Gear Ratio & Axle Ratio Calculator

Choosing axle and transmission gearing

• Numerically higher axle gears multiply torque and raise RPM at a given road speed.
• Overall ratio = transmission gear × axle — that is what the tire “feels” each shift.
• Pair gearing with tire diameter so cruise RPM lands in your engine’s comfort zone.

Diff and transmission swaps are about more than a catalog number. A 4.10 axle that pulls hard in first can leave you buzzing at 75 mph if the tire is short and the overdrive is shallow. This free gear ratio calculator stacks axle ratio, each transmission gear, and overall tire diameter so you can see overall ratio, road speed at a chosen RPM, and engine RPM at cruise before you order a ring and pinion.

Enter the axle ratio directly or derive it from ring and pinion tooth counts. Fill in the gear ratios for your 3- to 6-speed (or fewer), set overall diameter, then pick an interest RPM and a cruise speed. The transmission-bay ladder shows every gear side by side so you can compare launch overall against highway RPM in one glance.

The math is the same geometry used in RPM-at-speed worksheets: mph = (RPM × tire diameter in inches) ÷ (overall ratio × 336). Treat results as planning estimates — real load, converter slip, and tire growth under speed still move the needle. Cross-check with the RPM at speed and speedometer calibration tools when tires or gears change together.

Gear ratio for trucks, towing, and taller tires

Towing and heavy trucks care about staying in the torque band under load. Numerically higher axle ratios (3.73, 4.10, 4.56) multiply torque at the wheels and keep RPM up when you climb with a trailer — at the cost of higher highway RPM empty. Tall AT or mud tires do the opposite: they act like taller gears, so many lifted trucks re-gear after a tire swap to restore pull and transmission temps.

Use this gear and axle ratio calculator as a truck gearing worksheet: enter the tow or daily tire diameter, your transmission ratios (including overdrive), and a cruise speed you actually run. Compare empty highway RPM against a loaded estimate by watching whether cruise sits too low for the engine — if it does, step up a ring-and-pinion set before you buy. Pair with the RPM at speed tool when you only care about one gear and one road speed.

Taller tires vs axle ratio at 70 mph

A 35″ tire on stock 3.55s in 0.70 overdrive is a lazy highway — fine empty, often soggy with a trailer. Stepping to 4.10s on the same tire puts cruise RPM back toward where a 32″ tire on 3.73s used to live. These rows assume 35″ overall diameter and 0.70 OD.

How to use the gear ratio calculator

1. Set the axle ratio. Enter the final-drive number directly, or compute it from ring and pinion tooth counts.
2. Enter transmission ratios and tire diameter. Pick gear count (1–6), fill each ratio, and set overall tire diameter in inches or millimeters.
3. Read the gear ladder. Compare overall ratio, road speed at your interest RPM, and engine RPM at cruise for each gear.

Gear ratio and towing FAQ