Axle Gearing, Tire Size, Wheels, and Highway RPM

Bigger tires are not automatically better

A larger tire raises the vehicle, changes gearing effectively, increases rotating mass, and loads wheel bearings and brakes harder. Decide the smallest tire that meets your ground clearance and traction goals — then plan gearing to match.

Matching front & rear ratios

In a part-time 4WD system, engaged front and rear driveshafts must turn at the same speed on pavement — so front and rear axle ratios must match. This is not negotiable without an inter-axle differential.

4L60E behavior at cruise

• Overdrive ratio is approximately 0.70 — plan cruise RPM accordingly.
• Torque converter lockup is essential at highway speed with big tires — otherwise trans temperatures climb quickly.
• Crawl ratio = 1st gear × T-case low × axle ratio. Multiply your numbers, don’t guess.

Wheel considerations

• Backspacing affects scrub radius and steering effort.
• Load rating must exceed the corner load with margin — verify with a stamped load-index.
• Bearing load rises with tire diameter and offset. Big/heavy tires can shorten wheel-bearing service intervals.

Speedometer calibration

Tire and axle-ratio changes require reprogramming the PCM or intercepting the VSS with a signal converter. Plan for this before the first road test.