225/45R17 vs 245/40R18Tire Size Comparison

Safe

The 3.0% diameter difference is within the accepted 3% tolerance.

225/45R17634mm dia245/40R18653mm dia+9.5mm

Comparison Details

Overall Diameter634.3 vs 653.2 mm (+18.9 mm)
Diameter Difference+2.98%
Section Width225 vs 245 mm (+20 mm)
Sidewall Height101.3 vs 98.0 mm (-3.3 mm)
Circumference1992.7 vs 2052.1 mm
Clearance Change+9.5 mm
Speedometer Error+2.98%
Revs/Mile808 vs 784

Speedometer Correction

When switching from 225/45R17 to 245/40R18, your speedometer will differ from actual speed.

SpeedometerActual SpeedDifference
30 km/h30.9 km/h+0.9 km/h
40 km/h41.2 km/h+1.2 km/h
50 km/h51.5 km/h+1.5 km/h
60 km/h61.8 km/h+1.8 km/h
70 km/h72.1 km/h+2.1 km/h
80 km/h82.4 km/h+2.4 km/h
90 km/h92.7 km/h+2.7 km/h
100 km/h103.0 km/h+3.0 km/h
110 km/h113.3 km/h+3.3 km/h
120 km/h123.6 km/h+3.6 km/h
130 km/h133.9 km/h+3.9 km/h

Vehicles Using These Sizes

Vehicles with 225/45R17

2025 Hyundai ElantraLimited2024 Hyundai ElantraLimited2023 Hyundai ElantraLimited/N Line2022 Hyundai ElantraLimited/N Line2021 Hyundai ElantraLimited2020 Hyundai ElantraLimited/Sport

Vehicles with 245/40R18

2025 Audi A4Premium 40 TFSI2025 Audi A4Premium Plus 45 TFSI quattro2024 Audi A4Premium Plus 45 TFSI quattro2023 Audi A4Premium Plus 45 TFSI quattro2022 Audi A4Premium Plus 45 TFSI quattro2021 Audi A4Premium Plus 45 TFSI quattro2020 Audi A4Premium Plus 45 TFSI quattro2025 Mercedes-Benz C-ClassC 3002025 Mercedes-Benz C-ClassC 300 4MATIC2024 Mercedes-Benz C-ClassC 3002024 Mercedes-Benz C-ClassC 300 4MATIC2023 Mercedes-Benz C-ClassC 3002023 Mercedes-Benz C-ClassC 300 4MATIC

Individual Tire Size Details

225/45R17
Full specifications, compatible vehicles, and alternatives
245/40R18
Full specifications, compatible vehicles, and alternatives

Practical Guidance

Switching from 225/45R17 to 245/40R18 is within the industry-accepted 3% diameter tolerance. Your speedometer, ABS, and traction control should function normally. Note: these sizes require different rim diameters (17" vs 18"), so you will need different wheels.

The 3% diameter tolerance is a widely accepted industry guideline referenced by the ETRTO and most vehicle manufacturers. When in doubt, consult your vehicle's owner's manual or a qualified tire professional. For U.S. tire safety standards and recalls, see the NHTSA Tire Safety page.

Frequently Asked Questions

Safe. The overall diameter differs by 2.98%, width changes by 20mm.

Speedometer error will be 2.98%. At an indicated 100 km/h, actual speed is about 103.0 km/h.

Ground clearance changes by +9.5mm per side.