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Gearing Calculator

Your speed at any RPM, for any gear, with any tire size.

Your car's gearing is the mechanical multiplier between engine and wheels. Understanding it tells you your cruise RPM at any speed, what happens when you regear for towing or performance, and exactly how a tire size change affects your real-world speed and acceleration.

Road speed
96.2 km/h(59.8 mph)
Wheel RPM
804
Tire OD
634.3 mm
Tire circ.
1.993 m
Combined ratio
3.73

Typical usage: enter your cruise RPM (like 2,500 in 6th on the highway) and see your actual speed. Change the final drive to simulate a regear.

How Gear Ratios Work

Every gear in the transmission has a ratio — the number of times the input shaft (engine) must turn to rotate the output shaft (driveshaft) once. A ratio of 3.50 means the engine turns 3.5 times for every 1 rotation of the driveshaft. The final drive ratio then multiplies this again.

  • Combined ratio = gear ratio × final drive ratio
  • Higher combined ratio = more torque multiplication = more acceleration, higher cruise RPM, worse fuel economy
  • Lower combined ratio = less multiplication = less acceleration, lower cruise RPM, better fuel economy
  • Typical 1st gear: 3.5–4.5 (high torque multiplication for launch)
  • Typical 6th gear: 0.6–0.85 (overdrive — output faster than input)
  • Final drive (differential ratio): the permanent multiplier that affects all gears equally

Regearing: Numerically Higher vs Lower

When people say they 'went to taller gears' they mean a numerically lower ratio (e.g. 3.73 → 3.23). Higher numbers are 'shorter' gears — more rpm per mph but more torque.

  • Regear numerically higher (3.73 → 4.10): better for towing and acceleration; higher cruise RPM; worse MPG
  • Regear numerically lower (3.73 → 3.23): lower cruise RPM; better MPG; slower acceleration
  • Larger tire compensates for numerically higher gear: a 33" tire with a 4.10 drives similar to a 31" tire with a 3.73
  • Rule of thumb: for every inch of tire height added, numerically increase final drive by ~0.05–0.10

How Tire Size Changes Effective Gearing

Changing tire size is a cheap way to alter effective gearing. A larger tire has a longer circumference, so the wheel travels farther per revolution — equivalent to taller gearing. A smaller tire is like shorter gearing.

  • Upsizing from 225/45R17 to 245/40R18: ~3% taller effective gearing
  • Lift kit with 33" tires vs stock 29": ~13% taller — noticeable on low-end torque
  • Speedometer also reads low with larger tires (wheel covers more ground per rev)
  • Fuel economy may improve slightly with tall tires on flat roads; decreases with hills or towing

How to Use the Gearing Calculator

Use the calculator to find speed at a given RPM, or model a regear.

  1. 1
    Enter your engine RPM
    For a highway cruise check, enter your typical highway cruising RPM. For a top-speed estimate in a gear, enter your rev limit.
  2. 2
    Set the gear and final drive ratio
    Find gear ratios in your owner's manual or a spec database. The final drive ratio is in your axle spec or differential sticker.
  3. 3
    Enter your tire size
    The 225/45R17 format: width=225, aspect ratio=45, rim=17. The calculator determines your tire's overall diameter from these three numbers.
  4. 4
    Model a modification
    To model a regear, change the final drive ratio and see how cruise RPM shifts. To model bigger tires, change the tire dimensions and observe the speed change.

FAQ

How does changing the final drive affect my car?
Lower numerical ratio (e.g. 3.73 → 3.23) = taller gearing = lower RPM at highway speeds, better MPG, slower acceleration. Higher ratio (3.73 → 4.10) = snappier acceleration, higher cruise RPM, worse MPG.
Why does my new, bigger tire feel slower?
Bigger tire = longer wheel circumference = fewer revs per mile = effectively taller gearing. You lose acceleration but gain cruise efficiency.
How do I calculate RPM for a target speed?
RPM = (speed_km/h × 1000 × gear × finalDrive) / (60 × π × tireOD_m). Or just plug in guesses in this calc until the speed matches.
What does overdrive mean?
A gear ratio below 1.0 is called overdrive — the output shaft spins faster than the input shaft. Most modern 6-speed transmissions have the top 2 gears in overdrive (e.g. 0.84 and 0.67), lowering cruise RPM for fuel economy.

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