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Gear Ratio & Speed Calculator

Calculate speed from RPM, gear ratios, and tire size

Final Drive Ratio

3.55:1

@3000 RPM

205.4 mph

@60 mph

876 RPM

Diff Ratio

3.55:1

1.0 for direct drive (top gear), > 1.0 for lower gears

Results

205.4
mph @ 3000 RPM
876
RPM @ 60 mph
Final drive: 3.55:1

RPM vs Speed Table

RPMSpeed (mph)
1,500102.7 mph
2,000137.0 mph
2,500171.2 mph
3,000205.4 mph
3,500239.7 mph
4,000273.9 mph
5,000342.4 mph
6,000410.9 mph
7,000479.4 mph

Diff Ratio @ 60 mph

2.73:1674 RPM
3.08:1760 RPM
3.23:1797 RPM
3.55:1876 RPM
3.73:1921 RPM
4.1:11012 RPM
4.56:11125 RPM

Frequently Asked Questions

Q

What is final drive ratio and how is it calculated?

Final drive ratio is the overall gear reduction from engine crankshaft to the driven wheels. It equals the transmission gear ratio multiplied by the differential (ring and pinion) ratio. For example, a 1st gear ratio of 3.42 combined with a 3.73 diff gives a final drive of 3.42 × 3.73 = 12.76:1, meaning the engine turns 12.76 times for every one wheel revolution.

  • Final Drive = Transmission Gear Ratio × Differential Ratio
  • Lower gears (1st, 2nd) have higher numerical ratios for more torque
  • Top gear (5th, 6th) or overdrive has a ratio below 1.0 (e.g., 0.73)
  • A higher diff ratio (4.10 vs 3.23) gives faster acceleration but lower top speed
  • Transfer case ratio also multiplies in for 4WD vehicles in low range
GearTypical RatioFinal Drive (w/ 3.73 diff)
1st3.4212.76:1
2nd2.147.98:1
3rd1.395.18:1
4th1.003.73:1
5th (OD)0.732.72:1
Q

How do I calculate RPM at highway speed?

Use the formula: RPM = (Speed × Final Drive × 336) / Tire Diameter. For example, at 70 mph in 5th gear (0.73) with a 3.73 diff and 28" tires: RPM = (70 × 0.73 × 3.73 × 336) / 28 = 2,286 RPM. Lower RPM at cruise speed means better fuel economy and less engine noise.

  • RPM = (MPH × Gear Ratio × Diff Ratio × 336) / Tire Diameter (inches)
  • Most vehicles cruise at 1,800–2,500 RPM at 70 mph in top gear
  • Taller tires or lower diff ratio reduce cruising RPM
  • Diesel trucks often cruise at 1,600–1,900 RPM due to tall overdrive gears
  • Every 200 RPM reduction at cruise can improve highway MPG by 1–2%
Q

How do I choose the right differential ratio?

Differential ratio choice depends on your priorities. Lower numerical ratios (2.73, 3.08) favor fuel economy and highway cruising. Higher ratios (3.73, 4.10, 4.56) favor acceleration and towing. If you increase tire size, a numerically higher diff ratio compensates to keep RPM and acceleration similar to stock.

  • 2.73–3.08: highway-oriented, best fuel economy, slower acceleration
  • 3.23–3.55: balanced daily driving, moderate towing capability
  • 3.73–4.10: performance-oriented, good towing, higher RPM at cruise
  • 4.30–4.88: drag racing, heavy towing, large tires; poor highway MPG
  • Rule of thumb: increase diff ratio ~3–4% for every 1" of added tire diameter
Diff RatioBest ForRPM at 70 mph (top gear 0.73, 28" tire)
2.73Fuel economy, highway1,674
3.08Balanced daily driver1,888
3.55Light towing, spirited driving2,176
3.73Performance, moderate towing2,286
4.10Heavy towing, big tires2,513
Q

How do bigger tires affect my gear ratio and speed?

Bigger tires effectively lower your final drive ratio because each wheel revolution covers more ground. A 10% increase in tire diameter has the same effect as swapping to a 10% lower diff ratio—less acceleration, lower RPM, and a slightly higher top speed. To restore stock performance after a tire upgrade, install a proportionally higher diff ratio.

  • Larger tires reduce RPM at any given speed, like installing a numerically lower diff
  • Going from 28" to 33" tires (+17.9%) drops effective ratio from 3.73 to ~3.16
  • Loss of low-end acceleration is noticeable with 2"+ tire size increase
  • Re-gearing to 4.56 from 3.73 after 33" tires restores close-to-stock RPM
  • Speedometer reads low with bigger tires; GPS or recalibration needed

Example Calculations

1Truck with 3.73 Diff at Highway Speed

Inputs

Vehicle Speed70 mph
Top Gear Ratio0.73 (6th OD)
Diff Ratio3.73
Tire Diameter32"

Result

Engine RPM2,000 RPM
Final Drive Ratio2.72:1
Tire Revolutions per Mile631

Final drive = 0.73 × 3.73 = 2.72. RPM = (70 × 2.72 × 336) / 32 = 2,000 RPM. The 32" tires keep cruise RPM lower than stock 28" tires would.

2Muscle Car in 2nd Gear at Redline

Inputs

Engine RPM6,500 RPM
2nd Gear Ratio2.10
Diff Ratio3.73
Tire Diameter28"

Result

Vehicle Speed73.5 mph
Final Drive Ratio7.83:1

Final drive = 2.10 × 3.73 = 7.833. Speed = (6,500 × 28) / (7.833 × 336) = 182,000 / 2,632 = 69.1 mph.

3Re-Gear After 33" Tire Upgrade

Inputs

Stock Tire28"
New Tire33"
Stock Diff Ratio3.73
TargetRestore stock RPM

Result

Recommended Diff Ratio4.39:1
Tire Size Increase17.9%
RPM Change Without Re-Gear–15.2%

New ratio = Stock Ratio × (New Tire / Stock Tire) = 3.73 × (33/28) = 3.73 × 1.179 = 4.40. Closest available: 4.56 (slightly over-geared) or 4.30 (slightly under).

Formulas Used

Speed from RPM

Speed (mph) = (RPM × Tire Diameter) / (Final Drive × 336)

Calculates vehicle speed at a given engine RPM, accounting for transmission gear, differential, and tire size.

Where:

RPM= Engine revolutions per minute
Tire Diameter= Overall tire diameter in inches
Final Drive= Transmission gear ratio × differential ratio
336= Conversion constant for inches-per-mile and minutes-per-hour

RPM from Speed

RPM = (Speed × Final Drive × 336) / Tire Diameter

Calculates engine RPM at a given vehicle speed.

Where:

Speed= Vehicle speed in mph
Final Drive= Transmission gear ratio × differential ratio
Tire Diameter= Overall tire diameter in inches

Understanding Gear Ratios and Vehicle Speed

Gear ratio determines how engine RPM translates into wheel speed. The drivetrain multiplies engine torque through the transmission and differential before it reaches the wheels, and understanding these ratios helps you make informed decisions about performance modifications, tire upgrades, and fuel economy.

The speed at any RPM depends on three factors: the transmission gear ratio, the differential ratio, and the tire diameter. Changing any one of these changes how fast you go at a given engine speed. This is why swapping to larger tires without re-gearing makes the vehicle feel sluggish—the engine has to work harder to turn the larger tires.

Whether you are building a drag car that needs deep 4.88 gears, a highway cruiser that benefits from tall 2.73 gears, or re-gearing after a tire upgrade, this calculator helps you see exactly how each change affects speed, RPM, and acceleration across all gear ranges.

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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