245/45R18 to 255/40R19 gear ratio change
Effective ratio after tire change
Changing from 245/45R18 to 255/40R19 makes a 3.73 axle behave like 3.68. To restore the original cruise RPM and drivability, the math points toward about 3.78; verify real ratios, transmission gearing and vehicle limits before buying parts.
- Changing from 245/45R18 to 255/40R19 makes a 3.73 axle behave like 3.68.
- To restore the original cruise RPM and drivability, the math points toward about 3.78; verify real ratios, transmission gearing and vehicle limits before buying parts.
FAQ
- Effective ratio after tire change
- Changing from 245/45R18 to 255/40R19 makes a 3.73 axle behave like 3.68. To restore the original cruise RPM and drivability, the math points toward about 3.78; verify real ratios, transmission gearing and vehicle limits before buying parts.
- What should I verify before using this information?
- Use TireFitLab values as a sizing reference, then verify the vehicle handbook, tire placard, rim compatibility, load rating, and physical clearance before fitting.
| Original size | New size | Diameter delta | Reference axle ratio | Effective ratio after tire change | Target ratio to restore RPM |
|---|---|---|---|---|---|
| 245/45R18 26.68 in | 255/40R19 27.03 in | +1.31% | 3.73 | 3.68 | 3.78 |
| Nearest standard axle ratio | Cruise RPM at 100 km/h (62 mph) | Size verdict |
|---|---|---|
| 3.73 | 2920 → 2882 RPM | Within diameter tolerance |
Gear-ratio seed pages use the public TireFitLab size corpus and a fixed 3.73 reference axle so the math is comparable. They are drivetrain estimates, not vehicle-specific fitment advice.
RPM assumes top gear is 1:1. Overdrive transmissions, torque-converter slip and final-drive variants change real vehicle RPM.
How tire diameter changes gearing
Taller tires cover more distance per wheel revolution, so the engine turns fewer times per kilometre with the same axle ratio. Shorter tires do the opposite. The effective ratio expresses that change as if the axle gearing had been altered without opening the differential.
Related drivetrain references
Seasonal check
Planning a long summer drive?
Use the budget and running-cost tools before a trip, especially if the current tyres are worn or the replacement size changes diameter.
What changed
- Added curated gear-ratio change seed pages from deterministic tyre geometry.