Grimmjeeper Gear Calculator

GrimmJeeper Gear Ratio Calculator

The GrimmJeeper Gear Ratio Calculator is an essential tool for off-road enthusiasts, particularly those with Jeeps, looking to optimize their vehicle's performance. Understanding your vehicle's gear ratios is crucial for achieving the right balance between highway drivability, fuel efficiency, and off-road crawling capability.

Why Gear Ratios Matter

Your vehicle's gearing system consists of several components: the transmission, transfer case, and axle differentials. Each of these contributes to the overall final drive ratio, which dictates how many times your engine's crankshaft rotates for each rotation of your wheels. This ratio directly impacts:

  • Crawl Ratio: A lower (higher numerically) crawl ratio means slower wheel speed at a given engine RPM in low range, providing more torque for navigating challenging obstacles and precise control.
  • Highway Speed & RPM: The gear ratio affects your engine RPM at cruising speeds. Too high (numerically low) a ratio can lead to a "bogged down" feeling and poor acceleration, while too low (numerically high) can result in excessive RPM, poor fuel economy, and increased engine wear on the highway.
  • Power Delivery: Proper gearing ensures your engine operates within its optimal power band for various driving conditions, whether you're climbing a steep hill or cruising on the interstate.
  • Tire Size Compensation: When you install larger tires, you effectively "raise" your gear ratio (lower numerically), making the engine work harder. Re-gearing your axles (installing numerically higher axle ratios) compensates for this, restoring lost power and drivability.

How to Use This Calculator

This calculator helps you determine your vehicle's crawl ratio and estimated speed at a target engine RPM for each gear, in both high and low range. Input your vehicle's specifications:

  • Tire Diameter (inches): The actual measured diameter of your tires.
  • Axle Ratio: The gear ratio of your front and rear differentials (e.g., 3.73, 4.10, 4.88).
  • Transfer Case High Ratio: Typically 1.00 for 2H/4H.
  • Transfer Case Low Ratio: The reduction ratio when in 4-Low (e.g., 2.72, 4.00).
  • Transmission Gear Ratios: The individual ratios for each forward gear of your transmission.
  • Target Engine RPM: The RPM at which you want to calculate your speed (e.g., 2500 RPM for a typical cruising or off-road RPM).

The results will show your crawl ratio and the estimated speed in miles per hour (MPH) for each gear, allowing you to compare different setups and understand their impact on your vehicle's performance.

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Calculation Results

Enter your vehicle's specifications and click "Calculate Gear Ratios" to see the results.

function calculateGearRatios() { var tireDiameter = parseFloat(document.getElementById('tireDiameter').value); var axleRatio = parseFloat(document.getElementById('axleRatio').value); var tCaseHigh = parseFloat(document.getElementById('tCaseHigh').value); var tCaseLow = parseFloat(document.getElementById('tCaseLow').value); var trans1st = parseFloat(document.getElementById('trans1st').value); var trans2nd = parseFloat(document.getElementById('trans2nd').value); var trans3rd = parseFloat(document.getElementById('trans3rd').value); var trans4th = parseFloat(document.getElementById('trans4th').value); var trans5th = parseFloat(document.getElementById('trans5th').value); var trans6th = parseFloat(document.getElementById('trans6th').value); var targetRPM = parseFloat(document.getElementById('targetRPM').value); var resultDiv = document.getElementById('gearResult'); var output = '

Calculation Results

'; // Validate inputs if (isNaN(tireDiameter) || tireDiameter <= 0 || isNaN(axleRatio) || axleRatio <= 0 || isNaN(tCaseHigh) || tCaseHigh <= 0 || isNaN(tCaseLow) || tCaseLow <= 0 || isNaN(targetRPM) || targetRPM 0) { crawlRatio = (trans1st * tCaseLow * axleRatio).toFixed(2); output += 'Crawl Ratio: ' + crawlRatio + ':1'; } else { output += 'Crawl Ratio: N/A (1st Gear Ratio not provided)'; } output += 'Target Engine RPM: ' + targetRPM + ' RPM'; output += 'Estimated Speed (MPH) at ' + targetRPM + ' RPM:'; output += ''; var gearRatios = { '1st': trans1st, '2nd': trans2nd, '3rd': trans3rd, '4th': trans4th, '5th': trans5th, '6th': trans6th }; for (var gearName in gearRatios) { var transRatio = gearRatios[gearName]; if (!isNaN(transRatio) && transRatio > 0) { var finalDriveHigh = transRatio * tCaseHigh * axleRatio; var finalDriveLow = transRatio * tCaseLow * axleRatio; var speedHigh = (targetRPM * tireDiameter) / (finalDriveHigh * MPH_CONSTANT); var speedLow = (targetRPM * tireDiameter) / (finalDriveLow * MPH_CONSTANT); output += ''; } else { output += ''; } } output += '
GearHigh Range (MPH)Low Range (MPH)
' + gearName + '' + speedHigh.toFixed(2) + '' + speedLow.toFixed(2) + '
' + gearName + 'N/AN/A
'; resultDiv.innerHTML = output; } // Initial calculation on page load for default values document.addEventListener('DOMContentLoaded', function() { calculateGearRatios(); });

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