What is a gear ratio?
The axle (differential) gear ratio is the number of teeth on the ring gear divided by the number on the pinion gear. A 3.73 ratio means the driveshaft turns 3.73 times for every turn of the wheels. Higher numbers, often called "lower" gears, give stronger acceleration and towing but higher engine RPM on the highway.
Gear ratio and RPM formulas
RPM = mph × axle ratio × transmission ratio × 336 ÷ tire diameter (in)
Ratio for new tires = old ratio × new tire diameter ÷ old tire diameter
The constant 336 converts miles per hour and inches into revolutions per minute (63,360 inches per mile ÷ 60 minutes ÷ π).
Worked examples
Cruising RPM
A 41-tooth ring and 11-tooth pinion give a 3.73 ratio. On 31.6 inch tires in a 0.70 overdrive gear at 65 mph, the engine turns about 1,803 rpm.
Re-gearing after bigger tires
Going from 31 to 33 inch tires on a 3.73 axle makes the gearing taller, as if the axle were 3.50. To restore the original feel you need 3.73 × 33 ÷ 31 = 3.97, so the usual choice is 4.10 gears. At 65 mph in 0.70 overdrive on 33s, RPM goes from about 1,728 with 3.73 gears to about 1,899 with 4.10s.
Common axle ratios
| Ratio | Typical use |
|---|---|
| 3.08 to 3.42 | Highway fuel economy, light loads |
| 3.55 to 3.73 | All-round trucks and SUVs, light towing |
| 3.92 to 4.10 | Heavier towing, 33 to 35 inch tires |
| 4.56 to 5.38 | Large off-road tires, rock crawling |
Need the diameter of your tires? Get it from the size on the sidewall with the tire size calculator.
Frequently asked questions
How do I calculate gear ratio?
Divide the number of teeth on the ring gear by the number on the pinion gear. A 41-tooth ring and 11-tooth pinion give a 3.73 ratio.
What gear ratio do I need for 33 inch tires?
Multiply your current ratio by the new tire diameter and divide by the old one. From 31 inch tires with 3.73 gears, you need about 3.97, so most owners choose 4.10.
Is a higher gear ratio better for towing?
Yes. A numerically higher ratio, such as 4.10 instead of 3.55, multiplies torque at the wheels, which helps towing and acceleration at the cost of higher highway RPM and usually lower fuel economy.
How do I calculate engine RPM at a given speed?
Multiply the speed in mph by the axle ratio, the transmission gear ratio and 336, then divide by the tire diameter in inches.
More car calculators
Last updated October 2026.