How to Choose a Gear Ratio

The decision framework: restore-stock math, use-case bias, and the RPM window that keeps a dual-purpose rig honest.

Choosing a gear ratio is one calculation and one judgment call. The calculation is exact. The judgment call is where the cost lives — so do the calculation first, on paper, before forums get a vote.

Step one: restore-stock math

When you fit taller tires, your effective gearing gets taller by exactly the same percentage. The fix that returns factory behavior is:

Restore-stock ratio = factory axle ratio × (new tire diameter ÷ factory tire diameter)

Use actual measured diameters, not sidewall theater — a "35" is usually 34.5–34.8 inches, and the tire diameter calculator converts metric sizes honestly. A JL Sport (3.45 gears, 31.5-inch tires) going to true 35s needs 3.83 to feel stock again. There is no 3.83 in the catalog, which is why this is a decision and not a lookup.

Step two: bias by use

Restore-stock is the floor for a street-driven rig. From there:

Mostly daily driving / highway: pick the catalog ratio nearest restore-stock. Going deeper costs cruise RPM on every mile you own the vehicle.
Trail and overland: one step deeper than restore-stock. The crawl ratio gain is real, and the cruise penalty is a few hundred RPM.
Rock crawling or heavy towing: one to two steps deeper. Crawl and grade-holding matter more than 70-mph serenity, and a transmission that stops hunting gears on a grade is worth RPM.
Dedicated trailer/crawler: let the crawl ratio calculator drive. Cruise RPM is a rounding error in this vehicle's life.

Step three: check the RPM window

Whatever you choose, verify the cruise number before you order: use the regear comparison calculator and read RPM at your real highway speed. Most modern overdrive rigs are calm between roughly 2,200 and 2,800 RPM at 70–75. Below that window after a regear, you'll feel the downshifts you were trying to escape; above it, you'll hear the choice on every trip.

The mistakes that cost money

Gearing for the tire you might buy next year. Buy gears for the tires on the truck. A regear costs $1,000–$2,700 installed; doing it twice because of a hypothetical is the most expensive sentence in this hobby.
Ignoring the carrier break. On several common axles, crossing a ratio threshold means a new carrier or thick-cut gears. Check carrier breaks before the parts order, not during the install.
Forgetting the programmer. Electronic-platform regears aren't finished until the computer knows the new ratio and tire size — budget it in the cost guide.

The Decision System runs exactly this framework on your rig — worksheet, axle tables, budget and parts list — if you'd rather not assemble it by hand.