Regearing After Bigger Tires: How to Pick the Right Axle Ratio

Bigger tires make your axle ratio act taller, so the truck accelerates, tows and climbs like it has numerically lower gears than the ones listed on the axle tag. Regearing puts a numerically higher ring and pinion in both axles to win that back. This guide covers the math, when you can skip it, how to pick a ratio, and what the job involves and costs.

How bigger tires change your effective axle ratio

Your axle ratio is how many times the driveshaft turns for each turn of the wheels. A 3.73 ratio means 3.73 driveshaft turns per tire revolution. What the ratio can't account for is how far the truck travels on each of those revolutions, and that's set by the tire's diameter. Put on a taller tire and the truck covers more ground per revolution, which has the same effect as installing a numerically lower (taller) gear.

Two formulas cover nearly every regear decision:

Use real diameters, not the number printed in big letters on the sidewall. A 265/70R17 is about 31.6 in tall, and a tire sold as a "35" might measure 34.4 in. The tire maker's spec sheet lists overall diameter and revolutions per mile; if you want to be precise, measure the loaded height yourself. Our guide to reading a tire size shows how to convert metric sizes to inches.

What taller effective gearing does to your truck

Everything the engine does gets multiplied through the gears and then divided by the tire's radius. A taller tire cuts the force at the contact patch. Going from 31s to 35s drops it by about 11 percent before you account for the extra weight of the bigger tire and wheel. The symptoms are predictable:

• Slower acceleration. Every gear is effectively taller, and the heavier rotating mass takes more energy to spin up.
• Weaker towing. Manufacturer tow charts list ratings by axle ratio. Bigger tires push your effective ratio toward, or past, the tallest ratio on the chart. The sticker rating doesn't change, but the truck works like a lower-rated configuration. See our towing capacity guide for how those ratings are built.
• Gear hunting. Modern 8- and 10-speed automatics are programmed around a stock tire and ratio. With taller effective gearing, the transmission downshifts more often on grades and in headwinds, then upshifts, then downshifts again.
• Higher transmission temperatures. More downshifting and more torque converter slip mean more heat, and heat is what kills automatic transmissions. It's worst while towing.
• A speedometer that reads low. Unless the truck is reprogrammed, it thinks you're on the original tires.

A worked example: 3.73 gears, 31s to 35s

Take a half-ton that left the factory with 3.73 gears and 31-in tires, and a 10-speed automatic. Ford's 2026 F-150 10-speed, for example, has a 4.70:1 first gear and a 0.64:1 tenth gear. The owner installs a leveling kit and 35-in tires.

1. Find the effective ratio. 3.73 × 31 ÷ 35 = 3.30. The truck now behaves as though it has 3.30 gears.
2. Check highway rpm. Engine rpm = mph × axle ratio × transmission ratio × 336 ÷ tire diameter. (The 336 converts inches per minute to miles per hour.) At 70 mph in tenth gear on stock 31s: 70 × 3.73 × 0.64 × 336 ÷ 31 ≈ 1,810 rpm. On 35s: about 1,600 rpm. Two hundred rpm doesn't sound like much, but it's enough to drop many engines out of their comfortable cruising range and start the downshift dance on every hill.
3. Find the ratio that restores stock. 3.73 × 35 ÷ 31 = 4.21. There's no 4.21 gear set, so the choice is 4.10 or 4.30.
4. Compare the candidates. With 4.30s on 35s, the effective ratio is 3.81 and 70 mph comes in around 1,850 rpm, just above stock. With 4.10s, the effective ratio is 3.63 and cruise rpm is about 1,765, slightly below stock. A truck that tows or carries a camper should take the 4.30s. A lightly loaded daily driver will be happy on 4.10s.
5. Fix the speedometer. With no recalibration, an indicated 60 mph is really about 67.7 mph on 35s. Program the actual tire size and the new axle ratio into the truck's computer after the regear.

Suggested ratios for a truck that came with 3.73s on 31s

This table applies the formulas above to common upgrade sizes. "Restores stock" is the exact math; the suggested set is the closest common ratio, rounded deeper where the math lands between two sets. Not every ratio is made for every axle, so confirm availability for your specific front and rear axles before you settle on a number.

When you can skip regearing

Not every tire upgrade needs new gears. A modest bump of about one size, or roughly 3 to 5 percent taller, is usually livable, especially on a truck with a modern 8- or 10-speed automatic.

Those transmissions have deep first gears and tightly spaced ratios. Ford's 10-speed starts at 4.70:1, and the 8-speed automatic in the 2026 Jeep Gladiator and Ram Heavy Duty starts at 4.71:1. Compare that with an older four-speed automatic like GM's 4L60-E, which started at about 3.06:1. A 4.70 first gear with 3.73 axles gives 17.5:1 overall. Even after 35s knock the effective axle ratio down to 3.30, you still have about 15.5:1, well above the roughly 11.4:1 the old four-speed delivered with the same 3.73 gears on stock tires. That's why a lot of modern trucks on one-size-up tires drive fine without a regear.

Trucks that originally came with a numerically high ratio (a max-tow 3.73 or 4.10, for example) also have more margin than ones with highway-friendly 3.21 or 3.42 gears. And turbocharged engines with a broad, low torque curve tolerate taller effective gearing better than engines that make their power high in the rev range.

Crawl ratio and why trail rigs gear deeper

Crawl ratio is your lowest possible gearing: first gear × transfer case low range × axle ratio. It sets how slowly the truck can creep over a rock or down a steep grade without riding the brakes. Our glossary entry on crawl ratio covers the term in more detail.

The 2026 Jeep Gladiator Rubicon is a good example because every number is published. Its 8-speed first gear is 4.71, its Rock-Trac transfer case has a 4.0:1 low range, and it runs 4.10 axles. That's 4.71 × 4.0 × 4.10 = 77.2:1 on the factory LT285/70R17 tires, which are about 32.7 in tall. Bolt on 37s and the effective crawl ratio falls to about 68:1. Regear to 4.56 and you're back to about 76:1 effective; 4.88s give you about 81:1. For comparison, the 2026 Tacoma's 8-speed, 2.57:1 low range and 3.583 axles work out to about 40.6:1 on stock tires.

A deeper crawl ratio makes low-speed control easier and lets you use engine braking on steep descents. It's the reason trail builds often gear one step deeper than the "restore stock" math suggests.

Choosing from the common gear sets

The ratios you'll see most often are 3.21, 3.42, 3.55, 3.73, 4.10, 4.30, 4.56, 4.88, 5.13, 5.29 and 5.38. Availability depends on the axle, and a few rules help narrow the choice:

• Match stock feel for a highway truck. Pick the set closest to the "restore stock" number.
• Go one step deeper for towing, heavy armor and camping gear, manual transmissions, or lots of low-speed trail time.
• Watch carrier breaks. Numerically higher ratios use a smaller pinion, so the ring gear has to get thicker to reach it. Past a certain ratio the ring gear can't get any thicker, and the axle needs a carrier with a different mounting height. Crossing that carrier break adds a carrier (or a locker) to the bill.
• Order the right front gears. Many front axles need reverse-cut (high-pinion) gears. Make sure the front set is correct for your axle and not just the same ratio number.

Front and rear ratios must match

This one isn't negotiable. In 4WD, a part-time transfer case locks the front and rear driveshafts together. If the axle ratios differ, the front and rear tires try to turn at different speeds, and something has to give: tires scrub, the driveline binds, and parts break. Full-time and automatic 4WD systems suffer too, because their center differential or clutch pack has to slip or spin constantly to make up the difference, and it overheats and wears out. Regear both axles, to exactly the same ratio, at the same time. If you're adding lockers, this is the time to do it. See our guide to locking differentials.

What a regear job involves

Setting up a ring and pinion is precision work, and it's the reason most people pay a specialist. The basic process for each axle:

1. Teardown. Drain the oil, remove the axle shafts if needed, pull the differential carrier and remove the pinion. On many IFS trucks, the front differential comes out of the truck.
2. Pinion depth. Install the new pinion with the correct shim stack so its teeth sit at the right depth in the ring gear.
3. Pinion preload. New bearings, a new crush sleeve or solid spacer, and the pinion nut torqued to the correct bearing preload.
4. Ring gear and carrier. Bolt the ring gear to the carrier with new bolts and thread locker, install new carrier bearings, and set carrier bearing preload and backlash with shims. Backlash specs are often in the 0.006–0.010 in range, but use the spec for your axle and gear brand.
5. Check the pattern. Paint the teeth with marking compound, rotate the gears under load, and read the contact pattern. Adjust shims and repeat until the pattern is right.
6. Reassemble and fill. New pinion seal, new cover gasket or sealant, correct gear oil, plus friction modifier if the differential requires it.

A master install or overhaul kit supplies the wear parts. Kit contents vary, but they typically include new carrier and pinion bearings and races, a pinion seal and marking compound, and the fuller kits add shims, a crush sleeve and ring gear bolts. Reusing old bearings with new gears is a false economy.

Break-in matters

New gears need a gentle start to avoid overheating. Yukon Gear & Axle, for example, recommends letting the differential cool after the first 15 to 20 miles, waiting at least 500 miles before towing, keeping early towing trips short, and changing the gear oil after the first 500 miles. Follow your gear maker's instructions, and add that first oil change to your maintenance schedule.

What a regear costs

The gears are the cheap part. You're paying for someone who can read a contact pattern.

Prices vary a lot by region, shop, axle and brand, so treat these as ballpark ranges, not quotes. For a typical half-ton or midsize 4x4, the ring and pinion and a master install kit usually run a few hundred dollars per axle in parts, and labor commonly takes a shop four to eight hours per axle. All in, a front-and-rear regear at a reputable shop often lands somewhere around $1,800 to $4,000.

Expect to pay more for heavy-duty axles, IFS front differentials that have to come out of the truck, a carrier change at a carrier break, or new lockers, which can add a significant amount per axle in parts alone. Always get a written quote that spells out which kit, which gear brand, and whether the price covers both axles, fluids and tire-size programming.

Recalibrating the speedometer

After new tires and new gears, the truck's computer still thinks it has the original ratio and tire size. Depending on the truck, it calculates speed from the transmission output shaft, the ABS wheel-speed sensors, or both, and it uses the axle ratio for shift scheduling and other functions. Program the actual tire size and the new axle ratio with a dealer tool or a reputable programmer. On older trucks with a mechanical speedometer drive, the fix is a different drive gear in the transmission or transfer case.

If you haven't bought the tires yet, start with our comparison of leveling kits, lift kits and body lifts, then run your planned sizes through the gear ratio calculator above before you place an order.