2025 Tire Size Gear Ratio Calculator
Calculate how tire size changes affect gear ratio, speedometer accuracy, and vehicle performance. Input tire dimensions, axle ratio, and transmission ratio for precise results.
Tire and Gear Details
Tire Size and Gear Ratio Guidelines
| Parameter | Typical Range | Impact | Notes |
|---|---|---|---|
| Tire Width | 175-305 mm | Wider tires may slightly increase diameter | Common for passenger cars: 205-245 mm |
| Aspect Ratio | 30-70% | Lower ratios reduce sidewall height | Lower ratios for performance, higher for comfort |
| Rim Diameter | 14-22 in | Larger rims often paired with lower profiles | 16-18 in common for cars |
| Axle Ratio | 2.5-4.5 | Higher ratios increase torque, lower top speed | Check vehicle specs for accuracy |
| Transmission Ratio | 0.7-3.0 | Affects RPM at given speed | 1.0 is typical for direct drive |
Frequently Asked Questions
How does tire size affect gear ratio?
Larger tires increase the effective gear ratio, reducing engine RPM for a given speed, while smaller tires decrease it, increasing RPM.
How do I correct my speedometer after changing tire size?
Use the calculator to find the speedometer correction factor by comparing old and new tire diameters, then adjust the speedometer calibration or use a reprogramming tool.
What is the effective gear ratio?
The effective gear ratio accounts for tire diameter changes, calculated as (new tire diameter / old tire diameter) × original gear ratio.
Will changing tire size affect my vehicle's performance?
Yes, larger tires may improve handling but reduce acceleration, while smaller tires may improve acceleration but affect stability.
How do I find my vehicle's axle ratio?
Check your vehicle's owner manual, differential cover, or manufacturer specifications.
Tips for Tire Size and Gear Ratio Adjustments
- Verify tire dimensions from the sidewall (e.g., 205/55R16) for accurate inputs.
- Check your vehicle’s axle ratio in the owner’s manual or with a mechanic.
- Consider reprogramming the speedometer after significant tire size changes.
- Larger tires may require suspension adjustments to avoid rubbing.
- Consult a professional for major tire or gear ratio changes to ensure compatibility.
Disclaimer: Calculations are based on 2025 industry standards. Consult a mechanic or vehicle specialist for precise adjustments.