Gear Ratio & Tire Size Calculator
Calculate effective gear ratio and RPM at speed after a tire size change. Find the corrected axle ratio.
RPM at Speed Comparison
| Speed | Current RPM | New RPM | Difference | Change |
|---|---|---|---|---|
| 30 mph | 892 | 862 | -30 | -3.4% |
| 45 mph | 1,338 | 1,293 | -45 | -3.4% |
| 60 mph | 1,784 | 1,724 | -60 | -3.4% |
| 70 mph | 2,082 | 2,012 | -70 | -3.4% |
| 80 mph | 2,379 | 2,299 | -80 | -3.4% |
RPM calculated using effective gear ratio of 2.798 (3.73 axle x 0.75 trans x 1.0 t-case)
Suggested Re-Gear
To maintain the same RPM at speed with your new tires, you would need to change your axle ratio:
The nearest commonly available axle ratio is 3.91:1. This is numerically higher (shorter gearing), which will restore low-end power lost from the larger tires.
How It Works
When you change tire sizes, the effective gear ratio changes because larger tires cover more distance per revolution. This calculator shows you exactly how your RPM at various speeds will change.
The formula for RPM at speed is: RPM = (Speed x Gear Ratio x 336) / Tire Diameter, where 336 is the RPM constant that accounts for unit conversions.
The effective gear ratio combines your axle ratio, transmission gear, and transfer case ratio: Effective Ratio = Axle Ratio x Trans Gear x T-Case Ratio.
The corrected axle ratio is calculated as: New Axle = Original Axle x (New Tire Dia / Old Tire Dia).
How to Read a Tire Size
A tire size like P265/70R17 breaks down as:
- P — Passenger tire (LT = Light Truck)
- 265 — Section width in millimeters
- 70 — Aspect ratio (sidewall height is 70% of width)
- R — Radial construction
- 17 — Rim diameter in inches
Flotation sizes like 33x12.50R15 use inches: 33" overall diameter × 12.5" width on a 15" rim.
Frequently Asked Questions
Why does tire size affect my gear ratio?
Larger tires increase the effective gear ratio, making your vehicle behave as if it has taller (numerically lower) gearing. This is because the bigger tire covers more ground per revolution, so the engine doesn't need to spin as fast to reach the same speed. The trade-off is reduced low-end torque and slower acceleration.
How do I calculate the correct axle ratio for my new tire size?
To maintain the same effective gear ratio with larger tires, use this formula: New Axle Ratio = Original Axle Ratio x (New Tire Diameter / Original Tire Diameter). Our calculator above does this automatically and shows the corrected ratio.
What RPM should I be at while cruising on the highway?
Most modern trucks and SUVs are designed to cruise at 1,500-2,200 RPM at highway speeds (60-70 mph) in top gear. If your RPM is significantly lower after a tire size change, you may experience sluggish acceleration and transmission hunting. If it's significantly higher, you'll burn more fuel and the engine will be louder.
Do I need to re-gear after installing larger tires?
A re-gear is recommended if you increase tire diameter by more than 10-15%, especially on vehicles with automatic transmissions. Signs you need to re-gear include: sluggish acceleration, transmission hunting between gears, poor fuel economy, and the engine lugging at low RPM. Common re-gear ratios for 35" tires on a stock 3.73 axle are 4.56 or 4.88.