STEP 10
Gearing Should Match Tires
Gearing is the most underappreciated variable in CJ drivability. A Jeep that is geared correctly for its tire size feels relaxed, responsive, and capable. A Jeep geared too tall for its tires -- as most CJs with any tire size increase are -- feels lazy, refuses to get out of its own way, and is working harder than it needs to at every moment.
Step 10 is last in the sequence because the gearing decision depends on knowing the tire size, and the tire size decision depends on having the foundation right first. You cannot evaluate what gearing you need until you know what you are running. And you cannot responsibly increase tire size until the steering is tight (Step 6), the brakes are matched to the load (Step 8), and the AMC 20 shafts are converted (Step 9). Those supporting steps make Step 10 possible -- and make bigger tires safe rather than just bigger.
For a full breakdown of everything tire size changes beyond ground clearance -- gearing, steering load, braking, unsprung weight, and fuel economy -- see our CJ tire size guide. This step covers the gearing decision and what needs to be in place before you change tire size.
How Tire Size Changes Effective Gearing
When you increase tire diameter, you decrease the mechanical advantage between the engine and the ground. A taller tire travels farther per revolution than a shorter one, which means for the same engine RPM, the Jeep moves faster. That sounds like a benefit until you realize the engine now has less torque multiplication available to accelerate, climb grades, and maintain speed under load.
Think of it as a bicycle: shifting to a taller gear makes you go faster but requires more effort to pedal. Increasing tire size without changing axle gearing is the same thing -- you have effectively shifted the entire drivetrain into a taller gear without adding any additional engine power.
Stock CJ axle ratios are typically 2.73, 3.07, 3.31, 3.54, or 4.10 depending on the year and transmission. A CJ running 33-inch tires on stock 2.73 gears is dramatically undergeared. It will feel slow, require more throttle than it should for normal driving, make highway passing laborious, and run the transmission and engine hotter than they should from the constant effort. Re-gearing is consistently called the single best performance modification by experienced CJ owners. The improvement in drivability is not subtle.
Community-tested axle ratios by tire size. Based on the AMC 258 inline six.
| Tire Size | Recommended Ratio | Notes |
|---|---|---|
| 29-31 inch | 3.07-3.54 | Stock territory. Adequate for most driving if tires stay in this range. |
| 31-33 inch | 3.54-3.73 3.73 MINIMUM | 3.73 is the floor for comfortable driving. Stock 2.73 gears will feel gutless once you move past 31s. |
| 33-35 inch | 4.10-4.56 4.10 PREFERRED | 4.10 is the community sweet spot for 33s at lower elevations. High elevation owners often need 4.56 with the same setup. |
| 35-37 inch | 4.56-4.88 | 4.56 balances highway drivability and off-road performance. 4.88 improves crawl ratio but raises highway RPM. |
| 37 inch+ | 4.88 or deeper | At this size, axle strength and driveline upgrades become as important as gearing. |
Transmission matters. A CJ with a 4-speed non-overdrive transmission needs numerically higher gears than the same Jeep with a 5-speed overdrive. Quick rule of thumb:
tire diameter × 0.12 = starting ratio target | example: 33 × 0.12 = 3.96 → run 4.10s
Confirm your current axle ratio before deciding. Do not guess. The AMC 20 code is stamped into the housing next to the diff cover.
Signs Your Gearing Is Wrong for Your Tires
- Fifth gear on the highway feels useless or causes the engine to lug
- You avoid top gear entirely on anything but flat road
- The Jeep feels like it is always between gears on grades
- Engine RPM at 55-60 mph is noticeably low on the tachometer
- Fuel economy is worse than expected for the engine size
If you are seeing these symptoms and the tire size is larger than stock, gearing is the root cause. No carburetor tune, ignition upgrade, or exhaust modification will fix a gearing mismatch. The math has to match.
Tire and Lift Upgrades: The Right Order
Bigger tires and a lift kit are the most common CJ modifications -- and they are frequently done in the wrong order without the supporting work. Before you increase tire size:
- Confirm steering components are tight -- bigger tires amplify every bit of slop in the linkage
- Confirm brakes are adequate for the new tire size and weight
- Calculate whether your axle ratio needs to change for the new diameter
- Plan for caster correction as part of the lift installation, not as an afterthought
- Convert the AMC 20 to one-piece shafts before adding tire size and trail load
The community's proven sweet spot: a 2.5-inch lift with 33-inch tires. It clears the tires with minimal geometry compromise, keeps driveshaft angles manageable, and works well with a 4.10 or 4.56 gear ratio depending on transmission. It is not the only valid configuration, but it is the one with the most documented success across a wide range of CJ builds and use cases.
Lockers: After the Foundation, Not Before
Locking differentials are a meaningful off-road upgrade -- but they belong after everything else is sorted. A locker on a CJ with worn steering, marginal brakes, and incorrect gearing does not make the Jeep more capable. It makes a marginal condition more dangerous. A locker on a well-sorted CJ with an honest foundation is a legitimate upgrade that actually delivers what it promises.
Labor tip: if you are paying a shop to change the ring and pinion gearing in your differentials, and your budget allows, have them install the lockers at the same time. The differential is already open. The incremental labor to add the locker at that point is a fraction of what it costs to do it as a separate job later.
The Dana 300 Transfer Case
The Dana 300 transfer case found in 1980-1986 CJs is one of the genuine strengths of the platform. It has better low-range gearing than the earlier Dana 20, helical-cut gears that are quieter and stronger, and excellent aftermarket support.
The most common upgrade is the JB Conversions 4:1 low-range kit, which replaces the stock 2.62:1 low range with a 4:1 ratio -- a significant improvement for technical trail work where low-speed control matters more than ground speed. Paired with a twin-stick conversion for independent front and rear engagement, the Dana 300 becomes a very capable transfer case at a fraction of the cost of a replacement unit. If you have a 1980-1986 CJ, the Dana 300 is worth preserving and upgrading rather than replacing.
The Foundation Is Complete -- Now Build on It
Everything in Steps 1 through 10 is foundational. It is the work that makes a CJ a CJ: reliable, honest, safe, and drivable. The most common mistake in CJ ownership is skipping this foundation and going straight to capability upgrades -- new lift, new tires, new lockers, new bumpers -- and then spending the next two years troubleshooting problems that were already there but are now amplified by the modifications.
Do the foundation first. Build the capability on top of it. A CJ that has been worked through all 10 steps is a different machine than the one that came in at Step 1 -- and every capability upgrade you add from here will perform the way it is supposed to because the platform underneath it is honest.
Return to the CJ Foundation Checklist for the complete sequence overview, or go back to CJ Builds 101 to review any step.