High horsepower and torque are great, but gearing can really make the difference as to how you get the power to the ground, and how effectively you can negotiate the terrain you’re on. Whether it’s slow-speed ’crawling or high-speed racing, gearing makes the difference in how you get your vehicle to go where you want it. With a 4WD vehicle, gearing concerns arise in essentially three areas: transmission, transfer case, and axle gearing. Each of those can be tailored to optimize the type of ’wheeling you want to do and how you best utilize your motor torque.
When building a vehicle completely from scratch or when doing a complete drivetrain overhaul, you may have the ability to choose a wide range of ratios in these three areas. Of course, there is the option of two types of transmissions: manual and automatic. Manual transmissions may have the benefit of a low, granny first gear that is great to add additional low gearing on a 4WD. Automatic transmissions also vary a bit with respect to first-gear ratio. On the other end of the cogs, we can find manual and auto transmissions with overdrive top gearing (less than 1:1 final ratio) for highway use.
When planning your drivetrain, first decide how you want to use the vehicle. Will it be used often on the highway or will it be used only off-road or some combination of the two? This will determine at what end of the gearing spectrum you want to concentrate on and how to choose for low- and/or high-speed performance.
You may choose to build the vehicle to have a very wide range of usable speeds. This can often be done, but can depend on how many transfer case components you can accommodate in your chassis. But, with the wide range of transfer cases available today, along with aftermarket accessories, you can build a setup to accommodate a broad range of uses.
When choosing what hardware to use in your drivetrain, the first question to ask yourself is whether the 4WD will see much or any extended highway use. If so, then you’ll have to ensure that you have sufficient gearing to comfortably drive at speeds of 65 mph or faster. If the vehicle is mostly a trail rig you want to use for slow trail-running or crawling, then you can bias the gearing to optimize low speeds and place less emphasis on highway needs.
Manufacturers design vehicles with an overall drivetrain (or final drive) ratio that can provide a good compromise, providing reasonable acceleration (lower gearing) and good highway speed at a practical engine speed (higher gearing). The final drive ratio is what you end up with when you factor in the transmission and transfer case gearing, and the axle gear ratio. Tire size also plays a big role in the final results you get.
What do the proper gears do for your rig? They help maintain decent fuel economy, good acceleration off the line, better highway passing performance, and greater torque and smoother application of power to the tires. When dealing with an existing vehicle, it’s most common to swap only axle gears when needed to accommodate larger tires or other changes. When you’re building up a complete drivetrain, you may have the option to choose other gear ratios as well. We’ll discuss later the implications of those choices with respect to strength.
Changing axle gears to a numerically higher ratio to compensate for the addition of larger tires should not harm engine or transmission life. As long as these components are not rotating excessively fast, their lives will most likely increase with the gearing change. Engines will operate within their optimal powerband, providing smoother power delivery and avoiding excessively low end lugging that can wear bearings over extended time periods.
Automatic transmissions generally benefit due to less slip and heat build up as the result of running at too low of engine rpm. For those trannies with lock-up torque converters in the higher gears, a properly geared rig will more quickly shift up to these gears and lock up the converter, reducing heat buildup in the tranny. Manual transmissions can also benefit as clutches will wear less quickly and provide better performance with big tires if the proper axle gear ratios are used.
Late model vehicles use the speedometer reading as one input to the on-board computer or ECM (engine or electronic control module). Whenever you add larger tires and change the expected speed, as seen by the ECM, this will most likely affect your vehicle engine performance. Swapping to gearing that corrects the overall axle ratio to compensate for the larger tires will also correct the signal to the ECM.