Crawl ratios, low range and portal hubs

Crawl ratio = first gear × axle ring-and-pinion × transfer-case ratio × doubler ratio × portal ratio. Use 1 for an absent or direct-drive stage. For example, 4.00 × 4.88 × 2.72 × 1 × 1.22 = 64.77:1. A second 2.72 reduction raises that to 176.17:1. These are illustrative inputs, not a factory vehicle configuration.

In the calculator, keep the axle field as the actual ring-and-pinion ratio. Enter the portal ratio separately. If a supplier quotes an effective axle ratio that already includes the portals, separate it into its physical stages before using both fields.

Save both high and low transfer-case ratios, then select the active range. A separate reduction box can be included as a doubler. When entering a combined four-speed transfer-case ratio, leave the separate doubler off so the same reduction is counted only once.

Advance Adapters lists different Atlas low-range gearsets and a separate planetary stage for its four-speed cases. Use the actual tooth-count ratio or current assembly specification when precision matters; a marketed ratio can be rounded.

Ground speed depends on rolling revolutions per mile: speed in mph = engine RPM × 60 ÷ (total reduction × tire revs/mile). Use measured rolling revs/mile when available. Larger tires increase speed at the same engine RPM and mechanical ratio.

The displayed crawl ratio does not include torque-converter multiplication. Converter slip, tire slip, load and throttle can change real crawling behavior. A fixed slip percentage is a simplified speed estimate, not a stall or traction model.

Switch back to high range before judging highway RPM or calibrating the speedometer. Identify the originating speed sensor. Wheel sensors, driveshaft sensors and gearbox-output sensors respond differently to changes downstream of their pickup.

For four-wheel drive, verify compatible front/rear overall ratios and rolling tire dimensions. The calculator describes one selected axle; it cannot certify driveline compatibility.