How to Calculate Crawl Ratio: Formula, Worked Example & Low-Range Tables

For how to calculate crawl ratio, the point is this: Crawl ratio is the total gear reduction available in the lowest gear: first gear × low range × axle ratio, and it decides how slowly and how strongly a vehicle can move over an obstacle. A high crawl ratio lets the engine idle the vehicle forward at walking pace with a large torque multiplication at the wheels, which is what off-road driving needs when traction is poor and the surface is uneven. This page explains the formula, works through a complete example with stated numbers, and shows how changing the axle ratio or the transfer case changes the crawl ratio — with the reminder that every number here is an example to be replaced by the figures for the actual vehicle.

The formula is crawl ratio = first-gear ratio × transfer-case low-range ratio × axle ratio, and because it multiplies three numbers, a small change in any one of them moves the result noticeably. A vehicle with a 4.46 first gear, a 2.72 low range and a 4.10 axle ratio has a crawl ratio of 49.7:1, meaning the engine turns almost fifty times for each wheel revolution in low range. The worked example below shows the arithmetic, the table shows how the crawl ratio changes with different axle ratios, and the rest of the page covers what a high crawl ratio does on the trail, what it costs on the road, and the mistakes people make when they chase a bigger number. That is the point this page works through for how to calculate crawl ratio.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

how to calculate crawl ratio: How the gear ratio formula works

The arithmetic is straightforward once the four inputs are clear. RPM = (speed in mph × gearbox gear ratio × axle ratio × 336) ÷ tyre diameter in inches. The 336 folds together the 5,280 feet in a mile, the 12 inches in a foot and the 60 minutes in an hour, so it is a constant of the units rather than a property of the vehicle. If the gearbox is in a 1:1 gear, the gear ratio is 1.00 and the formula shortens to RPM = (speed × axle ratio × 336) ÷ tyre diameter. Rearranged, speed = (RPM × tyre diameter) ÷ (gear ratio × axle ratio × 336), which answers the opposite question: how fast is the vehicle going at a given engine speed. Every table in this article is built from those two formulas with the assumptions stated next to it, so any line can be re-checked. That is the point this page works through for how to calculate crawl ratio.

how to calculate crawl ratio: What a gear ratio is

On this page, how to calculate crawl ratio is the case in point: The axle ratio is the final multiplication in the driveline. The engine's torque passes through the gearbox, where each gear multiplies it by its own ratio, then through the differential, where the ring and pinion multiply it once more. A 3.73 ratio multiplies the torque arriving at the differential by 3.73 before it reaches the wheels, while a 4.10 multiplies it by 4.10. That extra multiplication is why a numerically higher ratio feels stronger off the line and holds speed better on a grade with a trailer behind it. The trade is engine speed: at the same road speed and in the same gearbox gear, the higher ratio turns the engine faster, which is why cruising RPM rises when a shorter ratio is fitted. The ratio itself is set by the tooth counts of the ring gear and pinion, so it is a physical fact about the axle rather than a setting that can be adjusted from the driver's seat.

how to calculate crawl ratio: Worked example: crawl ratio from first gear, low range and axle ratio

Taking how to calculate crawl ratio as the case in point: Take a gearbox with a 4.46 first gear, a transfer case with a 2.72 low range, and a 4.10 axle ratio. The crawl ratio is 4.46 × 2.72 × 4.10 = 49.7:1, so the engine turns 49.7 times for each wheel revolution in first gear and low range. At an idle of 700 RPM the wheels turn at 700 ÷ 49.7 = 14.1 RPM, and with 33-inch tyres that is a road speed of about 1.4 mph — slow enough to pick a line over rocks without touching the brakes. Change only the axle ratio to 4.56 and the crawl ratio becomes 4.46 × 2.72 × 4.56 = 55.3:1; change the low range to 4.00 and it becomes 4.46 × 4.00 × 4.10 = 73.1:1. Each input multiplies, so the table below shows the crawl ratio for the common axle ratios with the same example first gear and low range.

Crawl ratio table: axle ratio compared for how to calculate crawl ratio

On this page, how to calculate crawl ratio is the case in point: Assumption: first gear 4.46 and low range 2.72 (examples — use the figures for the actual gearbox and transfer case).

Axle ratio Crawl ratio Wheel RPM at 700 engine RPM
3.73 45.2:1 15.5 RPM
4.10 49.7:1 14.1 RPM
4.56 55.3:1 12.7 RPM
4.88 59.2:1 11.8 RPM

For how to calculate crawl ratio, the point is this: The table shows why the axle ratio is the easiest crawl-ratio change to make and why it is also the bluntest: moving from 3.73 to 4.88 lifts the crawl ratio from 45.3:1 to 59.2:1 with the example gearbox and transfer case, a 31% increase in torque multiplication and a 31% drop in crawl speed. A lower transfer-case ratio — a 4.00 low range instead of 2.72 — moves the number further still, but it is a bigger job and a bigger expense. The right crawl ratio is the one that lets the vehicle move slowly enough for the terrain without the engine labouring, and it is judged on the trail rather than on the chart.

how to calculate crawl ratio: How tyre diameter changes the answer

Tyre diameter is the third input and the one people forget. A larger tyre covers more ground per revolution, so at the same driveshaft speed the vehicle travels further — the gearing effectively gets taller, cruising RPM falls, and the engine has to work harder to accelerate the vehicle. That is exactly what happens when bigger tyres are fitted without changing the ratio: the speedometer reads low, the odometer under-records, and the engine sits below the RPM where it makes its best torque. Fitting a shorter ratio restores the balance by turning the driveshaft faster for the same wheel speed. The diameter figures used in this article are nominal new-tyre dimensions computed from the size code; real tyres vary slightly by brand, model, wear and inflation pressure, so treat the RPM figures as accurate to within a few dozen RPM rather than exact to the unit. That is the point this page works through for how to calculate crawl ratio.

What the ratio does on the road for how to calculate crawl ratio

Taking how to calculate crawl ratio as the case in point: A ratio is a trade between pulling power and cruising calm, and the tables below make the trade visible. Raising the number lifts the RPM at every speed: the engine works in a stronger part of its range, the vehicle accelerates harder and tows with less strain, but the motorway cruise gets busier and thirstier. Lowering the number drops the RPM everywhere: the cruise gets quieter and usually cheaper, while acceleration softens and the gearbox works harder on grades. The size of the effect is not subtle — moving from 3.21 to 4.10 is a 28% change in RPM at any speed — so a ratio change is one of the biggest single changes that can be made to how a vehicle drives. It is also why the choice should be made from the RPM table with the actual tyre diameter, not from the ratio number alone.

how to calculate crawl ratio: How to find the ratio already fitted

Taking how to calculate crawl ratio as the case in point: Finding the ratio already fitted is a matter of looking in the right places. The axle tag, usually a small metal tag bolted to the differential cover, often stamps the ratio directly. The door-jamb placard and the owner's manual list the axle and tyre specification for the vehicle as built, and the build sheet or window sticker carries the same information in coded form. A VIN decoder can also return the original axle ratio, though it reports the vehicle as built rather than as it stands now, so it will not catch a previous owner's axle swap. The hands-on check is to count teeth: mark the ring gear and pinion, count the teeth on each, and divide ring teeth by pinion teeth — 41 ring teeth and 11 pinion teeth give 41 ÷ 11 = 3.73. If the tag, the paperwork and the tooth count disagree, the tooth count is the one to trust, because it describes the axle actually fitted.

Common mistakes with gear ratios for how to calculate crawl ratio

Taking how to calculate crawl ratio as the case in point: The most common mistake is changing the ratio without checking the tyre diameter, or changing the tyres without checking the ratio — the two are one calculation, and doing half of it leaves the gearing wrong in the other direction. The second is comparing ratios across vehicles with different tyre sizes: a 3.73 on 35-inch tyres is effectively taller gearing than a 3.73 on 31-inch tyres, so the number alone does not describe the driving experience. The third is forgetting the gearbox: the formula needs the gear ratio of the gear in use, and overdrive gears below 1.00 change the answer, so a table built for top gear does not apply in third. The fourth is treating a calculated RPM as a promise about fuel use — RPM is one input to consumption, not the whole story, and load, aerodynamics and driving style move the result. And the fifth is assuming a ratio change is a fitment approval: it is a calculation, and clearances, load ratings and the placard still need checking with a professional.

how to calculate crawl ratio: Related pages

Frequently asked questions

How do I calculate crawl ratio?

Multiply the first-gear ratio by the transfer-case low-range ratio by the axle ratio. For example, 4.46 × 2.72 × 4.10 = 49.7:1. Use the figures for the actual gearbox, transfer case and axle — the example numbers on this page are illustrations, not the figures for any particular vehicle. That is the point this page works through for how to calculate crawl ratio.

What is a good crawl ratio for off-roading?

Taking how to calculate crawl ratio as the case in point: There is no single right number: it depends on the terrain, the tyre size and the engine. A crawl ratio around 50:1 suits general trail driving, while dedicated rock crawling often uses 70:1 or more. Higher is slower and stronger at the wheels, but it also means more drivetrain parts to maintain. Judge it by whether the vehicle can move slowly enough for the terrain without riding the clutch or the brakes.

Does crawl ratio matter in 4Hi?

In 4Hi the transfer case is in high range, usually 1.00, so the crawl ratio is just first gear × axle ratio — much lower than in low range. Low range is where the crawl ratio matters, because the transfer case multiplies the reduction. That is why low range exists: it gives the slow, high-torque movement that high range cannot. That is the point this page works through for how to calculate crawl ratio.

Does changing the gear ratio affect the speedometer?

On this page, how to calculate crawl ratio is the case in point: Yes, indirectly: the speedometer is calibrated for a rolling circumference, and a ratio change does not change the tyre, so the speedometer reading itself is unaffected by the ratio alone. What changes is the engine speed at a given road speed. A tyre-size change does affect the speedometer, because it changes the rolling circumference — see the speedometer error calculator for that calculation.

Can I change my gear ratio myself?

On this page, how to calculate crawl ratio is the case in point: A ratio change means replacing the ring gear and pinion and setting the backlash and preload precisely, which is a specialist drivetrain job with special tools. An incorrect setup fails quickly and expensively. Most owners have the work done by a drivetrain specialist, who can also confirm the ratio suits the axle, the gearbox and the way the vehicle is used.