YOU KNOW WHAT GRINDS MY GEARS? POOR RATIOS

The Rundown On Gears

Years ago, before a giant automotive aftermarket emerged and gave us almost limitless options for how we can modify our trucks, old gearheads were figuring out how to retrofit non-original (but factory-made) parts from other vehicles to customize their own rides the way they wanted. By increasing the engine power and changing gear ratios using parts that were made for other models, hot rodders were figuring out how to effectively increase their vehicle’s rate of acceleration. There were a number of ways to increase the power output of an engine, but proper gear multiplication was how to really take advantage of the engine power. Changing the overall gear ratio of a vehicle changes the top speed, the rate of acceleration, and the power to the wheels (the ability to do work).  With no gear multiplication, vehicle speeds would be limited to whatever the engine was churning out throughout its rpm band. That is why we have transmissions in vehicles that start us out in a low gear that revs the engine up quickly when more torque is required for acceleration, and then shift up to reach taller gears that keep rpms lower at higher speeds.
Some Terms
Let’s define a couple terms before we go any further: ‘overall gear ratio’ and ‘final drive ratio.’ The overall gear ratio (OGR) in a vehicle is the combination of the transmission gear, transfer case gear (when applicable), under drive (when applicable) and axle gearing—basically everything between the engine and wheels, versus one wheel revolution. This OGR changes each time you shift gears in your transmission, but if labeled with just a single ratio then you should assume that the OGR was calculated using the highest (typically overdrive) gear in the transmission and with the transfer case in high range (at 1:1). The final drive ratio (FDR) can be defined as the number of driveshaft revolutions it takes to make one revolution of the wheels. It is usually the same as the axle’s ring-and-pinion gear ratio (though the FDR would also need to include any planetary gearing reduction at the hubs, if an axle was so equipped). But neither overall gear ratio nor final drive ratio factor in tire size—and the tire diameter can greatly affect the power put to the ground as well as the number of revolutions an engine spins in a given driven distance. Look at a taller tire as a longer leverage arm. Increasing the distance between the axle’s center point and the ground makes the engine require more energy to turn that leverage arm and do the same amount of work. For practical purposes of this article, let’s refer to this ratio—with all gears and tire diameter factored in—as the ‘overall drive ratio’ (ODR). When we are referring to an ideal ODR, it is one that keeps the engine in the correct power band (close to stock) where it operates most efficiently. If the engine rpms are too high, you’ll be consuming too much fuel. If the rpms are too low, you’ll be struggling for power and also wasting fuel as the engine loads down. When you increase the size of your truck’s tires you are adding a larger leverage arm, subsequently altering the overall drive ratio and taking the engine out of its ideal power band. This can hurt your power output, your fuel economy, and your transmission as it tries to shift through gears based on your speed and engine rpm. By changing your axle gears, you can offset the ill effects of adding larger tires—all except for the added weight and inertia that the larger rolling mass makes.
What it means to you
Now that we’ve covered why proper gearing is important and why we should re-gear trucks with larger tires, let’s discuss how this pertains to you. Most gear changes are done with an intention to return the rpms to what they originally were (when stock) at cruising speeds. That being said, off-road enthusiasts tend to jump to the next highest gear ratio to keep the rpms a little higher for more available power and to offset for a heavier tire with more drag. Conversely, diesel trucks owners tend to keep gear ratios a little lower in an effort to load up the engine a little more and get the turbo spooled up, offering more boost and theoretically increasing efficiency while keeping available power up. Besides that, diesel engines typically create much more torque through the power band than their petroleum-fueled counterparts. This means that you can add larger tires to a diesel truck without as severe of consequences as when doing it to a gas truck. If your truck was equipped with 33-inch tire stock and you upgraded to a 35-inch tire, you can probably get away with skipping the gear job. If you’re adding a big lift and 40s under your diesel rig, you should definitely add new gears. You should do the simple calculations using your new tire size, old tires size, and original gear ratio to find what new ratio you should be running, but factors like engine power, transmission gearing, or even drag and resistance may lead you to deviate from the standard formulas.  If you need some help with your gear choices, don’t be afraid to ask a good drivetrain shop; That’s what they’re there for!
Ring & Pinion Tidbits

Ring and pinion gears can be machined with either a two-cut or a five-cut process. Two-cut teeth are the same height at the heel and toe of the gear (inside and outside of the gear teeth), while a five-cut gear tooth is shorter at the inside (the toe) of the gear and taller at the outside edge (heel) of the teeth. Ring-and-pinion gearsets in the drive axles of our vehicles fit together in a perpendicular orientation, transmitting the power from the driveshaft and redirecting it 90 degrees to the axle shafts. The ring gear (also called a crown gear) rides on a carrier—a differential with broached openings on both sides to accept splined axle shafts that the wheels bolt to. The ring gear and the pinion gear are hypoid gears with a fixed number of teeth that create a ratio signifying how many turns of the pinion gear (attached to the driveshaft) it takes for one revolution of the ring gear (or wheel). If the ring gear has 37 teeth and the pinion has 10, you divide the ring’s tooth count by the pinion’s tooth count for a remainder of 3.70 (essentially a 3.73:1 gear ratio, or close enough to work with that ratio in a two-axle 4WD setup).

Common factory ring & pinion gear ratios in trucks are 3.55:1, 3.73:1 and 4.10:1. These ratios mean that it takes 3.5, 3.7, or 4.1 turns of the pinion to accomplish one revolution of the ring gear (or driveshaft revolutions to turn the wheels one revolution). The most popular gear ratio changes for trucks are 4.56:1, 4.88:1, and 5.13:1 (or close to these ratios, depending what axle you’re working on).

You can use this simple formula to help figure out what gear ratio you should be running with your new bigger tires:
Common Axle Gear Ratios in Diesel Trucks for Selected Tire Sizes

Gear Installation

Proper gear installation is dependent on two things: backlash and pinion depth. Both of these affect how the ring and pinion gears mesh together for a proper pattern, as they are machined, lapped, and pre-run in sets (so you cannot interchange one ring or pinion for another). It is necessary to set up the proper gear pattern with the correct amount of gear mesh. The pinion depth refers to how far the pinion head is from the center point of the carrier. It is also known as the checking distance and is often marked on the pinion face. The pinion depth is always set first before installing the ring gear/carrier and setting the backlash. Backlash refers to how much the tooth space exceeds the thickness of the gear tooth when meshing together. In other words, backlash is how closely the gear teeth sit together and for street gears you should be between 0.008 and 0.012 inches. If an assembler is lucky, gears will set up correctly the first time. If not, you’ll have to pull the carrier back out and start again to try and get the correct pattern. Patterns are shown by marking the teeth with gear marking compound and spinning them together. The marking compound will show how the gears mesh and if the pattern is correct or favoring the toe or heel of the gear.

All this being said, if you’ve never set up gears before then don’t try it on your nice diesel truck. Take it to an experienced shop, or at the very least call a friend with prior gear installation experience before you delve into your own gear swap. Gear installation not only takes experience but also requires specific tools like a pinion depth setter and inch-pound torque wrench, and unless you’re planning on doing gears for a few more vehicles, the cost of the specialty tools and time involved can outweigh the cost of a professional installation. Plus, there’s no warranty if you screw up the install.
High Pinion Vs. Low Pinion Gears

Proper gear installation is dependent on two things: backlash and pinion depth. Both of these affect how the ring and pinion gears mesh together for a proper pattern, as they are machined, lapped, and pre-run in sets (so you cannot interchange one ring or pinion for another). It is necessary to set up the proper gear pattern with the correct amount of gear mesh. The pinion depth refers to how far the pinion head is from the center point of the carrier. It is also known as the checking distance and is often marked on the pinion face. The pinion depth is always set first before installing the ring gear/carrier and setting the backlash. Backlash refers to how much the tooth space exceeds the thickness of the gear tooth when meshing together. In other words, backlash is how closely the gear teeth sit together and for street gears you should be between 0.008 and 0.012 inches. If an assembler is lucky, gears will set up correctly the first time. If not, you’ll have to pull the carrier back out and start again to try and get the correct pattern. Patterns are shown by marking the teeth with gear marking compound and spinning them together. The marking compound will show how the gears mesh and if the pattern is correct or favoring the toe or heel of the gear.

All this being said, if you’ve never set up gears before then don’t try it on your nice diesel truck. Take it to an experienced shop, or at the very least call a friend with prior gear installation experience before you delve into your own gear swap. Gear installation not only takes experience but also requires specific tools like a pinion depth setter and inch-pound torque wrench, and unless you’re planning on doing gears for a few more vehicles, the cost of the specialty tools and time involved can outweigh the cost of a professional installation. Plus, there’s no warranty if you screw up the install.
Gear Ratios & Carrier splits

There is a carrier (differential) split that occurs between two gear ratios in most axles. At a certain ratio point, differential manufacturers offer a different carrier with a ring gear seat that is moved slightly closer to the pinion gear. This other version of the carrier is offered because the pinion gear head of a numerically high ratio is smaller and has less teeth, increasing the distance of gear tooth contact from the seat of the ring gear on the carrier. In a Dana 60, Dana 70, or GM 14-Bolt, the split is between 4.10:1 and 4.56:1. In a Dana 44 the split is between 3.73:1 and 3.92:1 and in a Dana 80 it’s between 3.73:1 and 4.10:1.

If you already have an aftermarket diff and are changing to a ratio beyond the carrier split, there are some manufacturers who make thick ring gear sets or even offer spacers to put a ratio like 4.88:1 on a 3.73:1 carrier.
Upgrading Differentials/Lockers

When changing gears, you have the option of adding some type of traction aiding device like a locker or a limited-slip differential. The differential (aka carrier) has to come out of the centersection for the gear change anyway, so you won’t be spending extra money on labor if you do the diff swap while you do the gear swap.

If you do choose to install a traction aid, we would suggest a selectable locker or a limited slip differential for best street performance. A limited slip differential deters a differentiation in wheel speed but does not totally stop it. A selectable locker locks both tires together but can be turned completely off to run like a normal open differential on the street. Either will be an excellent choice, but you might actually find the limited slip differential more preferable on the street than a totally open diff (which a selectable locker essentially is when unlocked).
Gear Ratios & Carrier splits

 Will a gear change affect my fuel economy? How can increasing the engine rpms versus the number of wheel rotations not decrease fuel economy?

Adding a numerically higher gear ratio increases the number of driveshaft rotations it takes to make a single wheel rotation. This subsequently means your engine will need to spin at a higher rpm to rotate the wheel at the same angular velocity as before you did your gear change (theoretically requiring more fuel). While this may seem counteractive, remember that you are using the gear ratio change to offset bigger tires. Bigger tires have a larger circumference and travel a further distance in a given wheel revolution than a smaller tire. A 33-inch tire makes 611 revolutions in a mile, while a 37-inch tire only makes 545 revolutions in the same distance. At any given speed, the angular velocity of the 37-inch tire will be slower than the 33-inch tire. When you add larger tires, you slow down your wheel’s angular velocity to accomplish the same work (the speed at which you are traveling), therefore lowering the engine rpm at any given speed. Therefore, the drop in rpms that larger tires cause will be counteracted by the increase in rpms that a higher gear ratio makes. In ideal circumstances, you would bring the cruising rpms back to near stock numbers.

Will a gear swap change my speedometer reading?

Yes; changing gears will affect your speedometer reading unless your vehicle is equipped with a GPS-controlled speedo. Changing gears after installing larger tires should return a more accurate reading to your speedo, assuming the gear change is done in effort to reach close-to-stock overall ratios. If you have already electronically adjusted your speedometer using a tuner, you will need to readjust the ECU to account for the gear ratio change.

Should I try changing my gears at home?

If you’re even asking this question then you’re probably not experienced enough to try changing your gears by yourself. Your best bet is to take it to a shop. If you insist on trying at home, lure/persuade a friend with gear experience to come over and help you. Experience is highly valuable when setting up gears.

Are certain gear ratios physically weaker than others?

The gear ratio is derived from the number of teeth on the ring gear versus the number of teeth on the pinion gear. When you start reaching higher ratios like 5.38:1 or 6.17:1, the pinion gear can start to get a little small in certain axles (conversely, a 4.56:1 gearset would use a larger pinion gear). While the pinion shaft and bearings and support sizes all remain the same, the smaller gear head can become a weak point in certain axles with high ratios. This is something you’ll likely never have to worry about with anything you drive on the street as we can’t imagine anyone using anything beyond a 5.13:1 gearset in the axles of any driven diesel truck.

What kind of price tag should I be expecting?

You should expect to pay between $200 and $400 for a good gearset, and you’ll also need an install kit for another $100 to $200 (per axle). This does not include the cost of labor, which can vary greatly depending on what shop, who you know, etc. Don’t be surprised by a quote of $500 for a good gear setup.

Do gears wear out?

Axle gears, like any other moving part, can eventually wear out. Without heavy abuse, you should see 100,000 miles or more out of a high-quality gearset before it goes bad. But some people hammer on their stuff so hard that they are able to chip teeth and cause premature wear to the setup in just a few thousand miles. Every application and driver is different.