How to Choose a Ring and Pinion Ratio for Bigger Tires
You put taller tires on the truck or Jeep and now it feels flat. It is slow off the line, the automatic downshifts on every grade and hunts between gears on the highway, the speedometer reads low, and fuel economy is worse than before. Nothing is broken. A taller tire covers more ground per turn, which makes your axle gearing act taller than the factory intended. The fix is a new ring and pinion. This guide walks through the same method gear shops use to pick the ratio for any vehicle and any tire size, step by step, with the math shown.
Step 1: Confirm Your Current Axle Ratio
Do not guess the ratio from the window sticker or a forum post. RANDYS Worldwide points to the owner's manual, online research or the tag on the differential cover as the usual places to find the stock ratio. If the tag is missing or the axle has been swapped, count the teeth. Sierra Gear and Axle gives the formula as ring gear teeth / pinion gear teeth = ratio. A 41-tooth ring and 11-tooth pinion, for example, is 41 / 11 = 3.73.
On a 4x4, check both axles. They must match, and a mismatched pair is its own problem you want to catch before ordering gears.
Step 2: Use the Real Tire Height, Not the Number on the Sidewall
This is where most people go wrong. Tire sizes are classifications, not measurements. RANDYS Worldwide warns that some tires can be "nearly a half an inch off their classification," and gives a real example: a 35x12.50-17 Mickey Thompson Baja Boss M/T measured 34.6 to 34.8 inches depending on whether it was mounted on a 9.5 or 10 inch rim. Pull the actual overall diameter from the tire maker's spec sheet for both your old tire and the new one. Half an inch on a 35 is about 1.4 percent, which is enough to push you between two available ratios.
Step 3: Calculate the Return-to-Stock Ratio
The starting number is the ratio that puts the engine back where the factory had it. Sierra Gear and Axle gives the formula as:
New tire height / original tire height x original ratio = return-to-stock ratio
Sierra's worked example: a truck going from 31.9 inch tires to 35s with a 3.08 axle. 35 / 31.9 = 1.097, and 1.097 x 3.08 = 3.37. That exact ratio does not exist, so the nearest real choices are 3.42 or 3.55. Crawlpedia runs the same formula on a bigger jump: 31 inch to 37 inch tires on a 3.73 axle gives (37 / 31) x 3.73 = 4.452, which rounds to 4.30 or 4.56.
Crawlpedia lists the common ratios you will be rounding to: 2.73, 3.08, 3.27, 3.31, 3.42, 3.54-3.55, 3.73, 3.90, 4.09-4.11, 4.30, 4.56, 4.88, 5.13 and 5.38. Not every ratio is made for every axle, so check which ones exist for your specific differential before you settle.
Step 4: Decide Whether to Round Up
The return-to-stock ratio is the floor, not the answer. Bigger tires are usually heavier, wider and mounted on a taller vehicle, and all of that costs power. Sierra Gear and Axle recommends choosing "a ratio that is at least one or two steps lower (higher numerically)" than your calculated number. Their guidance by use: highway driving points toward ratios like 3.73 or 4.11, while "hills and more off road use" call for "4.56 or even 4.88."
Apply that to Sierra's own example. The math said 3.37. One step up from the nearest real ratio lands on 3.55 or 3.73, and a truck that tows or carries a camper would lean to the deeper choice. For the 31 to 37 inch jump, the math said 4.45, so a loaded rig with 37s is really a 4.56 or 4.88 decision rather than 4.30.
Step 5: Check Cruise RPM Before You Buy
Before ordering, confirm what the engine will actually turn on the highway. The standard formula, as laid out by Crawlpedia's RPM calculator, is:
RPM = MPH x axle ratio x transmission gear ratio x 336 / tire diameter
The 336 is a constant that converts inches per mile into revolutions (Crawlpedia gives it as 336.13, from 63,360 inches per mile divided by 60 minutes x pi). As an illustration only, take a transmission with a 0.70 overdrive top gear at 70 mph:
- 3.73 axle on a 31 inch tire: 70 x 3.73 x 0.70 x 336 / 31 = about 1,980 RPM
- Same 3.73 axle on a 35 inch tire: about 1,755 RPM
- 4.56 axle on a 35 inch tire: about 2,145 RPM
That drop to roughly 1,755 RPM is why the automatic hunts and downshifts after a tire upgrade. Look up your own transmission's top gear ratio and run the numbers for each candidate ratio. RANDYS Worldwide recommends the same process with an RPM calculator: enter your stock setup first as a baseline at freeway speed, then enter the new tire height and adjust the gear ratio until the RPM lands where you want it. That ratio is your target when shopping for gear sets.
Step 6: Match the Front and Rear on a 4x4
Crawlpedia is direct about this: "If your vehicle is a 4x4, you will need to change the gears in both front and rear axles." The two ratios should be the same. Where an exact match is not offered for one axle, Crawlpedia notes that front and rear "can be different by 1% on highway and 2% off-road without sacrificing performance." Anything wider than that fights the transfer case in 4WD.
Gear sets also have carrier breaks. A carrier built for one ratio range may not accept a deeper set. When shopping for a locker or limited slip at the same time, the ratio range is printed in the part listing, for example this Eaton Detroit Truetrac for a rear Dana 44, 3.73 and down. Buy the gears first, then the carrier that fits that ratio.
The Fix: What to Order
A re-gear is not just two gears. Every new ring and pinion has to be set up with new shims for pinion depth, carrier bearing preload and backlash. Yukon's installation kit breakdown explains the options:
- Master overhaul kit: "carrier bearings and races, pinion bearings and races, a pinion seal, a complete shim kit, ring gear bolts, a pinion nut, a crush sleeve (if applicable), oil baffles and slingers (if applicable), thread locking compound, marking compound with brush, and a gasket." This is the right choice for most higher-mileage axles.
- Pinion install kit: for "lower mileage vehicles where the carrier bearings and races can be reused."
- Minimum install kit: for cases where "both the pinion and carrier bearings can be reused."
For a common application, a Revolution Gear and Axle master install kit for the JK Rubicon rear Dana 44 covers the bearings, seals and shims. For full-size trucks, the same type of kit exists for the 2000-2007 Ford 10.25/10.5 inch rear axle. If you want the gears and kits for both axles in one box, a package like the Yukon 4.56 gear and install kit package for the Jeep TJ Dana 30/Dana 44 keeps front and rear matched. Browse every set in our ring and pinion collection or the differentials collection.
Replace While You Are In There
- Carrier and pinion bearings and races, unless the axle is low mileage and they check out clean
- Pinion seal and axle seals
- Crush sleeve and pinion nut (where used)
- Ring gear bolts
- Cover gasket or sealant and fresh gear oil of the grade your axle maker specifies
- A locker or limited slip, if you were planning one, since the carrier is already out
Cost and Effort
Setting up gears is precision work: pinion depth, preload and backlash all have to be measured and adjusted, and the contact pattern has to be read with marking compound. It is typically a multi-hour job per axle for an experienced installer, and a 4x4 means doing it twice. Most owners have a gear shop do the setup and supply their own parts. The cheapest mistake to avoid is buying gears for the wrong ratio or an axle that has a carrier break you did not account for.
After installation, new gears need a break-in period before towing or hard use. Follow the break-in instructions that come with your gear set.
If you already know your tire size, our specific guides cover the best gear ratio for 35 inch tires on a Jeep Wrangler, the 4.56 vs 4.88 vs 5.13 choice for 37s, and the classic 3.73 vs 4.10 gear ratio comparison.
Frequently Asked Questions
What is the formula for a new gear ratio with bigger tires?
New tire height divided by original tire height, multiplied by the original axle ratio. That gives the ratio that restores stock performance. Round to the nearest ratio made for your axle, and consider going one or two steps numerically higher if the new tires are heavier or the vehicle tows or goes off road.
Do I have to re-gear both axles on a 4x4?
Yes. Both axles need the same ratio or the transfer case will bind in 4WD. Where an exact match is not available, Crawlpedia notes a difference of up to 1% on highway and 2% off road is acceptable.
Can I reuse my old bearings when I re-gear?
Only on a low-mileage axle where the bearings and races are in good condition. Yukon offers pinion and minimum install kits for that case. On most axles a master overhaul kit with new carrier and pinion bearings is the safer choice, since the axle is already apart.