Big Tires, Low Gears?
Q So I lifted my (any make and model of 4x4 here), and I went to buy big tires. The guy behind the counter asked me if I was going to regear my truck. I told him I have plenty of gear, I have a recovery rope, a fire extinguisher, a first aid kit, and a full cooler. (That last one was my own idea.) He shook his head and said, “No, I mean regear the axles.”
What is he talking about?
A You’re moving right along with your 4x4, Freddy. Gears are an important consideration when going to taller tires, but let’s lay some groundwork first.
Your engine, when running, turns gears in your transmission. The lower gears multiply the torque of the engine to get the mass of the truck moving. As you go up in gears, the multiplication reduces until you get to a drive gear of 1:1 or an overdrive gear of some fraction (usually around 0.75:1). In your transfer case the gearing is usually 1:1 in high range, but low range increase the multiplication of torque to something lower. Most factory transfer cases range from 1.95:1 to 2.73:1 low range. Some go as far as 4:1 low range, but those are rare. The lower the gearing, the more torque multiplication applied to the driveshafts and down to the axles, which is good for most off-road situations.
In addition to all the gearing in your transmission and transfer case, there are also gears in your axles. These are known as the ring-and-pinion gears, and they have a ratio the describes the number of rotations the pinion (which is attached to the driveshaft) turns for each single rotation of the ring gear (which is attached to the wheels and tires). Increasing the size of the tires requires more torque to get them moving, whether you are in low or high range, so it helps to “lower your gear ratio.” This means you want to increase the number of times the pinion turns for each ring gear rotation. An axle with 3.73:1 gears means the pinion turns 3.73 times for each rotation of the ring gear. If you go to taller tires, you want more rotations of the pinion gear, so you want a higher number such as 4.10, 4.88, 5.13, or 5.38. Going to a higher number is known as getting lower gears.
A simple tire gear calculator is to multiply your new tire diameter and your old gear ratio, then divide by your old tire diameter. Say you have a Ford with 30-inch tires and 3.50 axle gears, and you just lifted the truck to fit your new 35s:
(35 x 3.50) ÷ 30 = 4.08
This is very close to the common gear ratio 4.10, but we don’t stop there. Because you not only have a taller tire but also have a heavier tire and more rolling resistance to get it moving, I recommend you go down at least one gear ratio just to get back to factory performance, in this case a 4.56:1 ring-and-pinion. (If fuel economy is a priority, I would stick with the 4.10 ratio.) If you are going to spend more time in the dirt and crawling over rocks and stuff, then you may go even lower, such as 4.88:1.
To recap, you will need to know your current tire diameter and gear ratio and your new tire diameter to determine what your new gear ratio should be.
Locker or Limited Slip?
Q I went back to the 4x4 shop and told them I need lower axle gears. We discussed the best ratio for my (any make and model of 4x4 here), and as I was getting ready to order they asked if I wanted to keep it open, get a limited slip or maybe some lockers or a spool. I kind of went crossed-eyed with confusion. What are all those things?
A Freddy, now you’re getting to the good stuff. Lockers, limited slips, and spools are some of the most interesting components you can put in your 4x4. In fact, they are what make your vehicle into a true 4x4, where all four wheels are turning and clawing for traction.
Believe it or not, most two-wheel drive vehicles are only one-wheel drive because they have what is known as an open differential in the rear axle. This allows the wheels to turn at different speeds when rounding a corner, but it also sends driving power to the wheel with the least traction. So if one tire is on ice or in mud, it will spin, but the other tire will not, and you will go nowhere. Same thing if your 4x4 has open differentials front and rear. You really only have two-wheel drive (one wheel in front, one in the rear), even when you are locked in four-wheel drive.
To get true four-wheel drive, you need some sort of differential that evenly distributes driving power to each wheel. These are known as limited slips, lockers, and spools.
A limited slip has a system of clutches or gears that notice wheel spin and attempt to lock together both axleshafts for even drive distribution. A limited slip often requires special axle lubricant additive and can work great, but because of the clutch-style design they can wear out or lose their locking ability under hard load, and you may be back in two-wheel drive.
Lockers are the next step up the ladder from limited slips. These are designed to lock both axleshafts together, either automatically or by your selection, to evenly distribute power without clutches to fail. Now, some people get confused by lockers, locking hubs, and a locked transfer case. These are three very different things. Locking hubs are found on some front axles inside the wheels, and they allow drive power to turn the tires. A locking transfer case either locks the case to distribute power evenly to both front and rear axles, or unlocks the case to send power only to the rear axle, or has a variable all-wheel-drive setting. These AWD and 2WD settings are for use on hard surfaces, such as asphalt roads.
Within the locker or locking differential syndicate, we mentioned automatic or selectable lockers. Automatics sense any tire spin and lock the axles together automatically. They can unlock when you’re making turns, but can also produce odd on-road characteristics if the tires are not identical in size, so watch those tire pressures. Selectable lockers use a cable, air pressure, or an electric switch to engage the lockers, allowing zero differentiation between wheel speed from side to side and even power distribution to each wheel, even if one is off the ground. Selectable lockers are better on-road because you can turn them off, resulting in an open locker, but they are often more expensive and rely on cables, hoses, or wires, which can be misinstalled or damaged. Plus, you may simply forget to switch it on.
A spool is a locking device that allows for no differentiation between wheel speeds, and it cannot be turned off. Spools can result in difficult steering and increased tire wear, but they are tough and cheap. Spools are either purchased, or made by welding up the internal part of an open differential, but in that case they are only as strong as the welds and original carrier.
I prefer a selectable locker if the driver is skilled, knows how and when to use it, and can have it properly installed. But an automatic locker is hard to beat for “install and forget about it” off-roaders. Spools are best for 4x4s that only go off-road or drive minimally on the street.