Re-Gear Calculator (Axle Ratio for Bigger Tires)

Symptoms beat formulas: constant downshifting on mild highway grades, needing one gear lower than before for the same hill, transmission temperature creeping while towing, and around-town acceleration that feels asthmatic. As arithmetic: under 5% tire-diameter increase rarely justifies the cost; 8–12% (33→35, 31→34) is the classic re-gear zone; beyond 15% the driveline is genuinely mismatched and re-gearing transforms the truck.

Round deeper (numerically higher) if you tow, crawl or run heavy bumpers and armor — the extra 100–200 cruise RPM is cheaper than lugging. Round taller if the truck is a highway commuter wearing big tires for looks. Example: restore says 4.13 — a tower buys 4.30s, a commuter 4.10s. Remember both axles must match exactly on 4WD; you're buying two gearsets and two installs.

Because lower RPM only helps when the engine isn't fighting: big tires add weight (10–20 lb per corner of rotating mass), aerodynamic frontal area from the lift, and rolling resistance from aggressive tread — then the too-tall gearing forces wide throttle openings and converter unlock to hold speed. Re-gearing often IMPROVES economy on heavily-tired trucks: the engine returns to its efficient island even though RPM rises. The odometer also under-counts miles after taller tires, fabricating part of the apparent loss.

Parts run $200–400 per gearset and quality installs $400–800 per axle (setup of pinion depth and backlash is craftsman work — bad setup whines or fails). While the diff is open: bearings, seals and — the popular bundle — a locker or limited-slip, since the labor overlaps almost entirely. Afterward, recalibrate the speedometer for both the tires and any electronically-known axle ratio; our speedometer-error tool quantifies why.