Final Drive Calculator
See the overall reduction between engine and wheels for one transmission gear and differential/final-drive combination.
How to use this calculator
- Enter the transmission ratio for the exact gear you want to analyze. A different gear produces a different overall reduction even though the axle ratio is unchanged.
- Enter the differential or final-drive ratio. If the driveline includes an active transfer-case reduction, multiply that additional stage into the overall ratio separately.
- Enter engine RPM and torque at the same operating point if you want wheel-side speed and torque estimates that correspond to each other.
- Use an efficiency value only as good as your source. The field represents combined mechanical losses and is not a universal constant for all drivetrains.
- Remember that wheel torque is not the same as force at the road surface; tire rolling radius must be included to convert torque into tractive force.
Formula and assumptions
Overall ratio = transmission ratio × final drive. Wheel-side RPM = engine RPM ÷ overall ratio. Wheel-side torque ≈ engine torque × overall ratio × efficiency, before accounting for tire leverage.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
A 1.40:1 transmission gear and 3.73:1 final drive produce 1.40 × 3.73 = 5.222:1 overall reduction. At 3,000 engine rpm, the axle or wheel-side rotational speed is about 574 rpm. With 250 N·m engine torque and 90% assumed drivetrain efficiency, estimated wheel-side torque before tire-radius leverage is about 1,175 N·m.
Frequently asked questions
Is wheel torque the same as tractive force?
No. Tractive force at the contact patch is approximately wheel torque divided by loaded tire radius, with additional real-world effects such as tire grip and compliance.
Should I include transfer-case ratio?
Yes when a transfer-case reduction is active. Multiply it into the transmission and final-drive ratios to obtain the complete mechanical reduction.
Why does first gear show so much more wheel torque?
First gear usually has a higher numerical transmission ratio. That provides greater torque multiplication but reduces wheel speed for the same engine RPM.
Can this compare two axle ratios?
Yes. Hold engine RPM, engine torque, selected transmission gear and efficiency constant, then compare the final-drive values. This isolates the mathematical effect of the axle ratio.
Does this include tire size?
No. This calculator stops at rotational wheel-side quantities. Use an RPM/speed calculator when tire circumference and road speed are part of the question.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
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