OK, so you’ve got your new Jeep, what’s the first thing you want to do (other than get it muddy, of course)? You want to go put that lift and those big knobby tires on it. Now you’ve got your lift and tires on, its lookin’ sweet, and it’s time for a test drive. Just a spin around the block at first, but something is wrong. You go to step on the gas and the response is sluggish, you take it out on the highway and you find you have to keep shifting down just to keep up with traffic. So what happened? Your new lifted Jeep performs like a snail. The missing piece to this equation is gears. By changing the size of your tires you’ve just raised the effective gear ratio of your vehicle. But I digress; let’s start at the very beginning.
It’s all about the transfer of torque from the engine to the tires and the various components that affect the application of such. Here’s the problem: a typical engine generates between 200 and 300 ft. lbs. of torque, but an average Jeep weighs between 4,000 and 5,000 lbs. So how does 250 lbs of force move 4K lbs of weight? The answer is torque multiplication. Through the use of gear reduction, torque is multiplied to generate exponentially more power. Here is a simple example: an engine with 250 ft.-lbs. of torque driven through a transmission with a 4:1 first gear ratio will produce 1000 ft.-lbs. of torque at the driveshaft. Take that 800 and reduce it again through a typical 3.73:1 differential and you have 3730 ft.-lbs. of torque at the axles. The final piece of the gearing is the tire itself. The smaller diameter the tire the less torque it requires to turn it, while the larger the diameter the more force needed.
Mileage VS Power
When a vehicle is designed by the manufacturer it is engineered for maximum efficiency; the gear ratios are perfectly matched to the tire size typically with gas mileage in mind. Our government mandates that the vehicles they produce meet average MPG standards and therefore most vehicles are not geared with power in mind from the factory – this includes Jeeps. This is why there is such a dramatic effect on performance when going to an even slightly bigger tire.
The solution is to change out differential gears to a lower (numerically higher) ratio. A good rule of thumb is to go up .5 in ratio for every 2 inches in increased tire diameter. But that will just bring you back to a factory, efficiency biased gearing. If you plan to take your Jeep off-road it’s usually best to over-gear a bit. For example, if your Jeep has a factory 3.73 ratio and you are going from a 31” to a 33” tire a 4.56 might be a good choice. Below is a chart to help match tire sizes with appropriate gear ratios.
Re-gearing a vehicle is no small undertaking and must be performed by a skilled technician with special tools. Typically, the labor to install the gears costs as much or more than the parts themselves. It’s also a good idea to consider what other modifications you plan to do in the future, as a re-gear alone can cost in the $2,000 range, and you don’t want to be having to do it multiple times. This is the time to prepare for larger tires, lockers, and bigger axle shafts.
One great (yet more expensive) solution to the gearing issue is to modify or replace the transfer case to get lower ratios for off-road while keeping a more mileage-friendly ratio in the differential. Most factory 4 low t-case ratios are in the 2.5:1 range. However, new gear sets can modify a factory case to 4:1 and aftermarket cases can be bought to go as low as 6:1. Consider how that much torque multiplication can effect final torque numbers.
Lower gearing not only solves the on-road performance issues caused by larger tires but it gives you more control, more power, and decent slower speeds. It can make that lift kit and bigger tires more than just show, but a real performer off-road.