# Transmission Gear Ratios Calculator Calculate transmission gear ratios, overall drive ratios, MPH per gear at redline, and cruise RPM at 60 mph. Works for 3 to 6 speed manual and automatic gearboxes. ## What this calculates A transmission gear ratio tells you how many turns of the engine produce one turn of the driveshaft. Multiply that by the axle ratio and you get the overall drive ratio, which sets how fast the car goes at any given engine speed. This calculator takes up to six gear ratios plus the final drive and tire size, and returns the overall ratio for each gear, the top speed in each gear at redline, and the cruise RPM at 60 mph in top gear. ## Inputs - **1st Gear Ratio** — min 0 — First gear ratio (input shaft turns this many times per output turn). - **2nd Gear Ratio** — min 0 — Second gear ratio. - **3rd Gear Ratio** — min 0 — Third gear ratio. - **4th Gear Ratio** — min 0 — Fourth gear ratio (1:1 direct drive on most 4-speeds). - **5th Gear Ratio** — min 0 — Fifth gear ratio (overdrive on most modern boxes). Enter 0 if not applicable. - **6th Gear Ratio** — min 0 — Sixth gear ratio (deep overdrive). Enter 0 if not applicable. - **Final Drive (Axle) Ratio** — min 0 — Differential / rear-end ratio. - **Tire Diameter** (in) — min 0 — Overall tire diameter. A 225/70R16 is about 28.4 inches. - **Engine Redline** (RPM) — min 0 — Engine RPM used for the per-gear top-speed column. ## Outputs - **1st Overall Ratio** — 1st gear ratio x final drive (total reduction in 1st). - **2nd Overall Ratio** — 2nd gear ratio x final drive. - **3rd Overall Ratio** — 3rd gear ratio x final drive. - **4th Overall Ratio** — 4th gear ratio x final drive. - **5th Overall Ratio** — 5th gear ratio x final drive. - **6th Overall Ratio** — 6th gear ratio x final drive. - **Top Speed in 1st** (mph) — MPH at redline in 1st gear. - **Top Speed in 2nd** (mph) — MPH at redline in 2nd gear. - **Top Speed in 3rd** (mph) — MPH at redline in 3rd gear. - **Top Speed in 4th** (mph) — MPH at redline in 4th gear. - **Top Speed in 5th** (mph) — MPH at redline in 5th gear. - **Top Speed in 6th** (mph) — MPH at redline in 6th gear. - **Engine RPM at 60 mph (top gear)** (RPM) — Engine RPM cruising at 60 mph in the highest active gear. ## Details How transmission gear ratios work A transmission gear ratio is the number of times the input shaft spins per single turn of the output shaft. A 3.58:1 first gear means the engine turns 3.58 times for every one turn of the driveshaft. The overall drive ratio in any gear is the gear ratio multiplied by the final drive (axle) ratio: overall = gear ratio x final drive. With the overall ratio and the tire diameter you can compute road speed from engine RPM: MPH = (engine RPM x tire diameter x pi) / (overall ratio x 1056) The 1056 constant comes from 12 inches per foot times 5280 feet per mile divided by 60 minutes per hour. Flip the equation to get engine RPM from MPH: RPM = (MPH x overall x 1056) / (tire x pi). Typical transmission gear ratios Gearbox1st2nd3rd4th5th6th Muncie M21 (4-speed)2.201.641.281.00-- Tremec T-5 (5-speed WC)2.951.941.341.000.63- Tremec TR-6060 (6-speed)2.972.071.431.000.840.56 Ford 6R80 (automatic)4.172.341.521.140.870.69 Choosing gear ratios and final drive Lower gears (higher numbers like 3.5:1) give strong acceleration but lower top speed in that gear. Overdrive gears (less than 1:1 like 0.78) let the engine cruise at low RPM on the highway for better fuel economy. The final drive ratio shifts every gear at once: a 4.10 axle instead of a 3.55 trades top-end MPH for quicker acceleration throughout the rev range. For a street car you usually want the redline in top gear to happen somewhere above the vehicle's actual max speed so you are not RPM-limited. For a drag car you want first and second gear to place peak horsepower RPM right near the trap speed of each gear. Enter your candidate ratios, tire size, and redline, and the per-gear MPH column tells you whether the plan hangs together. Tire size matters as much as gear ratio Swap a 26 inch stock tire for a 33 inch off-road tire and your effective final drive drops by 33 / 26 = 1.27x. Acceleration slows, top speed rises, and the speedometer reads low. Regear the axle from 3.55 to 4.56 to restore the original effective ratio. Play with the tire diameter input to see the effect in real time. ## Frequently Asked Questions **Q: What are transmission gear ratios?** A: A transmission gear ratio is the number of input shaft revolutions per one output shaft revolution. A 3.58:1 first gear means the engine turns 3.58 times for every turn of the driveshaft. Lower gears have higher numbers (more torque multiplication, lower speed), overdrive gears have numbers below 1.00 (less torque, higher speed). **Q: How do I calculate overall drive ratio?** A: Multiply the gear ratio by the final drive (axle) ratio. A 2.97:1 first gear paired with a 3.73 axle gives an overall ratio of 2.97 x 3.73 = 11.08:1. At 6500 RPM with a 26 inch tire that is about (6500 x 26 x pi) / (11.08 x 1056) = 45.3 MPH in first gear at redline. **Q: What is a good cruise RPM for highway driving?** A: Most modern vehicles cruise between 1800 and 2400 RPM at 70 mph in top gear. Diesel trucks often run under 1700 RPM to maximize fuel economy. If this calculator returns an RPM at 60 mph above 2800 in top gear, you probably want a taller (numerically lower) final drive or a deeper overdrive gear. **Q: How do I pick gear ratios for a swap?** A: Start from the top gear: pick a final drive and top-gear ratio so that redline equals roughly the car's max speed (or the speed you want the tach to hit at the dyno). Then space the lower gears so each shift lands in the engine's power band. A close-ratio box overlaps gears more tightly for track use; a wide-ratio box gives a strong first and a deep overdrive for street and towing. **Q: Why does tire size change my transmission gear ratios?** A: Tire diameter does not change the gear ratios themselves, but it changes the effective final drive. A taller tire (larger diameter) reduces the effective reduction and raises top speed in each gear while cutting acceleration. The formula in this calculator uses tire diameter directly, so larger tires mean the MPH at redline goes up and the cruise RPM at 60 mph goes down. **Q: What is the difference between 1:1 and overdrive gear?** A: A 1:1 gear (direct drive, typical of 4th in most 4-speeds) means the input and output shafts turn at the same speed. Any ratio below 1:1 (like 0.78 or 0.60) is an overdrive, where the output shaft spins faster than the input. Overdrive gears are used for highway cruising to reduce engine RPM and improve fuel economy. --- Source: https://vastcalc.com/calculators/physics/transmission-gear-ratios Category: Physics Last updated: 2026-04-08