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Getting the Ratio Right; Correcting the Axle Gear Ratio for Oversized Tires

RATIOnale

John Lehenbauer
Mar 14, 2019
Photographers: John Lehenbauer and Steve Flores
For part two of our series about transforming Jared Lehenbauer’s new-to-him ’01 Ford F-250 into a truck that’s better suited to his needs, we’re detailing how to select the correct gear ratio for two- and four-wheel-drive trucks with big tires.
There are probably thousands of reasons why people don’t install the correct rearend gears to compensate for larger tire sizes. Some may not want to spend the money (easily more than $1,000 including labor) while others may feel a diesel truck has an abundance of power and torque so it will not be affected by the tire-size increase. We admit not changing the gears does not have a huge effect on a truck that’s just driven to the store and around town. However, if it tows or hauls heavy loads, there may be a noticeable difference.
Jared purchased the truck with an inherent gear problem. The previous owner lifted the rig and installed 37-inch tires but left the stock 3.73 gears intact. The ratio is fine with the original (approximately) 32-inch tires, as Ford engineers developed this combination (tire size and gear ratio) along with the transmission gearing to establish a drive ratio that provides the best power, economy, and driveability. Even though the 5-inch size difference doesn’t seem like much, a 37-inch tire reduces the 3.73 gear ratio down to about a 3.19.
Photo 2/72   |   When we arrived at Steve Flores’ place, he wasted no time getting the rear of Jared Lehenbauer’s ’01 Ford F-250 up on jack stands and removing the tires. You may be curious about the F-350 badge on the tailgate. No, we did not make a mistake; the truck is definitely an F-250 (the Dana 50 front axle is a giveaway). Somewhere along the line, the badge got switched.
From the factory, a truck’s engine and drivetrain are engineered to work together to provide the maximum amount of power in the most efficient way. The engine revs to the correct rpm and the transmission shifts to the right gear at the proper speed, ultimately providing the most power and economy available. When an incorrect tire-and-gear combination is used, it throws that balance off. Engine rpm and gearshifts will not coincide correctly with the vehicle’s speed on the highway (as originally engineered). The engine’s rpm will be lower, possibly taking the engine out of its prime power range. Now, fueling must be increased and the engine has to be pushed harder to make the same power it made at the previous speed. It can also require a driver or the transmission to downshift in order to get back into the power range (but at a slower speed). It is also possible to find the transmission “between gears” and hunting for the right one, which can be hard on the gearbox if it happens too frequently.
Correcting the gear ratio for compatibility with tire size allows engine speed to return to the proper rpm range and the transmission into the correct gear for the speed being traveled (like stock). This is critical when hauling or towing heavy loads that can put even more strain on the engine and transmission when not in the correct power range.
It is not always easy to know the right gear ratio to use with a particular tire size, so we went to the Internet and searched for a “gear-ratio calculator.” The search brought up a number of different charts posted by manufacturers and businesses, which allow you to figure out the best ratio for the tires you are using. Also, most shops that install gears are good sources of information regarding the proper setup.
Once we understood what was needed, we gave Nitro Gear & Axle a call and ordered a set of 4.30 gears for the Dana 50 front and 10.5-inch Sterling rear axles (as well as new bearings and seals) that will bring the ’01 Ford’s drivetrain as close to stock as possible. Mag-Hytec’s differential covers are being added to give the upgrade a clean finish.
We contacted our buddy Steve Flores (who has been setting up axles for years) for help with the installation. After a little bribery of food and drink, Steve agreed to help us out and give us his expert insight on the proper way to set up front and rear axles with new bearings, seals, and ring and pinion gears.
Photo 3/72   |   Steve pulls the differential cover to drain all the gear oil. Some sludge was found in the housing. Then he removes the driveshaft and checks the yoke to see what condition the pinion bearings are in. Some play and noise are detected, so it’s a good thing the differential is getting all new bearings.
Photo 4/72   |   Correcting The Axle Gear Ratio Diff Yoke
Photo 5/72   |   The left and right axleshafts are popped out so the differential can be removed.
Photo 6/72   |   Steve makes alignment marks on the bearing caps and axlehousing before removing the caps to ensure they are correctly oriented during reassembly.
Photo 7/72   |   The carrier comes out with an easy tug. Normally, a large prybar and some grunting are involved in getting extracting one. Steve says when carrier bearings become worn, they lose the preload that’s exerted on them by the housing and shims. While the centersection is in good overall shape, components such as bearings, gears, and such are just fatigued.
Photo 8/72   |   Correcting The Axle Gear Ratio
Photo 9/72   |   The stock limited-slip differential is in good shape, but the ring gear has seen better days. There is a lot of pitting and some rust on the teeth. When Jared got the truck, he serviced both differentials and found the gear oil in the rear resembled chocolate milk. The oil was contaminated with water, which accounts for the pitting and rust found on the gears.
Photo 10/72   |   Correcting The Axle Gear Ratio Intake Battery Tray
Steve removes the pinion nut and then uses a puller to separate the yoke from the pinion. With the yoke removed, he uses a dead-blow hammer to tap the pinion free from the bearings.
Photo 14/72   |   The pinion seal and front bearing are removed from the centersection.
Photo 15/72   |   Correcting The Axle Gear Ratio Housing Bearing
The bearing races are removed from the housing with a hammer and punch. Some metal shrapnel is found in the front cavity of the housing between the races. Upon inspection of the pinion and bearings, we see the rear bearing (nearest the gear) has spun on the pinion shaft and eaten the bearing shims. This explains the play Steve found on his initial inspection of the yoke/pinion.
Photo 19/72   |   Normally, when replacing the pinion gear and bearings with new pieces, there is no need to remove the old bearing. In this instance, Steve is using the old bearing as a setup bearing (to properly shim the new pinion before pressing on the new bearing). He opens up the inner race with a barrel sander, enabling it to slide on and off the pinion for changing shims easily and not damaging the new bearings.
A bearing puller is used to remove the carrier bearings. The bearing on the ring-gear side did not want to come off with just the puller, so Steve heated the inner race before reinstalling the puller.
Photo 23/72   |   All the new pieces are unboxed for installation.
After removing the bearings and ring gear from the carrier, Steve begins the assembly process. First, he installs the new ring gear with the supplied bolts, making sure to use thread-locking compound on each one. Once the bolts are installed, Steve torques them to specifications.
Photo 28/72   |   New bearing races are installed in the differential housing.
Photo 29/72   |   Steve measures shim thickness to determine what he will use on the pinion. The same-thickness shims are used under the crush-sleeve eliminator. Once Steve gets the test package close to a fitment he thinks might work, he installs the pinion and bearings in the clean axle centersection.
Photo 30/72   |   Correcting The Axle Gear Ratio Diff Pinion Installed
The differential is lifted into the housing then tapped into place with a brass punch and hammer. Steve installs approximately the same number of shims on the outside (between bearing and housing) to get the ring gear and pinion gear mesh close. The bearing caps are then installed.
Photo 34/72   |   The ring gear is painted with a marking compound before the differential is spun to see what the mesh pattern between the ring and pinion looks like. Then, a dial indicator is positioned to determine how much backlash there is so Steve knows what adjustments he needs to make.
Photo 35/72   |   Correcting The Axle Gear Ratio Ring Gear Dial Indicator
Photo 36/72   |   The carrier and pinion are removed and installed two or three more times as Steve tries to determine the correct shimming for all the bearings to center the gears and achieve the mesh pattern he wants.
Photo 37/72   |   Correcting The Axle Gear Ratio Workbench Shims
Photo 38/72   |   When Steve finally gets the gears centered with a good mesh pattern and .007 inches backlash, the carrier and pinion are pulled out one last time.
Photo 39/72   |   Once the proper shimming is determined, the setup bearing is removed from the pinion, and the new bearing is pressed on with the shims.
Photo 40/72   |   Before installing the pinion gear, Steve smears grease on the bearings to ensure they have some lube and will not dry out before gear oil gets to them. The pinion with bearings, shims, and oil slinger is installed with the yoke (and nut), but no seal. An inch-pound torque wrench is used to determine the bearing preload on the pinion and also helps verify all the shimming is correct. The new bearings are set at 30 lb-in. No seal is installed at this time because it adds extra drag to the yoke/pinion and Steve wants an exact measurement of preload.
Photo 41/72   |   Correcting The Axle Gear Ratio Torque Wrench Yoke
Photo 42/72   |   The carrier is installed in the housing with the correct shims, bearing caps, and thread-locking-compound-coated bolts. Bolts are torqued to specification.
A new pinion seal is installed along with the yoke, pinion nut, and washer. Thread-locking compound is used on the pinion nut to make sure it stays in place. Steve taps the yoke with a dead-blow hammer to make sure everything is seated properly. The final step before installing the driveshaft is to torque stripe the pinion nut.
Photo 46/72   |   We put the stock Sterling 10.5 differential cover and the Mag-Hytec replacement side by side for a comparison. The aluminum Mag-Hytec piece holds more fluid and has ports for temperature sensors, a dipstick, and a magnetic drain plug that is definitely a plus when it’s time to service the differentials.
Photo 47/72   |   Before installing the Mag-Hytec cover, the O-ring is installed with a dab of silicone spread over it. Stainless steel hex-head bolts and washers that come with the cover are used.
Photo 48/72   |   Correcting The Axle Gear Ratio Diff Cover Install Housing
Photo 49/72   |   The axleshafts are reinstalled with the bolts torqued to specification. After the driveshaft is set in place, the tires and wheels are put back on and the differential is filled with the proper gear oil.
Photo 50/72   |   Correcting The Axle Gear Ratio Driveshaft Steve Wrench
Photo 51/72   |   Correcting The Axle Gear Ratio Rearend Truck Diff
Photo 52/72   |   With the rear differential now sporting 4.30 gears, it’s time to pull the front end apart. The front-axle steering knuckles make this disassembly a bit more complicated.
Steve begins taking the Dana 50 front axle apart by removing the driver-side tie-rod end, so he can access the differential cover (to drain the oil and remove it). He then removes the locking hub and axleshaft C-clip, which facilitates removing the unit-bearing hub from the knuckle. The axleshaft is then removed. We discovered the outer seal that supports the stub shaft in the knuckle and the thrust washer were worn out, so we had to track down new parts before putting it back together. We also replaced the outer axle seal on the passenger side.
Photo 58/72   |   A dead-blow hammer, a prybar, and a piece of square metal tubing (to protect the sealing surface of the differential housing from damage and provide a better prying angle) are used to remove the front differential.
Photo 59/72   |   With the differential out, Steve begins removing the bearings and ring gear.
Photo 60/72   |   Correcting The Axle Gear Ratio Workbench Ring Gear Bearings Seals
Photo 61/72   |   Reassembling the carrier for the front Dana 50 axle is the same basic process as the rear, with a couple of steps slightly altered. The new ring gear is heated with a torch (to expand the metal slightly for easier fitment) before being installed on the carrier. Another difference is the carrier-bearing shims are under the bearings, which makes them difficult to interchange for setup when the bearing is pressed on. So, the old bearings are opened with a barrel sander so they can be used as setup bearings.
Photo 62/72   |   Correcting The Axle Gear Ratio Carrier Torque Wrench
Photo 63/72   |   On Dana front axles, the shaft seals are pressed into the centersection to keep oil out of the axle tubes. The seals, along with the old pinion-bearing races, are removed and the housing is cleaned of any old oil and debris. With everything cleaned up, Steve taps new axle seals into place with a seal driver. Note: Make sure to use the proper tools to install the axle seals. They are prone to leaking if they’re installed improperly.
Photo 64/72   |   The ring gear is painted with marking compound to check the mesh pattern to see what adjustments need to be made. The carrier and pinion come out several times for shimming until the pattern is correct.
Photo 65/72   |   Once the correct shims for the carrier and pinion are found, Steve removes the setup bearings and presses on the new ones. Like the front pinion, a crush sleeve is not used with the pinion bearings—an eliminator is used instead.
Photo 66/72   |   Correcting The Axle Gear Ratio Pinion Bearings
Photo 67/72   |   Before installing the differential, grease is applied to the axleshaft seals.
Photo 68/72   |   The carrier is installed with the bearing caps and bolts (with thread-locking compound applied). The backlash and preloads are rechecked.
Photo 69/72   |   After installing the yoke and pinion nut, the differential cover is installed. The larger Mag-Hytec cover is designed so it does not interfere with the tie rods when the steering is turned full left or right.
Photo 70/72   |   Finally, the axleshafts, (care is taken not to damage the seals), unit-bearing hubs, locking hubs, and brakes along with the wheels and tires are reinstalled.
Photo 71/72   |   Correcting The Axle Gear Ratio Front Axle Diff Cover Tire

What Difference Did the Gears Make?

Before we installed the gears in the ’01 Ford F-250, we hooked up Jared Lehenbauer’s 30-foot toy hauler loaded with off-road machines, firewood, fuel, and all the essentials for a couple of days away. Our towing route took us on a section of California State Highway 14, which has a couple of good hills that would give us a feel for how well or poorly the truck will climb a grade. On the road, the Banks iDash 5-inch monitor we installed in part one of our series was used to record data during the trip to see how the truck performed. This also gave us an opportunity to look for any other problems the truck may have that might need to be addressed.

The run with the stock 3.73 gears had the truck working hard to get up to speed on the highway. Once there, EGT ranged between 800 and 1,000 degrees Fahrenheit, transmission temperature was around 180 degrees, and engine-oil temp was 208, with an average of about 60 percent load on the engine in Third gear at 60 mph. When we hit the biggest hill on the highway, the transmission had to be dropped into Second gear with 100 percent engine load at 2,800 rpm to keep the truck close to 50 mph. The turbocharger’s boost shot up from 10 psi to 20 psi and the EGT tipped 1,200 degrees F. The transmission temperature only went up slightly, while the oil temp crept up to 220 degrees.

After installing the 4.30 gears in the truck’s axles, we hooked up the trailer again and took the same route to see what type of improvements the gears made. The truck definitely got up to speed quicker and easier. We also noticed accelerating to pass was easier. The readings going down the highway were in the same ballpark as before, even with outside temperatures a bit warmer. But the biggest difference was seen on the hills. On the largest grade, the truck was able to climb in Third gear at 2,600 rpm going more than 50 mph with some pedal left. The EGT was slightly higher at 1,300 degrees F while the transmission, engine, and boost only crept up a fraction.
Photo 72/72   |   Correcting The Axle Gear Ratio Truck Trailer Desert

Sources

Mag-Hytec
Van Nuys, CA 91402
818-786-8325
www.mag-hytec.com
Nitro Gear & Axle
Sacramento, CA
916-673-6464
www.nitro-gear.com

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