How Much Does Tire Size Affect Gear Ratio

| How-To

Why Modify Gear Ratios?

If the Tires Are Bigger, the Gears Should Exist Lower

If you're new to the off-road world or unfamiliar with the manner the axles on your truck piece of work, then you lot might exist unfamiliar with why y'all'd want to bother changing the gear ratios in your truck's axles.

Modifying the axles' gear ratios changes the corporeality 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 farther) at the same engine rpm (theoretically using the same amount of energy). Unfortunately, the theoretical earth does not conduct over to reality in some cases. Calculation a taller tire puts more infinite betwixt the ground and the axle center signal, which lengthens the leverage arm. This decreases the amount of force practical at the end of the arm (torque to the footing 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, calculation larger tires to your truck almost never helps economic or acceleration operation. 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 tin actually get amend fuel economic system and run across almost no ability loss.

To counteract the effects of larger tires, you tin change the gears in your differential(southward) to increment the ratio of how many revolutions of the driveshaft information technology takes to brand i 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 mill.

This will normally create the optimum balance for best fuel economy and best power while at the same time correcting the speedometer (in virtually vehicles), realigning proper shift points in automated transmissions, and relieving stress on the rest of the powertrain prior to the pinion.

What does all this hateful? Well, a happier You, really. Having the correct gear ratio for your truck and its tires will requite you the best fuel economic system, improve acceleration, throttle response, and your ability to maintain highway speeds.

Find What Gears To Run

Later you add taller tires to your truck or SUV, Information technology's fairly uncomplicated to figure out what gear ratio you'll demand 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 irresolute gears for taller tires, you are heading to a numerically higher gear ratio. The college the gear ratio, the smaller the pinion gear. You lot do not want go too low in an effort to increment power to the wheels and/or save stress in the rest of the drivetrain. Too small of a pinion, and it volition go the weak point in your drivetrain.

For many axles, pinions start to get a little scrawny at the 5.xiii:1 or 5.29:ane ratios.

Pictured here is a smaller 5.13:ane pinion gear (left) next to a larger 4.56:1 pinion gear (right) for a Super Duty rear axle.

While Yous're In There…

If you're paying to do a gear modify, you may want to take advantage of the fact that it will exist little or no price in labor to add a locking or express skid differential at this signal. The old gears take to exist removed and new ones set regardless, and it may actually exist easier and less fourth dimension 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.