Find Your New Axle Ratio
See how a tire-size change affects effective gearing, cruise RPM, and speedometer accuracy—then compare a stock-feel ratio with a deeper trail-oriented starting point.
A 4.18:1 mathematical target restores the original overall gearing. The closest ratio in the common comparison list is 4.10:1.
Planning estimate only. Actual gear availability depends on the axle, carrier, differential, transmission, engine, and intended use. Confirm the final ratio with a qualified drivetrain installer. Automatic-transmission converter slip is not included.
Measure First. Then Do the Math.
Moving to a taller tire makes the axle ratio feel numerically lower. That can reduce acceleration, change transmission behavior, and move the engine below its preferred RPM range. This calculator shows the ratio needed to restore the same overall gearing.
- Measure both tires. Use loaded diameter from the ground to the top of the tire when possible—not only the size printed on the sidewall.
- Enter the current axle ratio. Check the build sheet, axle tag, differential tag, or count driveshaft and tire revolutions.
- Add top-gear ratio and cruise speed. These values estimate engine RPM. A 1.00 input represents direct drive with no overdrive.
- Compare the recommendation styles. “Restore stock feel” follows the tire-size correction. “Extra crawl / power” adds a clearly labeled 8% deeper planning target.
Common Regearing Questions
Use these answers as a starting point, then verify the final combination for your exact drivetrain.
How do I calculate gear ratio for bigger tires?
Multiply the current axle ratio by the new measured tire diameter, then divide by the old measured tire diameter. Example: 3.73 × 37 ÷ 33 = 4.18. That 4.18:1 figure is the mathematical target that restores the original overall gearing; the actual gear set must be selected from ratios available for your axle.
Should I use advertised tire size or measured diameter?
Use measured, loaded diameter whenever possible. Actual diameter can vary with manufacturer, wheel width, air pressure, tread wear, and vehicle weight. Using the sidewall size is still useful for a quick comparison, but it is less precise.
What does the “extra crawl / power” option do?
It adds 8% to the stock-feel mathematical target before finding the closest ratio in the common comparison list. This can be a helpful planning reference for a heavier trail build, towing, or more responsive acceleration, but it is not a universal prescription.
What will my speedometer read after regearing?
The calculator estimates the displayed speed at your selected real cruise speed using the new tire diameter and recommended replacement gear ratio. A small difference can remain because the closest available gear set might not exactly match the mathematical target. After installation, program the measured tire diameter and actual axle ratio into the Jeep computer. On supported models, a compatible Z Automotive Tazer can enter both values to help recalibrate the speedometer and transmission shift points. Verify compatibility for your exact Jeep year and model.
Do I need to regear both axles on a 4WD Jeep or truck?
Yes. The front and rear axle ratios must match before four-wheel drive is used. Running mismatched ratios can create drivetrain bind and damage. Always verify both axle assemblies before ordering parts.
How accurate is the cruise RPM estimate?
The estimate uses tire diameter, road speed, axle ratio, and transmission top-gear ratio. It does not include torque-converter slip, tire growth, transfer-case reduction, or calibration error, so real-world RPM can differ slightly.
Build It for the Trail
Protect the drivetrain and find technical resources for the next stage of your build.
