Crawl Ratio Calculator (4×4 Gearing)

How it works

crawl ratio = 1st gear × transfer-case low × axle ratio · speed at idle (mph) = idle rpm × tyre diameter (in) ÷ (336 × crawl ratio) · wheel torque ∝ crawl ratio

Crawl ratio is the total gear reduction between the crankshaft and the wheels in the lowest gear, and it decides two things off-road: how slowly the truck can move with the engine at a comfortable speed, and how much torque reaches the tyres before the clutch or converter has to slip. Multiplying first gear, low range and the axle ratio gives it directly; the tyre diameter then turns it into a ground speed at idle, which is the number you feel on a ledge. Automatics need less numeric reduction because the torque converter multiplies at low speed, and the cheapest way to add a lot is a lower transfer-case gear set, which leaves highway gearing alone — unlike axle gears, which change everything.

Worked example

A Jeep with a 4.46 first gear, 2.72 low range and 4.10 axles has a crawl ratio of 49.7:1, creeping at 1.5 mph at a 700 rpm idle on 35″ tyres — a strong trail ratio for an automatic. Swapping in a 4.0:1 low range lifts it to 73:1 (+47%) and idle speed drops to 0.97 mph; 4.88 axle gears instead would give 59:1 but also change highway RPM.

Frequently asked questions