5.38 Gear Ratio Guide: Maximum Torque for 40-Inch Tires and Competition Builds

The 5.38 gear ratio is where off-road building gets serious. This is competition-grade gearing — designed for rigs running 40-inch and larger tires, dedicated rock crawlers, and builds where trail performance takes absolute priority. At Godspeed Off-Road, a 5.38 regear is a relatively rare request compared to 4.88 or 5.13, and that rarity tells you something: this ratio is for builders who know exactly what they need and why.

What 5.38 Gears Are Designed For

A 5.38 ratio provides 31% more torque multiplication than factory 4.10 gears and 10% more than 4.88s. That extra mechanical advantage is necessary when you’re turning tires that are 40 inches tall and can weigh over 100 pounds each. The rotational inertia of four 40-inch tire-and-wheel assemblies is enormous — without aggressive gearing, your engine simply cannot accelerate them effectively, and your transmission becomes the sacrificial component.

The 5.38 ratio also dramatically increases your low-range crawl ratio. On a Jeep JK with a stock transfer case, 5.38 gears produce a crawl ratio of approximately 68.9:1 — compared to 47.7:1 with stock 3.73 gears. That’s 44% more wheel torque at any given engine speed, which means slower, more controlled crawling over technical obstacles.

Highway RPM at 65 MPH by Tire Size

With 40-inch tires, 5.38 gears put you at approximately 2,030 RPM at 65 MPH — a perfectly reasonable highway cruise. The common assumption that 5.38 gears are “too aggressive for the street” is simply wrong when paired with appropriately large tires. It’s the same principle as 4.88 gears with 35-inch tires — the ratio matches the tire size.

Who Should Run 5.38 Gears?

• 40-inch and larger tire builds: This is the primary application. If you’ve committed to 40-inch tires, 5.38 gears are the standard recommendation for maintaining highway drivability.
• Dedicated rock crawlers: Even with 37-inch tires, some dedicated trail rigs run 5.38 gears for the additional crawl ratio. The higher highway RPM (~2,190) is acceptable when the vehicle spends 80%+ of its time on trails.
• Competition vehicles: Rock racing, Ultra4, and other competitive off-road vehicles frequently run 5.38 or steeper ratios paired with 37–42-inch tires.
• HEMI and LS-swapped rigs: Vehicles with engine swaps producing 400+ horsepower can use 5.38 gears with 37-inch tires without the highway RPM feeling excessive, because the engine’s power band is broader.

Required Drivetrain Modifications

A 5.38 regear is never a standalone project. At this gear ratio and tire size, the entire drivetrain needs to support the forces involved:

Full-float rear axle or axle upgrade: Factory semi-float axles are at their structural limit with 40-inch tires. A full-float rear axle conversion (or a complete aftermarket axle like a Dynatrac, Currie, or RuffStuff housing) is strongly recommended. The wheel bearings carry the vehicle weight independently of the axle shafts, so a broken shaft doesn’t mean losing a wheel.

Heavy-duty front axle: Dana 30 front axles are completely inadequate for 40-inch tires. Even Dana 44s are marginal. Most 40-inch builds run a Dana 60 front — either a swap from a donor vehicle or an aftermarket unit. At minimum, a trussed and sleeved Dana 44 with chromoly shafts and high-steer is required.

Aftermarket driveshafts: Factory driveshafts will not survive the torque loads and angles of a 40-inch tire build with 6 inches of lift. Tom Wood’s, Adams, or JE Reel driveshafts with 1350 or 1480 U-joints are standard at this level.

Upgraded brakes: Stopping 40-inch tires from highway speed requires significantly more braking force than stock components provide. Big-brake kits — larger rotors, multi-piston calipers — are a safety requirement, not an option.

Heavy-duty steering: Hydraulic assist steering is common on 40-inch tire builds. The effort required to turn 40s at low speed exceeds what most factory power steering systems can deliver reliably.

5.38 vs. Other Gear Ratios for Big-Tire Builds

• 5.13 vs. 5.38: For 37-inch tires, 5.13 is the daily-driver choice and 5.38 is the trail-priority choice. For 40-inch tires, 5.38 is the standard. The RPM difference is about 100 RPM at highway speed.
• 5.38 vs. 5.71/5.86: For 42-inch and larger tires, ratios steeper than 5.38 are available from Yukon and other manufacturers. These are pure competition ratios — highway driving is an afterthought.

Compare all ratios on our gear ratio comparison chart.

Availability and Gear Set Sourcing

5.38 gear sets are less commonly stocked than 4.88 or 5.13 ratios, which can mean slightly longer lead times. Yukon Gear & Axle and Nitro Gear are the primary aftermarket sources. OEM manufacturers don’t typically produce 5.38 ratios — this is exclusively aftermarket territory. We keep common applications in stock or can source them within a few business days.

The Investment Reality

A 40-inch tire build with 5.38 gears is a significant investment in your vehicle. The regear itself follows the same process as any ring-and-pinion swap, but the supporting modifications — axle upgrades, driveshafts, brakes, steering — add up quickly. We always walk customers through the full scope of what a 40-inch build requires before starting work. There’s no point in installing 5.38 gears if the axle shafts are going to break on the first trail ride.

At Godspeed Off-Road, we approach these builds holistically. We’ll evaluate your vehicle, your goals, and your budget, then lay out a build plan that makes sense — whether that’s doing everything at once or staging the build over multiple phases.

Frequently Asked Questions

Are 5.38 gears too aggressive for street driving?

Not when paired with the right tire size. With 40-inch tires, 5.38 gears produce approximately 2,030 RPM at 65 MPH — perfectly comfortable for highway driving. The ratio only becomes “too aggressive” when paired with tires that are too small, like 33s or 35s.

Can I run 5.38 gears with 37-inch tires?

Yes, with a caveat. At highway speed, you’ll be at approximately 2,190 RPM — higher than the 5.13/37-inch combination but still within the acceptable range. This setup makes sense for vehicles that spend most of their time off-road and want maximum crawl ratio. For primarily street-driven rigs with 37s, 5.13 is the better balance.

What axle do I need for 5.38 gears?

5.38 ring-and-pinion sets are available for Dana 30, 35, 44, and 60 axle housings. However, just because a gear set fits doesn’t mean the axle is strong enough. For 40-inch tires, we recommend Dana 44 rear minimum (ideally Dana 60) and a built Dana 44 or Dana 60 front. For 37-inch applications, Dana 44s with chromoly shafts are adequate for most use.

How much does a 5.38 regear cost?

The regear investment itself is comparable to any other ratio — labor is the same, and the gear set premium over more common ratios is modest. However, a 5.38 regear rarely happens in isolation. Budget for axle shaft upgrades, driveshafts, and potentially brake and steering improvements. We provide detailed build estimates so you know the full investment before any wrenches turn.

Will 5.38 gears void my warranty?

Aftermarket gear installations do not automatically void your vehicle warranty — the Magnuson-Moss Warranty Act requires the manufacturer to prove that the modification caused the specific failure. However, any drivetrain warranty claim will likely be scrutinized more closely. For most 40-inch tire builds, the vehicle is well out of warranty anyway.

Ready for a Big-Tire Build?

A 40-inch tire build with 5.38 gears is the pinnacle of off-road capability. It’s not for everyone — but if this is your goal, you want a shop that’s done it before and knows exactly what your vehicle needs. We’ll build it right the first time.

Call Dan at (941) 706-0071 or visit us at 2858 Clark Road, Sarasota, FL.