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TL;DR
- New axle ratio ≈ old ratio × new tire diameter ÷ old tire diameter: a 3.73 truck going from 31- to 35-inch tires needs about a 4.21 to restore stock rpm.
- Taller tires make the speedometer read low by the same ratio: on 35s instead of 31s, an indicated 60 mph is about 67.7 mph, and the odometer under-counts by about 11%.
- Fluid intervals vary by maker: Ford's 2020 F-150 needs axle fluid only after a leak or submersion, while the 2020 Super Duty shortens it to 30,000 miles for hot, sustained towing on non-synthetic fluid.
- If an axle has been under water, Ford says to check the axle lubricant and change it if necessary.
- Any lift or leveling kit needs an alignment afterward, and Spicer says U-joint operating angles should stay at 3° or less and within 1° of each other at each end of the shaft.
Regearing After Bigger Tires: The Math
A taller tire travels farther with each turn, so it has the same effect as a numerically lower axle ratio. The relationship is proportional: effective ratio = actual ratio × original tire diameter ÷ new tire diameter. To restore stock gearing, reverse it: new ratio = original ratio × new diameter ÷ original diameter. The same numbers give engine speed. At a given road speed in a 1:1 transmission gear, rpm = mph × axle ratio × 336 ÷ tire diameter in inches, where 336 converts inches per wheel turn into miles per hour (63,360 inches in a mile ÷ 60 minutes ÷ π, rounded). Drivetrain makers' calculators, such as Spicer's transmission-ratio rpm calculator, take exactly these inputs.
| Stock axle ratio (31-in tires) | Effective ratio on 35s | Ratio that restores stock rpm |
|---|---|---|
| 3.55 | 3.14 | 4.01 |
| 3.73 | 3.30 | 4.21 |
| 4.10 | 3.63 | 4.63 |
Worked through for the 3.73 truck at 65 mph: about 2,630 rpm on the stock 31s, about 2,330 rpm on 35s, and back to about 2,630 rpm with a 4.21 ratio. Axles are sold in set ratios, so choose the nearest one offered for your axle, and use measured tire diameters, since a tire's real height often differs from its nominal size. The regear calculator runs these numbers for any combination.
Speedometer and Odometer Error After a Tire Change
The speedometer and odometer count wheel revolutions calibrated to the factory tire, so a size change skews both by the ratio of the diameters. True speed = indicated speed × new diameter ÷ old diameter. On 35s instead of 31s, an indicated 60 mph is really about 67.7 mph, and the odometer records roughly 88.6 miles for every 100 driven. That matters for maintenance: a service reminder set for 7,500 miles on the dash comes due after about 8,470 real miles. Ask your dealer or tuner how your truck's speedometer is recalibrated for a new tire size.
Fluid Change Intervals: Two Ford Manuals Compared
Manufacturers turn towing and off-road use into service intervals very differently. Ford's 2020 manuals for its two pickup lines show how differently one maker can treat it:
| Condition | 2020 F-150 | 2020 Super Duty |
|---|---|---|
| Normal driving | Axle and transfer-case fluid changes or checks not required | Change axle fluid every 150,000 mi |
| Long periods of towing above 70°F, or long stretches at wide-open throttle above 45 mph | No change; same leak/submersion rule | Change rear axle fluid every 30,000 mi if it is non-synthetic; 75W-140 synthetic to Ford spec WSL-M2C192-A keeps the 150,000-mi interval |
| Off-road operation | Inspect steering linkage, ball joints and U-joints; lubricate grease fittings | Every 50,000 mi: change rear axle fluid on Dana rear axles (some F-350s, all F-450s and F-550s) and transfer-case fluid, inspect front axle fluid |
| Axle submerged in water | Check or change the axle and transfer-case fluid | Check the axle and power-transfer-unit lubricant and change if necessary |
The lesson is to read your own manual's special-operating-conditions schedule rather than apply a blanket rule. Water is the one case both treat alike: gear oil contaminated by a water crossing should be checked promptly. See fluid change intervals for off-road driving for more models, and towing and payload for the towing ratings those schedules assume.
Lift Installs: What Changes and What It Takes
Rough Country, which makes lift and leveling kits, says a lift changes suspension geometry, steering angles, brake-line travel and alignment settings. Its guidance puts a leveling kit at about 2 to 4 hours and a full suspension lift at a full day or weekend for an experienced DIY installer. Before driving, it says to check brake-line length and routing, ABS sensor wire slack, and driveshaft and CV-axle angles, because raising the truck increases suspension travel and operating angles. It calls for an alignment after any change in ride height, warning that skipping it leads to uneven tire wear, steering pull and reduced stability, and suggests a shop experienced with lifted vehicles for lifts of 4 inches or more. NHTSA notes that a proper alignment maximizes tire life and keeps the vehicle from drifting to one side on a straight, level road. See suspension and lift kits for choosing the kit.
Driveshaft and U-Joint Angles After a Lift
Lifting the body or suspension usually steepens the driveshaft. Spicer, the Dana driveline brand, says U-joint operating angles should be 3° or less for vibration-free running, and that the angles at each end of a shaft should be equal within 1° (within ½° for shafts in front of a transfer case). Any angle above 3° lowers U-joint life and may cause vibration. The allowable angle also falls as driveshaft speed rises:
| Driveshaft rpm | Spicer's maximum operating angle |
|---|---|
| 1,500 | 11.5° |
| 2,500 | 7.0° |
| 3,500 | 5.0° |
| 4,500 | 3.7° |
| 5,000 | 3.2° |
In a 1:1 transmission gear the driveshaft turns at engine speed, so the 3.73 truck on 35s at 65 mph spins its shaft at about 2,330 rpm. Rough Country says quality lift kits account for these geometry changes with brackets or extensions, so check what yours includes for your lift height. Ford's own off-road schedules add U-joint inspection and grease-fitting lubrication, and the 2020 Super Duty manual adds inspecting and lubricating the U-joints every 7,500 miles in dusty or sandy conditions.
After a Hard Trail Day
Ford's off-road schedules give a checklist worth borrowing for any truck: inspect the steering, suspension ball joints and tie rods, lubricate any grease fittings, and inspect the U-joints. The 2020 Super Duty manual also has owners check that the drain holes in the exhaust tips and under the shield near the right rear tire are clear of debris. Add a look at skid plates and sliders for dents that could now touch a component behind them, and a check of fasteners on anything bolted to the frame.
Wheel Bearing Noise and Wear
Timken's troubleshooting guide separates bearing noises from other causes. A humming, rumbling or growling bearing noise is heard driving straight but gets louder when the steering wheel is turned slightly, and the side opposite the rumble is usually the bad one. A grinding noise when turning or when the load shifts points to roller or raceway damage, while snapping or clicking in sharp turns can mean excessive bearing endplay or a worn outer CV joint. Timken notes that uneven tire wear usually comes from worn suspension parts, misalignment or wrong inflation rather than the bearing itself. The 2020 Super Duty's dusty and off-road schedules include replacing wheel-bearing grease and seals on two-wheel-drive trucks with non-sealed bearings.
How to Use This Section
Start with the numbers for your own truck: measured tire diameters and your axle ratio for regearing and speedometer correction, your owner's manual's special-conditions schedule for fluids, and the lift maker's instructions and torque specs for suspension work. The guides below cover each job in detail.
How We Rank Maintenance & DIY Gear
Every tool and part in this section is weighed on its manufacturer specifications and the gear-ratio, fluid-interval and alignment facts above, cross-checked with owner installation reports. We don't perform these installs or maintenance jobs ourselves; see how we rank for the full method and our affiliate disclosure for how the site is funded.
Sources
- Interco Tire — Tire Size Change Gear Ratio Calculator — A new gear ratio is calculated from the new tire diameter, the old tire diameter and the current ratio; bigger tires cost power.
- Spicer (Dana) — Transmission Ratio RPM Calculator — Engine rpm and vehicle speed are calculated from transmission ratio, tire height, mph and ring-and-pinion ratio — the inputs of the rpm formula on this page.
- Yukon Gear & Axle — How to Use an RPM Calculator to Find the Right Gear Ratio — Larger tires change engine rpm at a given speed and affect acceleration, towing, fuel economy and drivetrain stress.
- Ford — 2020 F-150 Owner's Manual: Special Operating Conditions Scheduled Maintenance — Axle and transfer-case fluid changes or checks not required unless a leak is suspected or the assembly has been submerged, including under towing; off-road use calls for inspecting steering linkage, ball joints and U-joints and lubricating grease fittings.
- Ford — 2020 Super Duty Owner's Manual (PDF) — Axle fluid every 150,000 mi; rear axle fluid every 30,000 mi under long hot towing or sustained wide-open throttle above 45 mph with non-synthetic fluid, waived with 75W-140 synthetic (WSL-M2C192-A); off-road 50,000-mi axle and transfer-case items; check and change axle lubricant after submersion; U-joint and wheel-bearing items in the dusty and off-road schedules; exhaust-tip drain-hole inspection.
- Rough Country — How to Prep for a Lift Kit Install — Lift kits change geometry, steering angles, brake-line travel and alignment; leveling kit 2–4 hours, full lift a day or weekend; alignment required after any ride-height change; check brake lines, ABS wires, driveshaft and CV angles; re-torque after break-in; experienced shop for 4 in or more.
- Spicer (Dana) — Driveline Operating Angle Calculator — U-joint operating angles of 3° or less; equal within 1° at each end (½° in front of transfer cases); above 3° lowers U-joint life; maximum angle by driveshaft rpm.
- NHTSA — Tires (TireWise) — A wheel alignment maximizes tire life and keeps the vehicle from veering left or right on a straight, level road.
- Timken — Symptoms of a Worn Wheel Hub Bearing (Tech Tip) — Bearing noise symptoms (humming, rumbling or growling that intensifies when turning slightly; grinding; snapping or clicking) and the usual causes of uneven tire wear.