Gear Ratio Speed Calculator
RPM, tire diameter, transmission gear, and axle ratio in — vehicle speed out, across every gear, so you know exactly what a tire swap or re-gear actually changes.
Analyze the mechanical advantage of your specific drivetrain configuration to uncover absolute top speeds and final wheel rotations.
Swap to bigger tires without changing your axle ratio and the truck immediately feels slower — not because it lost power, but because the effective gearing changed. The same engine torque now has to rotate a larger tire, which acts exactly like dropping to a numerically lower axle ratio. The gear ratio speed calculator above puts numbers on that relationship: enter your RPM, transmission gear, differential ratio, and tire diameter to see vehicle speed in any gear — and to find out what re-gearing would actually fix.
How Vehicle Speed Is Calculated from Gear Ratios
Vehicle speed depends on four variables working together: engine RPM, transmission gear ratio, final drive (axle) ratio, and tire diameter. The formula: MPH = (RPM × Tire Diameter) ÷ (Final Drive × Transmission Gear × 336). The constant 336 applies to manual transmissions and locked torque converters; use 355 for automatics with an unlocked converter, which accounts for converter slippage.
At 3,000 RPM in 3rd gear (1.33 ratio) with a 3.73 axle and 31-inch tires: MPH = (3,000 × 31) ÷ (3.73 × 1.33 × 336) = 55.7 mph. Change to 35-inch tires without re-gearing and the same scenario becomes 63.1 mph — the truck reads faster on the speedometer but actually lost torque multiplication at the wheels.
What Each Ratio Does to Speed and Torque
| Axle Ratio | Character | Cruising RPM at 70 mph (31″ tire, 0.73 OD) | Best For |
|---|---|---|---|
| 2.73 | Tall — low RPM, low torque multiplication | ~1,640 RPM | Highway fuel economy, light loads |
| 3.08 | Balanced highway | ~1,850 RPM | Daily driving, moderate towing |
| 3.55 | Balanced all-around | ~2,130 RPM | Mixed use, moderate towing |
| 3.73 | Performance-oriented | ~2,240 RPM | Towing, hauling, spirited driving |
| 4.10 | Deep — high torque multiplication | ~2,460 RPM | Heavy towing, larger tires |
| 4.56 | Very deep | ~2,740 RPM | Drag racing, extreme towing, very large tires |
| 4.88+ | Extreme reduction | ~2,930 RPM | Off-road crawling, maximum tire diameter compensation |
Tire Size Changes Your Effective Gearing — Here’s By How Much
Every inch of added tire diameter effectively lowers your final drive ratio by roughly 3%. Going from 31-inch to 35-inch tires (a 13% diameter increase) is equivalent to dropping your axle ratio from 3.73 to approximately 3.28 — a meaningful loss in off-the-line torque, towing capability, and low-speed response. The rule of thumb: increase your axle ratio by 3–4% for every additional inch of tire diameter to maintain stock-equivalent performance.
| Stock Tire → New Tire | Diameter Increase | Stock Ratio | Recommended Re-Gear Target |
|---|---|---|---|
| 31″ → 33″ | +6.5% | 3.55 | 3.73 |
| 31″ → 35″ | +12.9% | 3.73 | 4.10–4.30 |
| 33″ → 35″ | +6.1% | 3.73 | 4.10 |
| 33″ → 37″ | +12.1% | 3.73 | 4.56 |
| 35″ → 40″ | +14.3% | 4.10 | 4.88 |
Gear Reduction for Non-Vehicle Applications
Outside of vehicles, gear ratio problems follow the same math but with different inputs. A motor running at 1,750 RPM driving a 60-tooth gear meshed with a 20-tooth gear produces a 3:1 reduction — output shaft turns at 583 RPM with three times the input torque (minus friction losses, typically 2–5% per stage). For multi-stage reductions, multiply the individual ratios: a 3:1 first stage driving a 4:1 second stage gives 12:1 overall. Output RPM = input RPM ÷ 12. Output torque = input torque × 12 × combined efficiency.
Real-world efficiency per stage runs 96–98% for helical gears, 93–97% for spur gears, and 85–95% for worm gears. On a 3-stage worm reduction at 90% per stage, the actual output torque is the theoretical value × 0.9³ = 72.9% of ideal. That gap matters for sizing motors and designing around thermal limits.
What Cruising RPM Tells You About Your Setup
Most modern engines are most efficient between 1,600–2,500 RPM at highway cruise. Below 1,600 in overdrive, the engine may lug under load. Above 2,800 RPM at cruise, fuel economy suffers and long-distance trips become noisy. If your current setup puts you outside that window at highway speed, the calculator shows you exactly which ratio change — axle, transmission gear swap, or tire size adjustment — closes the gap most efficiently.
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A 4.10 axle ratio spins the driveshaft 4.10 times per wheel revolution; a 3.73 spins it 3.73 times. The 4.10 multiplies torque more aggressively — stronger acceleration and better towing — but at higher cruising RPM and lower fuel economy. On a truck with 33-inch tires, the 4.10 runs roughly 300 RPM higher at 70 mph than the 3.73. Which is better depends on use: 3.73 suits mixed highway driving; 4.10 suits heavy towing or larger tires.
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MPH = (RPM × Tire Diameter in inches) ÷ (Transmission Gear Ratio × Axle Ratio × 336). Use 336 for manual transmissions and locked torque converters; use 355 for automatics with an unlocked converter. Example: 3,000 RPM in 1st gear (3.82), 3.73 axle, 33-inch tires: (3,000 × 33) ÷ (3.82 × 3.73 × 336) = 20.5 mph.
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Yes — larger tires cover more ground per revolution, which acts like numerically lowering your axle ratio. A 10% increase in tire diameter has the same effect on torque delivery as dropping your ratio by 10%. Installing 35-inch tires on a truck geared for 31s makes it feel sluggish off the line. A re-gear to a numerically higher ratio compensates.
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Most dedicated off-road builds target a crawl ratio of at least 50:1 — calculated as first gear × transfer case low range × axle ratio. Serious rock crawling setups run 70:1 to 100:1 or higher. A stock Jeep Wrangler Rubicon achieves around 73:1. Higher crawl ratios give more control at ultra-low speeds but raise the minimum manageable speed on technical terrain.