305/40R18
305/40R18 is uncommon as OEM fitment but appears as an aftermarket upsize option. At 27.61″ diameter with a 4.80″ sidewall (40% of section width), this size turns 731 revolutions per mile.
About 305/40R18
305/40R18 is less common as a factory fitment but is available as an aftermarket size, so SKU selection is narrower than mainstream OEM sizes. At 27.61″ overall diameter, 305/40R18 turns 731 revolutions per mile, which is the reference point the factory speedometer and odometer were calibrated against. The 40% aspect ratio gives it a low-profile sidewall that delivers sharp steering response but transmits road imperfections. The 18″ rim pairing is typical for full-size truck, premium SUV, and sport-sedan platforms, so compatible wheels are easy to source new or used. Going one step shorter to 295/40R18 drops diameter by 1.14%, which stays within the ±3% fitment safe zone for most vehicles. Stepping up to 285/45R18 adds 1.78% diameter — also inside the safe window, with marginally taller ride height. If you are shopping for a direct replacement, any tire within roughly ±0.7″ of the 27.61″ stock diameter (about ±2% in percentage terms) will keep ABS, TPMS, and odometer readings inside the vehicle's designed tolerance.
Compare with another size
275/65R18 or "33" for inch aliases305/40R18
- Diameter
- 27.61″ / 701.2mm
- Section Width
- 12.01″
- Sidewall
- 4.8″ / 122mm
- Circumference
- 86.73″
- Revs/Mile
- 731
285/40R18
- Diameter
- 26.98″ / 685.2mm
- Section Width
- 11.22″
- Sidewall
- 4.49″ / 114mm
- Circumference
- 84.75″
- Revs/Mile
- 748
Speedometer & Diameter Impact
At an indicated 60 mph, your actual speed is 58.63 mph (94.36 km/h). Your speedometer reads 2.28% high versus actual speed.