Gear Ratio Calculator Find the right gear ratio for your new tire size
Original Tire Size
→
New Tire Size
Current Effective Ratio
3.37:1
With 3.73:1 gears + new tires
Recommended Ratio
4.13:1
To restore stock performance
Closest Available
4.1:1
Standard gear ratio
Tire Diameter Change
+10.84%
RPM at Highway Speed (Top Gear)
| Speed | Stock RPM | New Tires RPM | Difference |
|---|---|---|---|
| 45 mph | 1,484 | 1,339 | -145 |
| 50 mph | 1,649 | 1,488 | -161 |
| 55 mph | 1,814 | 1,636 | -177 |
| 60 mph | 1,978 | 1,785 | -193 |
| 65 mph | 2,143 | 1,934 | -210 |
| 70 mph | 2,308 | 2,083 | -226 |
| 75 mph | 2,473 | 2,231 | -242 |
| 80 mph | 2,638 | 2,380 | -258 |
How to Use This Calculator
Select your original (stock) tire size, the new tire size you're installing, your current axle gear ratio, and your transmission's top gear ratio. The calculator shows:
- Current effective ratio: What your gearing effectively becomes with larger tires (lower number = taller = less torque).
- Recommended ratio: The axle gear ratio that would restore your original driving characteristics.
- RPM chart: Engine RPM at common highway speeds for stock tires vs. new tires, so you can see whether you'll be lugging the engine.
Gear Ratio Formula
Effective Ratio = Axle Ratio × (Original Circumference / New Circumference)
Recommended Ratio = Axle Ratio × (New Circumference / Original Circumference)
RPM at Speed = (Speed × 26822) / Circumference × Axle Ratio × Trans Ratio
Recommended Ratio = Axle Ratio × (New Circumference / Original Circumference)
RPM at Speed = (Speed × 26822) / Circumference × Axle Ratio × Trans Ratio
Example
Chris — re-gearing after installing 35" tires on his Jeep Wrangler
Chris's JK Wrangler had stock 255/75R17 tires (32.0" diameter) with 3.21:1 axle gears. He installed 315/70R17 tires (35.0" diameter) — a 9.4% increase. His effective gear ratio dropped from 3.21 to 2.93, making the Jeep sluggish with high RPMs just to maintain highway speed. The calculator recommends 3.51:1 gears. The closest available option is 3.55:1, which Chris installed. Now his RPM at 65 mph is within 50 RPM of where it was with stock tires.
Frequently Asked Questions
Your vehicle's effective gear ratio depends on both the axle gear ratio and the tire circumference. Larger tires have a greater circumference, so the wheel covers more ground per revolution. This effectively lowers the gear ratio (making it "taller"), reducing torque at the wheels and lowering engine RPM at any given speed. This is why trucks and Jeeps with larger tires often need to re-gear — to restore the power delivery and driving characteristics they had with stock tires.
Use this calculator to find the recommended ratio. Enter your stock tire size, new tire size, and current gear ratio. The calculator shows what effective ratio your current gears provide with the larger tires and what new gear ratio would restore stock-equivalent performance. Match the recommended ratio to the closest available gear set for your differential.
With larger tires and stock gears, you'll experience: lower engine RPM at highway speeds (potentially below the power band), sluggish acceleration especially from a stop, the transmission hunting between gears on hills, increased fuel consumption as the engine works harder in lower gears, and premature transmission wear. A 10% tire size increase typically requires going from about 3.73 to 4.10 gears to compensate.
The axle ratio (also called ring and pinion ratio or final drive ratio) is the gear reduction in the differential — typically between 2.73:1 and 5.38:1 for trucks and SUVs. The transmission ratio is the gear multiplication within the gearbox for each gear. In top gear (overdrive), this is typically 0.63-0.80:1. The overall drive ratio = transmission ratio × axle ratio. This calculator accounts for both to show accurate RPM at speed.