Calculators · Tyres and gearing

Gearing change, gear spacing and shift point calculator

Changing an axle ratio, comparing gearboxes or choosing when to change gear all come down to ratios between gears. These calculators show how a new final drive changes cruising RPM and wheel torque, how far RPM drops between two gears, which RPM to shift at to land in the right part of the rev range, and the crawl ratio and crawl speed of an off-road drivetrain.

Effect of a new final drive ratio

Shows how engine RPM at a given road speed changes when you fit a different axle or differential ratio. A numerically higher ratio gives stronger acceleration and pulling power at the cost of higher cruising RPM.

Results
New RPM at the same speed—rpmEngine speed with the new ratio.
Change in RPM and wheel torque—%RPM and wheel torque both change by this percentage.

Formula
new rpm = old rpm × new ratio / old ratio

RPM drop between gears

When you change up, road speed stays the same, so engine speed falls in proportion to the two gear ratios. Close ratios keep the engine near peak power; wide ratios suit relaxed cruising and heavy vehicles with broad torque.

Results
RPM after the shift—rpmEngine speed as the next gear engages.
RPM drop—rpmHow far the revs fall.
Ratio step—%How much lower the next gear is than the current one.

Formula
rpm after = shift rpm × next ratio / current ratio
step = current / next − 1

Shift RPM to land at a target RPM

Works out when to change gear so the engine lands at a chosen RPM in the next gear — for example just below peak torque. Useful for setting shift lights or planning gear changes in racing.

Results
Shift at—rpmChange gear at this engine speed. Check it is below the redline.

Formula
shift rpm = target rpm × current ratio / next ratio

Crawl ratio and crawl speed

The crawl ratio multiplies first gear, the transfer case low range and the axle ratio. It decides how slowly and controllably an off-road vehicle can climb over obstacles; the crawl speed shows how fast it moves with the engine at idle.

Results
Crawl ratio—: 1Total reduction in the lowest gear.
Crawl speed—Vehicle speed at that engine speed in the lowest gear.

Formula
crawl ratio = first × low range × axle
speed = rpm / crawl × π × D × 60 / 1,000,000

Worked example: regearing a pickup

  1. Swapping a 3.55 axle for 4.10 raises RPM by 4.10 / 3.55 = 15.5%: 2,500 rpm becomes 2,887 rpm at the same speed.
  2. Wheel torque rises by the same 15.5%, which helps with bigger tyres or towing.
  3. Shifting from a 2.00 to a 1.50 gear at 6,500 rpm drops the engine to 4,875 rpm.
  4. With a 4.71 first gear, 2.72 low range and 4.10 axle, the crawl ratio is 52.5 : 1 — about 2.3 km/h at idle on 800 mm tyres.

Typical crawl ratios

VehicleCrawl ratio
Two-wheel-drive pickupabout 12 – 18 : 1
Four-wheel drive in high rangeabout 12 – 20 : 1
Four-wheel drive in low rangeabout 35 – 60 : 1
Dedicated rock crawlerabout 80 – 150 : 1

Related

Gear ratio, road speed and wheel torque calculatorTyre size calculator and comparisonWheel torque, tractive force and drivetrain loss calculator

About these results

Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.

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