# Gear Ratio Calculator

> Calculate engine RPM at a given speed after changing differential or transmission gear ratios

**Category:** Physics
**Keywords:** gear ratio, transmission, differential, RPM, engine speed, drivetrain, final drive, tire size
**URL:** https://complete.tools/gear-ratio-calculator

## How gear ratio calculations work

The core formula connecting engine RPM to vehicle speed is:

```
RPM = (Speed x Final Drive x Gear Ratio x 336.13) / Tire Diameter
```

Where speed is in miles per hour, tire diameter is in inches, and 336.13 is a unit-conversion constant that accounts for the relationship between linear tire travel per revolution and distance units. To solve for speed instead:

```
Speed = (RPM x Tire Diameter) / (Final Drive x Gear Ratio x 336.13)
```

The **effective gear ratio** for any given transmission gear is simply the product of the transmission ratio and the final drive ratio. For example, a first gear of 2.97 combined with a 3.73 final drive yields an effective ratio of 11.08:1. This means the crankshaft turns 11.08 times for every single revolution of the drive wheels in that gear.

Tire diameter plays a critical role because it determines how much ground the vehicle covers per wheel revolution. Larger tires effectively act as a taller gear, reducing engine RPM at a given speed. The calculator computes tire diameter from the standard metric tire designation (width/aspect ratio/rim diameter) using:

```
Tire Diameter = (2 x (Width_mm x Aspect_Ratio / 100) / 25.4) + Rim_Diameter
```

For instance, a 225/45R17 tire has a sidewall height of 225 x 0.45 = 101.25 mm (3.99 inches), giving a total diameter of 2 x 3.99 + 17 = 24.98 inches.

## Common gear ratio changes

**Differential gear swaps** are the most popular drivetrain modification. A typical example is changing from a 3.55 to a 3.73 or 4.10 ring-and-pinion set. Higher numerical ratios (shorter gears) increase engine RPM at any given speed, which improves acceleration by keeping the engine closer to its torque peak. The tradeoff is higher highway cruising RPM and slightly reduced fuel economy. Going from 3.55 to 4.10 gears increases RPM by roughly 15 percent at every speed.

**Tire size changes** have a similar but inverse effect. Fitting taller tires (for off-road capability or aesthetics) effectively lowers your gear ratio. Many truck and SUV owners who install larger tires also re-gear their differential to compensate for the lost acceleration and restore factory-like driving behavior. For example, upgrading from 31-inch to 35-inch tires on a Jeep Wrangler with 3.73 gears might call for a swap to 4.56 gears.

**Transmission swaps** are less common but still relevant, especially in classic car builds or racing applications. Upgrading from a 4-speed to a 6-speed transmission provides additional gear ratios that can improve both acceleration and highway fuel economy. The extra overdrive gear in a modern 6-speed allows cruising at lower RPM on the highway while still offering aggressive lower gears for launches.

**Transfer case considerations** apply to four-wheel-drive vehicles. The transfer case low-range ratio further multiplies the overall gear reduction. When calculating gear ratios for off-road use, multiply the effective ratio by the transfer case low-range ratio to get the crawl ratio.

## How to use this calculator

1. Select your tire size from the preset list or enter custom dimensions. The tire width in millimeters, aspect ratio, and rim diameter in inches will auto-populate when you choose a preset. The calculator displays the computed tire diameter.
2. Choose a transmission preset that matches your vehicle, or select "Custom" and enter each gear ratio manually. You can add or remove gears as needed.
3. Enter your differential (final drive) ratio. Common values include 3.08, 3.23, 3.42, 3.55, 3.73, 3.90, 4.10, and 4.56. Check your vehicle's build sheet or rear axle tag for the exact ratio.
4. Set your engine redline RPM. This determines the maximum speed in each gear.
5. Enter the vehicle speed you want to analyze and choose between mph or km/h.
6. Review the results table showing RPM in each gear at your chosen speed, along with the maximum speed achievable in each gear at redline.
7. To compare two setups, switch to "Compare Two Setups" mode. Configure both Setup A (your current drivetrain) and Setup B (your planned changes) to see RPM differences at a glance.

## FAQs

**Q:** How does tire size affect engine RPM?


**A:** Larger tires reduce engine RPM at a given speed because the wheel covers more ground per revolution. Each additional inch of tire diameter lowers RPM by roughly 3 to 4 percent. This is equivalent to installing a numerically lower (taller) gear in the differential. Conversely, smaller tires raise RPM, similar to installing shorter gears.


**Q:** What happens when I change my differential ratio?


**A:** Changing to a numerically higher differential ratio (for example, from 3.55 to 4.10) increases engine RPM at every speed in every gear. This improves acceleration because the engine stays closer to its power band, but it raises highway cruising RPM. A numerically lower ratio (such as going from 3.73 to 3.08) reduces RPM everywhere, improving fuel economy at the cost of acceleration.


**Q:** How do I find my vehicle's gear ratios?


**A:** Transmission gear ratios are published in your vehicle's service manual and are often available online from enthusiast forums and manufacturer databases. For the differential ratio, check the RPO codes on your vehicle's build sheet, look for a tag on the differential housing, or count ring and pinion teeth. Many vehicles also list the axle ratio on the original window sticker.


**Q:** What is an effective gear ratio?


**A:** The effective gear ratio is the total mechanical reduction from the engine to the wheels in a given transmission gear. It equals the transmission gear ratio multiplied by the final drive (differential) ratio. A first gear of 3.27 with a 4.10 final drive gives an effective ratio of 13.41:1, meaning the engine turns 13.41 times for each wheel revolution.


**Q:** Does this calculator account for drivetrain losses?


**A:** No. This calculator uses the theoretical relationship between RPM, speed, gear ratios, and tire diameter. Real-world values may differ slightly due to tire deflection under load, torque converter slip in automatic transmissions, and minor frictional losses through the drivetrain. The calculated values are typically within 1 to 2 percent of actual measured values.

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