Final Drive Ratio Calculator uses the formula FDR = ring gear teeth ÷ pinion gear teeth to find axle ratio, then estimates speed, highway RPM, torque multiplier, and travel per engine rev.
The Number That Decides Whether Your Gear Swap Was Worth It
Drop a 4.10 ring-and-pinion into a truck running 35s, expecting a torque bump, and instead watch highway RPM climb past where the engine likes to live. The ring and pinion teeth count, the gear you’re sitting in, and the actual rolling diameter of the tire all feed into one ratio — and that ratio decides cruising RPM, top-end speed, and how much mechanical advantage actually reaches the wheels.
Final Drive Ratio Calculator Used Formula
Final Drive (Differential) Ratio: FDR = Ring Gear Teeth ÷ Pinion Gear Teeth
Total Drivetrain Ratio: Total Ratio = FDR × Transmission Gear Ratio
Vehicle Speed: MPH = (Engine RPM × Tire Diameter) ÷ (Total Ratio × 336.135)
Metric Speed: km/h = MPH × 1.609344
Speed per 1,000 RPM: MPH₁₀₀₀ = (1,000 × Tire Diameter) ÷ (Total Ratio × 336.135)
Engine RPM at a Target Speed: RPM = (Target MPH × Total Ratio × 336.135) ÷ Tire Diameter
Torque Multiplier: Multiplier = Total Ratio × 100 (per 100 lb-ft of input torque)
Tire Circumference: Circumference = Tire Diameter × π
Travel per Engine Revolution: Distance = Circumference ÷ Total Ratio
Wheel Revolutions per Mile: Revs/Mile = 63,360 ÷ Circumference
What Each Field Actually Controls
- Ring Gear Teeth / Pinion Gear Teeth: the differential’s gear set. This pair alone sets the Final Drive Ratio shown in the headline result.
- Transmission Gear Ratio: whichever single gear you want to evaluate. Enter 1.00 for direct/overdrive-equivalent gears, or the specific ratio for any other gear in the box.
- Target Engine RPM: the RPM point the speed calculation is built around — useful for checking redline speed or a cruising RPM you’re targeting.
- Overall Tire Diameter: the actual rolling diameter in inches, not the size printed on the sidewall. This single number drives circumference, speed, and revs-per-mile together.
Why “Diff,” “Trans,” and “Total” Aren’t the Same Number
The headline Final Drive Ratio is the differential alone — Ring ÷ Pinion — independent of whatever gear the transmission is in. Speed, RPM, and torque outputs, however, are all driven by the Total Ratio, which multiplies that differential number by the Transmission Gear Ratio you entered. With the default 1.00 transmission value the two numbers match, which is why it’s easy to assume Diff and Total always move together. They don’t — drop in a lower (numerically higher) transmission gear and Total climbs well past the differential ratio alone, even though the axle itself hasn’t changed.
Worked Example
41-tooth ring, 10-tooth pinion, transmission ratio of 1.00, 26-inch tire, evaluated at 3,000 RPM:
- FDR = 41 ÷ 10 = 4.10:1, and Total Ratio = 4.10 × 1.00 = 4.10:1
- Speed at 3,000 RPM = (3,000 × 26) ÷ (4.10 × 336.135) ≈ 56.60 mph (91.08 km/h)
- RPM at a steady 60 mph = (60 × 4.10 × 336.135) ÷ 26 ≈ 3,180 RPM
- Forward travel per engine revolution = (26 × π) ÷ 4.10 ≈ 19.92 inches
- Wheel revolutions per mile = 63,360 ÷ (26 × π) ≈ 776 revs, unaffected by gearing — tire size alone decides it
What the Result Doesn’t Account For
Every output here is geometric math, not a dyno reading. There’s no allowance for drivetrain friction, tire slip, torque converter behavior, or shift timing — the Torque Multiplier card shows theoretical gear multiplication only, before any mechanical losses. Speed and RPM figures are only as accurate as the tire diameter entered; a tire that’s worn, oversized, or undersized relative to its rated diameter will shift every downstream number with it.
Common Questions
Why does the Final Drive Ratio stay the same no matter what transmission gear I select?
Because that headline figure is the differential’s ring-and-pinion ratio only. Changing gears moves the Total Ratio, RPM, and speed figures — the differential itself hasn’t changed.
Should I enter the tire size from the sidewall or the measured diameter?
Use the actual rolling diameter. Sidewall-stated sizes and real rolling diameter can differ enough to throw off every speed and RPM figure the calculator produces.