Gear ratio
Also called axle ratio, ring and pinion ratio, final drive ratio, crawl ratio, regearing
A gear ratio, usually called the axle ratio in a truck, is how many times the driveshaft turns for one turn of the wheels, and a numerically higher ratio trades top speed for torque, which is what a low crawl ratio is built from by multiplying it with the transfer case and first gear.
What is a gear ratio?
Inside every differential a small pinion gear drives a larger ring gear bolted to the axle. Count the teeth on each and divide, and you have the axle ratio. Spicer, an axle maker, describes its own calculator for exactly this: the ratio between the ring gear and pinion gear in the differential housing in the rear end axle of a vehicle. A pinion with 11 teeth turning a ring gear with 41 gives 41 divided by 11, or 3.73:1, meaning the driveshaft turns 3.73 times for every single turn of the wheel.
A numerically higher ratio, 4.56:1 against 3.73:1, means more driveshaft turns per wheel turn: more torque multiplication at the tire and a lower top speed for a given engine RPM. A numerically lower ratio does the opposite: less torque, better fuel economy, a higher top speed. Neither is better in the abstract. It is a tradeoff a manufacturer makes for the vehicle's job, and it is one a builder can undo.
Crawl ratio: what three ratios multiply into
The axle ratio is only one of three multipliers stacked in a four-wheel-drive truck's driveline. Multiply the axle ratio by the transfer case's low-range reduction and by the transmission's first-gear ratio, and the result is the crawl ratio: the total torque multiplication available at the wheels in first gear, low range, before the brakes or the throttle touch anything.
Jeep's own numbers for the Wrangler Rubicon are a usable example of the scale involved: a maximum and improved crawl ratio of 77.2:1 on petrol models and 70.3:1 on diesel, built in part from the Rock-Trac transfer case's 4.0:1 low range. That number is why a stock Rubicon idles up terrain that would stall a taller-geared truck in the same gear, and why the same vehicle in high range, where the transfer case's multiplier drops out, drives like an ordinary truck on the highway.
Why builders regear
Fitting larger tires changes the effective ratio without touching a single gear. A bigger tire covers more ground per revolution, which is the same thing as making the final drive numerically taller, exactly the wrong direction for a truck that just got heavier and wants more clearance, not less torque. The symptoms are familiar to anyone who has done it: sluggish acceleration, an engine that labours and runs hot climbing a grade, a transmission hunting for the right gear.
Regearing, installing a numerically higher ring and pinion to match the new tire diameter, restores the driveline's original crawl ratio and driveability. It is axle work, not a bolt-on, which is why it tends to happen after the tire size is decided rather than before.
See also
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Sources
- Gear Ratio Calculator, Spicer Parts, Dana Incorporated, read 2026-09-24An axle manufacturer's own plain description of what a gear ratio is and how it is counted, from the teeth on the ring gear and the pinion gear.
- All-New Jeep Wrangler, press kit, Stellantis Media, Jeep brand press office, read 2026-09-24The manufacturer's own description of the Rubicon's electronic front sway-bar disconnect, and its published crawl ratio figures of 77.2:1 on petrol models and 70.3:1 on diesel. media.stellantis.com blocks plain fetch and was read through a rendered browser. A press kit is marketing copy rather than an engineering document, so its superlatives are its own; the mechanical description and the published numbers are what we cite it for.