If you’re new to the off-road world or unfamiliar with the way the axles on your truck work, then you might be unfamiliar with why you’d want to bother changing the gear ratios in your truck’s axles.
Modifying the axles’ gear ratios changes the amount of revolutions of the driveshaft per each revolution of the wheel. Going to lower gears (ironically, a numerically bigger ratio) will increase the rpms of an engine (taking for granted the same diameter tire and going the same speed).
When you add taller tires to your truck or SUV, the larger circumference of the tires allows your vehicle to travel a further distance at the same engine rpm. This effect is taking better advantage of the work being done by allowing you to do more work (travel further) at the same engine rpm (theoretically using the same amount of energy). Unfortunately, the theoretical world does not carry over to reality in some cases. Adding a taller tire puts more space between the ground and the axle center point, which lengthens the leverage arm. This decreases the amount of force applied at the end of the arm (torque to the ground at the contact patch of the tire). Making matters worse, it takes more torque (which uses more fuel) to move a heavier wheel-and-tire combo, and bigger tires are almost always heavier. Therefore, adding larger tires to your truck almost never helps economic or acceleration performance. The small exception this rule may be the diesel truck. Some diesels trucks have so much torque that the effects of adding a mildly taller and heavier tire are negligible and they can actually get better fuel economy and see almost no power loss.
To counteract the effects of larger tires, you can change the gears in your differential(s) to increase the ratio of how many revolutions of the driveshaft it takes to make one full revolution of the wheel (raising the engine rpm at the same speed). Swapping out to “lower” gears will offset some effects of larger tires and (if you gear appropriately) will put your engine rpm versus speed close to where it was from the factory.
This will usually create the optimum balance for best fuel economy and best power while at the same time correcting the speedometer (in most vehicles), realigning proper shift points in automatic transmissions, and relieving stress on the rest of the powertrain prior to the pinion.
What does all this mean? Well, a happier YOU, really. Having the correct gear ratio for your truck and its tires will give you the best fuel economy, improve acceleration, throttle response, and your ability to maintain highway speeds.
Find What Gears To Run
After you add taller tires to your truck or SUV, It’s fairly simple to figure out what gear ratio you’ll need or what rpm you’ll be cruising at with these two formulas:
rpm= mph x gear ratio x 336 / tire diameter
(new tire / old tire) x original axle ratio = new axle ratio
Don’t Go Too Low
When changing gears for taller tires, you are heading to a numerically higher gear ratio. The higher the gear ratio, the smaller the pinion gear. You do not want go too low in an effort to increase power to the wheels and/or relieve stress in the rest of the drivetrain. Too small of a pinion, and it will become the weak point in your drivetrain.
For many axles, pinions start to get a little scrawny at the 5.13:1 or 5.29:1 ratios.
Pictured here is a smaller 5.13:1 pinion gear (left) next to a larger 4.56:1 pinion gear (right) for a Super Duty rear axle.
While You’re In There…
If you’re paying to do a gear change, you may want to take advantage of the fact that it will be little or no cost in labor to add a locking or limited slip differential at this point. The old gears have to be removed and new ones set up regardless, and it may actually be easier and less time consuming for your drivetrain specialist to set up a new gearset around a new differential.
This ARB differential added a little more cost to the setup of our WCD gears since it required running an air line out of the axlehousing.