
- Odometer: for every 10,000 miles it shows, you'll have driven about 10,298 miles, 298 more than shown.
- Engine speed: wheel revolutions per mile go from 808 to 784, so at the same road speed the engine turns about 2.89% slower in any given gear.
- Width: 245/40R18 is 0.79 in (20.0 mm) wider. Centered in the same place, each sidewall moves about 0.39 in (10.0 mm) outward; wheel width and offset decide the real position.
- 245/40R18 needs 18 in wheels; it will not mount on 17 in wheels. This is a plus-one size change. On the larger wheel the sidewall is 0.13 in (3.3 mm) shorter. A shorter sidewall generally gives a firmer ride over sharp bumps and leaves less rubber protecting the rim.
- Both sizes have a low profile (45 and 40 series).
- The 2.98% diameter change is above 2%, enough to consider speedometer recalibration. Anti-lock brake, traction and stability control systems also use wheel-speed signals, so check the vehicle maker's guidance for this size.
- A 20 mm width change can move the approved rim-width range far enough that a wheel chosen for the old size falls outside it. Confirm the range for the exact tire model before buying wheels or tires.
- For scale, the 0.74 in diameter change is more than the roughly 0.5 in a tire loses as its tread wears from a typical new 10/32 in to the 2/32 in wear bars. This swap changes the speedometer reading more than a full tread's wear does.
Going from 245/40R18 to 225/45R17 instead: 2.89% shorter; an indicated 60 mph is about 58.26 mph. Open 245/40R18 to 225/45R17 in the calculator
How the setup changes
Proposed 245/40R18 is 0.74 in taller overall.
Shared center and scale. Circles compare diameter and rim size, not loaded ride height.
Speedometer
Indicated vs actual
| Dash reads | Estimated speed |
|---|---|
| 30.00 mph | 30.89 mph |
| 60.00 mph | 61.79 mph |
| 70.00 mph | 72.09 mph |
Assumes the speedometer was accurate with the current tires. Uses diameter ratio.
Detailed comparison
Changes are proposed minus current. Nominal rev/mile uses geometric circumference, not measured rolling circumference. Values round independently; rounded columns may not subtract exactly. Wheel specifications retain industry notation.
| Measurement | 225/45R17 | 245/40R18 | Change |
|---|---|---|---|
| Tire size | 225/45R17 | 245/40R18 | — |
| Nominal section width | 8.86 in (225.0 mm) | 9.65 in (245.0 mm) | +0.79 in (+20.0 mm) |
| Rim diameter | 17 in | 18 in | +1.00 in (+25.4 mm) |
| Overall diameter | 24.97 in (634.3 mm) | 25.72 in (653.2 mm) | +0.74 in (+18.9 mm) |
| Radial sidewall estimate | 3.99 in (101.3 mm) | 3.86 in (98.0 mm) | -0.13 in (-3.3 mm) |
| Geometric circumference | 78.45 in (1,992.7 mm) | 80.79 in (2,052.1 mm) | +2.34 in (+59.4 mm) |
| Revolutions per mile | 808 (nominal) | 784 (nominal) | -23 |
| Diameter change | — | — | +2.98% |
| Ride-height estimate | — | — | +0.37 in (+9.5 mm) |
Physical checks
- Approved rim-width range for the exact tire model.
- Load index and speed rating equal to or greater than vehicle requirements.
- Brake-caliper, hub, and wheel-fastener compatibility.
- Fender, liner, suspension, and full-lock steering clearance.
- Clearance at full compression and under expected load.
- Alignment, tire pressure, and manufacturer guidance.
- Professional inspection when variance or clearance is uncertain.
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Review record
Automated dimensional check against the FitMyTires publication policy (v1) on August 19, 2026. The check covers size data and calculations, not vehicle fitment. Read the editorial and corrections policy.