The Effect of Larger Tires on a Final Drive Ratio

Big tires are a popular mod, but final-drive effects must be taken into account.

Replacing a car's wheel-and-tire package with a larger diameter tire can give it a whole new look and performance envelope. However, the dynamic effect of tire replacement should be taken into account when purchasing.

Final Drive

A vehicle's final-drive ratio is determined by multiplying the transmission gearing and axle gearing. This will dictate engine RPM to wheel RPM at any given speed.

Wheel Travel

How far a wheel will travel per revolution is determined by its diameter. For instance, a tire that is 25 inches tall will travel 78.5 inches per revolution, and one that is 30 inches tall will travel 94.2 inches.

Gearing Effects

If you were to replace a 25-inch tire with a 30-inch tire, the car would travel approximately 20 percent farther per revolution.

Final-drive Effects

If that same vehicle were in top gear with a 3.5:1 final-drive axle ratio, it would act as though those gears were actually 2.8:1. This would cause the engine to run at a lower RPM than it originally did at the same speed.

Side Effects

Because a car with larger-than-stock tires is effectively deceived into believing it's traveling slower than it really is, the stock speedometer will need to be re-calibrated to read correctly.

References

• 4x4xplor.com: Changing Your Jeep Wrangler Speedometer Gear
• MyjeepTJ.com: Tire Size to Final Drive Ratio Chart
• Ecomodder.com: Tire Size Discussion

Resources

• Classic Crusher (Video)