Best Gear Ratio for 37 Inch Tires: 4.56 vs 4.88 vs 5.13

You moved up to 37s and now the rig feels like it lost half its engine. It is slow to get rolling, the automatic keeps downshifting on mild grades and never settles into top gear, the speedometer reads low, and fuel economy drops. None of that means the transmission is failing. A 37-inch tire changes your effective final drive ratio a lot, and the fix is a re-gear. The real question is which ratio: 4.56, 4.88 or 5.13. The answer depends on your engine, your transmission and how much weight the vehicle carries. This guide shows each one with numbers.

Why 37s Need a Re-Gear

Yukon Gear and Axle explains that "installing larger tires effectively changes your final drive ratio," so the engine has to work harder to turn the bigger tire at the same road speed. Yukon also makes an important point: "The goal isn't always lower RPM. The goal is to put the engine in a range where it performs best." Most vehicles leave the factory with tall, fuel-economy gearing. Add 37s on top of that and an automatic will start gear hunting.

Yukon's formula for the ratio that only brings back your current feel is to multiply your current ratio by new tire diameter / old tire diameter. For example, a Jeep already geared 4.56 for 35s that moves to 37s needs 4.56 x (37 / 35) = 4.82. The closest real ratio is 4.88. If you are coming from stock gears on stock tires, the gap is much larger. That is why hardly anyone on 37s runs anything shallower than 4.56.

4.56 vs 4.88 vs 5.13: Who Should Run Which

The Exodus 4x4 regear chart gives ratios for 37-inch tires by exact engine and transmission:

Source: Exodus 4x4 regear chart. OffroadMe's Jeep chart agrees: 5.13 for a 3.6L JK automatic on 37s, 5.38 for a trail-used 3.8L JK automatic, and 4.88 for the JL and Gladiator 8-speed.

Run 4.56 on 37s if...

You have a lot of torque. Exodus lists 4.56 for every Hemi and EcoDiesel Wrangler and Gladiator on 37s. The extra torque makes up for the taller effective gearing, and 4.56 keeps highway RPM down. On a V6, 4.56 with 37s is the "it works, but it is soft" choice. It is only reasonable for a light build that mostly sees highway miles.

Run 4.88 on 37s if...

You drive a JL with the 3.6L or 2.0L Turbo and the 8-speed automatic. That is the ratio both Exodus and OffroadMe recommend. OffroadMe says it "helps the transmission use its upper gears instead of constantly hunting." For a daily-driven JK or JL on 37s, it adds that "4.88 keeps cruising RPM lower."

Run 5.13 on 37s if...

You drive a V6 JK automatic, any V6 manual, or a V6 Gladiator. The Gladiator is longer and heavier than a Wrangler, which explains why Exodus puts even the 8-speed JT at 5.13. OffroadMe notes that 5.13 "gives more torque multiplication at the cost of higher RPM at road speed." Weight is the tiebreaker. Exodus's rule is: "For each 300LB add-on... go UP ONE gear ratio." Steel bumpers, a winch, armor and a loaded roof rack easily add that much, so a JL 8-speed that would be fine on 4.88 when bare often belongs on 5.13 once it is built.

Check the Math: Cruise RPM on 37s

OffroadMe publishes the standard formula: RPM = (MPH x axle ratio x top-gear ratio x 336) / tire diameter. It lists 0.67 overdrive for the JL/JT 850RE 8-speed and 0.83 for the JK 3.6L W5A580 5-speed automatic. It also notes that a 37-inch tire usually measures about 36.5 inches. Using 36.5 inches at 70 mph:

The math shows why the answers are different. The 8-speed's deep overdrive leaves plenty of room, so going from 4.88 to 5.13 costs only about 110 rpm at 70 mph. The JK's 0.83 top gear already spins the engine faster, but the 3.6L JK still needs 5.13 on 37s to pull that top gear without downshifting. For comparison, a stock 3.45 JL on the same tire would run about 1,490 rpm at 70, which is why it hunts.

Make Sure the Axles Can Take the Ratio

Both axles get the same ratio, and you order by the housing on your axle tag, not by the model year alone. Yukon points out that 4.88, 5.13 and 5.38 Jeep gears "all use the same 8-tooth pinion design," so the deeper ratio is not the weak link that older articles claim. What you do need to check is the carrier break. Yukon warns that "a Dana 44 built around 3.73s won't take 4.88s without the right carrier or a thick-cut set." Confirm this before you order, not partway through the install. If you are deciding whether your front axle is worth re-gearing at all, read our Dana 44 vs Dana 60 comparison before you spend the money.

The Fix: A Complete Front and Rear Re-Gear Kit

Buy gears, install kits and covers as one kit so both axles get matched gears and new bearings in a single job:

• JL / Gladiator JT with Dana 44s, V6, loaded build, 5.13: Yukon Stage 2 JL/JT Dana 44 5.13 re-gear kit (YGK069STG2)
• JL non-Rubicon Dana 30/35, 8-speed, 4.88: Yukon Stage 2 JL Dana 30/35 4.88 re-gear kit (YGK074STG2)
• JL / JT Dana 44, Hemi or EcoDiesel, 4.56: Yukon Stage 2 JL/JT Dana 44 4.56 re-gear kit (YGK067STG2)
• JK non-Rubicon Dana 30/44, 3.6L automatic, 5.13: Yukon JK non-Rubicon Dana 30/44 5.13 re-gear kit (YGK014STG2)

Every ratio and axle we carry is in our ring and pinion collection.

Replace While You Are In There

• Carrier and pinion bearings, shims, crush sleeve and pinion seal: never put new gears on old bearings. The kits above include master install parts.
• Diff covers: 37s put the housing closer to rocks. The Stage 2 kits above include covers.
• Locker or limited slip: the carrier is already out, so this is the cheapest time to add traction. See our differentials and lockers.
• Gear oil: fresh fill at install, then a change after break-in.

Cost, Effort and Break-In

A re-gear means setting pinion depth, backlash and bearing preload with shims on two axles. Most owners have a shop do it, and many do it at the same time as the lift and tire install because much of the labor overlaps. Recalibrate the speedometer for the new tire size afterward.

Exodus 4x4's break-in procedure: drive 15 to 20 minutes in town, let the axles cool completely, and repeat that two to three times. Then drive on the highway at 65 mph or less with cooling stops, and change the gear oil at 500 miles.

If the transmission still shifts badly after a correct re-gear and speedometer calibration, look at the transmission itself. Our Jeep Wrangler JK and JL automatic transmission guide shows which unit your year uses. Still on 35s? See our best gear ratio for 35 inch tires guide.

Frequently Asked Questions

Is 4.88 or 5.13 better for 37s on a JL?

For a JL with the 3.6L or 2.0L Turbo and the 8-speed automatic, Exodus 4x4 and OffroadMe both recommend 4.88. Go to 5.13 if you have a 6-speed manual or a heavy build. Exodus says to go up one ratio for every 300 lb of add-ons.

Is 4.56 enough for 37 inch tires?

On a Hemi or EcoDiesel Wrangler or Gladiator, yes. Exodus lists 4.56 for those engines on 37s. On a 3.6L V6, 4.56 with 37s leaves the engine below its best range, and the automatic will tend to hunt on grades.

Are 5.13 gears weaker than 4.56 gears?

Not in the way people assume. Yukon notes that common Jeep ratios of 4.88, 5.13 and 5.38 all use the same 8-tooth pinion design. The axle housing and shafts matter more than the step from 4.88 to 5.13.

Sources

• Exodus 4x4 - Don't ReGear Your Jeep Without Reading This
• OffroadMe - Jeep Gear Ratio Calculator and Chart (JK, JL, JT)
• Yukon Gear and Axle - How to Select the Right Gear Ratio for Your Vehicle
• Yukon Gear and Axle - Picking Ratios: A Jeep JK Re-Gearing Story