Rear End Gear Ratio Calculator

Enter your vehicle’s details to calculate engine RPM at a given speed or find the ideal rear end gear ratio for your setup. Our Rear End Gear Ratio Calculator is easy to use.

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Engine RPM at different speeds for various rear end gear ratios, assuming Tire Diameter: 27” and Trans Ratio: 0.70.

Gear Ratio	RPM at 55 MPH	RPM at 65 MPH	RPM at 75 MPH
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What is a Rear End Gear Ratio?

The rear end gear ratio (also known as the differential gear ratio or axle ratio) is a crucial specification in any vehicle with a driveshaft connecting the transmission to the drive wheels. It represents the ratio between the number of teeth on the ring gear and the number of teeth on the pinion gear within the differential. For instance, a 4.10:1 rear end gear ratio means the driveshaft (connected to the pinion gear) turns 4.10 times for every one rotation of the rear wheels (connected to the ring gear).

This ratio is a multiplier for torque from the engine and transmission before it reaches the wheels, and it also dictates how fast the engine turns (RPM) relative to the vehicle’s speed (MPH). A higher numerical ratio (e.g., 4.10) is considered a “lower” or “shorter” gear, providing more torque multiplication for better acceleration but resulting in higher engine RPM at highway speeds. A lower numerical ratio (e.g., 3.08) is a “higher” or “taller” gear, offering less torque multiplication but lower engine RPM at highway speeds, potentially improving fuel economy.

A rear end gear ratio calculator is a tool used by mechanics, racers, and car enthusiasts to understand the relationship between tire size, transmission gearing, rear end gear ratio, engine RPM, and vehicle speed. By inputting known values, you can calculate the unknown, such as the engine RPM at a specific speed with a certain gear ratio, or determine the ideal rear end gear ratio to achieve a target RPM at a desired cruising speed.

Who should use a Rear End Gear Ratio Calculator?

• Performance Enthusiasts: To optimize acceleration or top speed by changing the rear end gear ratio.
• Off-Roaders: To adjust gearing for larger tires and maintain crawling ability or highway performance.
• Truck Owners: To select the right ratio for towing heavy loads while managing RPM and fuel economy.
• Restorers & Builders: To match components for a custom vehicle build.
• Anyone Changing Tire Size: To see how different tires affect the effective gear ratio and speedometer accuracy.

Common Misconceptions

• Higher number means faster top speed: False. A higher numerical ratio (e.g., 4.10 vs 3.08) means more acceleration but generally a lower top speed (or reaching it at max RPM sooner) compared to a lower numerical ratio, assuming the engine has enough power.
• All gear ratios affect fuel economy the same: False. Lower numerical ratios (taller gears) often lead to better highway fuel economy by reducing engine RPM, while higher numerical ratios (shorter gears) can reduce it due to higher cruising RPM.
• It’s just about the rear end: The final drive ratio is a combination of the transmission gear ratio and the rear end gear ratio. Overdrive transmissions significantly affect the final drive.

Rear End Gear Ratio Formula and Mathematical Explanation

The core relationship between these factors can be expressed with a few key formulas. The most common is calculating Engine RPM based on other factors:

RPM = (MPH * Rear End Gear Ratio * Transmission Gear Ratio * 336.13) / Tire Diameter (inches)

Where 336.13 is a constant derived from inches per mile (63360) and minutes per hour (60), divided by pi (to relate diameter to circumference), simplified (63360 / (pi * 60) is roughly 336.13).

We can rearrange this formula to solve for other variables:

Rear End Gear Ratio = (RPM * Tire Diameter) / (MPH * Transmission Gear Ratio * 336.13)

MPH = (RPM * Tire Diameter) / (Rear End Gear Ratio * Transmission Gear Ratio * 336.13)

Tire revolutions per mile can be calculated as: Tire Revs/Mile = 63360 / (Tire Diameter * PI) ≈ 20168 / Tire Diameter.

Variables Table

Variable	Meaning	Unit	Typical Range
RPM	Engine Revolutions Per Minute	RPM	500 – 8000+
MPH	Vehicle Speed	Miles Per Hour	0 – 200+
Rear End Gear Ratio	Ratio of ring to pinion gear teeth	Ratio (e.g., 3.73)	2.50 – 5.13+
Transmission Gear Ratio	Ratio of the selected transmission gear	Ratio (e.g., 0.67)	0.50 – 4.00+
Tire Diameter	Overall diameter of the tire	Inches	20 – 40+

Practical Examples (Real-World Use Cases)

Example 1: Calculating RPM at Highway Speed

John has a car with 27-inch tires, a 0.70 overdrive transmission ratio, and a 3.73 rear end gear ratio. He wants to know his engine RPM at 65 MPH.

• Tire Diameter = 27 inches
• Transmission Ratio = 0.70
• Rear End Gear Ratio = 3.73
• Speed = 65 MPH

Using the formula: RPM = (65 * 3.73 * 0.70 * 336.13) / 27 ≈ 2110 RPM.
So, John’s engine will be turning at approximately 2110 RPM at 65 MPH.

Example 2: Choosing a Gear Ratio for Cruising

Sarah is building a hot rod with 30-inch tall tires and an overdrive transmission with a 0.64 top gear. She wants the engine to run around 1800 RPM at 70 MPH for comfortable cruising.

• Tire Diameter = 30 inches
• Transmission Ratio = 0.64
• Target RPM = 1800
• Target Speed = 70 MPH

Using the formula: Rear End Gear Ratio = (1800 * 30) / (70 * 0.64 * 336.13) ≈ 3.59.
Sarah should look for a rear end gear ratio close to 3.55 or 3.73, with 3.55 being very close to her target.

How to Use This Rear End Gear Ratio Calculator

Our Rear End Gear Ratio Calculator is designed to be intuitive:

1. Enter Tire Diameter: Input the overall diameter of your vehicle’s drive tires in inches. You can find this on the tire sidewall or use a tire size calculator.
2. Enter Transmission Top Gear Ratio: Input the ratio of your transmission’s highest gear (or the gear you are interested in, usually top gear for cruising). For overdrive, this will be less than 1.00 (e.g., 0.67, 0.70).
3. Enter Rear End Gear Ratio: Input your current or desired rear end gear ratio (e.g., 3.08, 3.73, 4.10).
4. Enter Speed (MPH): Input the speed at which you want to calculate the engine RPM.
5. Enter Desired RPM and Speed (Optional): If you want to find an ideal gear ratio, fill in your target RPM and the speed at which you want to achieve it.
6. View Results: The calculator instantly shows:
   • The engine RPM at the specified speed with the given gear ratio.
   • The required rear end gear ratio to achieve your target RPM at your target speed.
   • Intermediate values like tire revolutions per mile.
7. Analyze Table and Chart: The table and chart show RPM at various speeds for the entered gear ratio and two nearby ratios, helping you visualize the impact of different gearing.

The results help you decide if your current gearing is suitable or what changes might be needed to meet your performance or fuel economy goals with your rear end gear ratio.

Key Factors That Affect Rear End Gear Ratio Results

Several factors influence the relationship between your rear end gear ratio, RPM, and speed:

1. Tire Diameter: Larger tires effectively make the overall gearing “taller” (lower numerical ratio), reducing RPM at a given speed. Smaller tires do the opposite.
2. Transmission Gear Ratios: The gear ratios within the transmission, especially the top gear (overdrive), significantly impact the final drive ratio and thus cruising RPM.
3. Rear End Gear Ratio: This is the direct multiplier in the differential. Higher numbers mean more torque multiplication and higher RPM at speed.
4. Engine Power Band: The ideal rear end gear ratio should keep the engine within its optimal power band for the intended use (e.g., cruising at a torque peak for efficiency or keeping it in the horsepower peak for racing). Check out our engine horsepower calculator for more.
5. Vehicle Weight and Aerodynamics: Heavier vehicles or those with poor aerodynamics might require shorter gearing (higher numerical ratio) to maintain performance, even if it means higher cruising RPM.
6. Intended Use: A vehicle used mainly for highway cruising benefits from taller gearing for lower RPM and better fuel economy, while a drag racing car needs much shorter gearing for maximum acceleration. Consider using our quarter-mile calculator to see how gearing affects ET.
7. Converter Stall Speed (Automatics): For automatic transmissions, the torque converter’s stall speed can influence the “feel” and initial acceleration, interacting with the gear ratio.
8. Fuel Economy vs. Performance: There’s often a trade-off. Shorter gears improve acceleration but can hurt fuel economy at higher speeds due to increased engine RPM. Taller gears do the opposite.

Frequently Asked Questions (FAQ)

What is a good rear end gear ratio for fuel economy?

Generally, a lower numerical ratio (taller gear, e.g., 2.73, 3.08) combined with an overdrive transmission will yield better highway fuel economy by keeping engine RPM low at cruising speeds.

What is a good rear end gear ratio for acceleration?

A higher numerical ratio (shorter gear, e.g., 3.73, 4.10, 4.56) provides better torque multiplication for quicker acceleration, especially from a stop.

How does changing tire size affect my effective rear end gear ratio?

Increasing tire diameter effectively lowers your numerical gear ratio (makes it taller), reducing RPM at a given speed. Decreasing tire diameter effectively raises it (makes it shorter), increasing RPM. You might need a speedometer calculator to adjust for tire changes.

Can I change just the rear end gear ratio?

Yes, you can change the ring and pinion gears within the differential to alter the rear end gear ratio, but it’s a job that requires precision and specialized tools.

What’s the difference between rear end gear ratio and final drive ratio?

The rear end gear ratio is the ratio within the differential. The final drive ratio is the product of the transmission gear ratio (in a specific gear) and the rear end gear ratio. So, in overdrive, the final drive ratio is lower than the rear end ratio.

Will a different rear end gear ratio affect my speedometer?

Yes, changing the rear end gear ratio or tire size will affect speedometer accuracy unless it’s recalibrated. The speedometer calculator can help estimate the error.

Is a 3.73 gear ratio good for a daily driver?

It can be, especially if paired with an overdrive transmission. It offers a good balance between decent acceleration and manageable highway RPM for many vehicles.

How do I find my current rear end gear ratio?

Sometimes there’s a tag on the differential cover, or you can look up your vehicle’s VIN or build sheet. Alternatively, you can jack up the rear end, mark the driveshaft and tire, rotate the tire one full revolution, and count the driveshaft rotations (this is less precise).