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Rock Crawling Gear Calculator

2026-09-27 6 min

Competition-style crawl ratio and idle-crawl speed at 1000 RPM for big tires.

Crawl Ratio
82.08 :1
Speed At 1000 RPM
1.34 mph

Calculator

Transmission Ratio(:1)
:1
Transfer Low Ratio(:1) Help
:1

Aftermarket cases reach 4-5:1

Axle Ratio(:1)
:1
Tire Diameter(in)
in
Results update live as you type

Live statistics

Crawl Ratio

82.08 :1

Speed At 1000 RPM

1.34 mph

Results

Optimal

Crawl Ratio

82.08:1

Speed At 1000 RPM

1.34mph

Planning estimate from standard automotive formulas. Validate with dyno, timeslip or datalog data when precision matters.

Visualizations

Results overview

82.1Crawl Ratio
1.3Speed At 1000 RPM

Crawl Ratio vs. Transmission Ratio

Crawl Ratio

82.1:1
20200

Formula

Standard gear-to-speed identity evaluated at 1000 RPM in the deepest low-range stack.

MPH @1000 = 1000 x tire / (crawl x 336)

Variables

SymbolNameUnitDescription
336MPH constant-Unit conversion for tire-in and revs
CCrawl ratio:1trans x transfer x axle

Explanation

1000 x 37 / (82.08 x 336) = 1.34 mph — true walking-pace crawling on 37s.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Record the inputs.

Transmission Ratio:4 :1
Transfer Low Ratio:4 :1
Axle Ratio:5.13 :1
Tire Diameter:37 in
Result: 4 inputs ready
Step 2

Apply the formula.

MPH @1000 = 1000 x tire / (crawl x 336)
Result: Primary result = 82.08 :1
Step 3

Read every derived output.

Crawl Ratio:82.08 :1
Speed At 1000 RPM:1.34 mph
Result: Crawl Ratio = 82.08 :1
Final Answer
82.08

Worked Examples

See how the calculator works with different scenarios.

Scenario

Built crawler with 4:1 case.

Inputs

Trans:4.00
Transfer:4.00
Axle:5.13
Tire:37 in

Calculation

Crawl 82.08; 37,000/27,579 = 1.34 mph

Result

82.08:1, 1.34 mph at 1000 RPM.

Interpretation

Idle-crawls ledges with minimal brake work.

Understanding Your Result

Your Result: Crawl Ratio: 82.08 :1

Typical range: Crawl 80-150:1; speed 0.8-2 mph at 1000 RPM.

What This Means

Deepest crawl plus the actual ground speed it produces at fast idle.

Why It Matters

Speed-at-RPM tells whether the rig walks obstacles or lunges — the number drivers feel.

Professional Interpretation

Treat the primary result as a planning estimate, then validate on a dyno, at the track, or with logged sensor data. If measured numbers disagree by more than ~10%, re-check inputs (units are the usual culprit) before changing parts.

Recommendations

  • •Double-check units on every input before trusting the result
  • •Compare against the worked example below to sanity-check magnitude
  • •Validate with measured data (dyno, timeslip, datalog) when it matters

Learn — fuel economy, deeply

Rock gearing converts idle RPM into inches-per-minute of deliberate, controllable motion.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

4 · Measure like a pro (the 3-tank protocol)

  1. Fill to first click at the same pump, record the odometer (Tank 0).
  2. Drive normally for 2–3 tanks; always refill to first click at the same pump.
  3. MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
  4. Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
  5. Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

  • •Size gears to tires first, then the case
  • •Doublers stack two lows for 150:1+
  • •Watch oiling in deep aftermarket cases

Common Mistakes

Mixing units (inches with mm, psi with kPa)
Convert every input to the unit shown on its field before calculating.
Trusting a single output blindly
Cross-check with the worked example and, when it matters, a dyno or datalog.

Assumptions

This calculator makes the following assumptions to provide estimates:

Steady-state model: Formulas assume stabilized conditions; transients, heat soak and traction limits are excluded.
Standard factors: Empirical constants (BSFC, VE, drivetrain loss) use typical values — your combination may differ.

Limitations

Be aware of these limitations when using this calculator:

Estimate, not measurement: Predictions carry real-world scatter of roughly ±5–15% depending on the model.
No transient effects: Shift time, wheelspin, spool lag and heat soak are not modeled here.

Frequently Asked Questions

Answers to common questions about this calculator.

Where does 336 come from?
It bundles unit conversions (inches, miles, minutes): mph = rpm x tire-dia / (ratio x 336) is the standard automotive gearing identity.
Can crawl be too deep?
Yes — past ~150:1 wheel speed gets too slow for ledges needing momentum, and engine braking can get abrupt.

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Related Guides

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Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG
Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km
Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe
Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE
Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance
Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag
Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling
Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II
On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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