โ๏ธ Gear Ratio Calculator
See how bigger tires affect your engine RPM and whether you need to re-gear. Enter your tire diameter, axle ratio, and transmission type.
๐ Tire
โ๏ธ Drivetrain
๐ฃ๏ธ Driving
๐ง How It Works
Engine RPM is calculated as: RPM = (Speed ร Gear Ratio ร Transmission Ratio ร 336) รท Tire Diameter. The constant 336 comes from converting units (mph to inches per minute, factoring ฯ). When you install larger tires, the effective gear ratio drops, causing lower RPM at cruise โ which can lead to lugging and poor acceleration.
How to use: Enter your tire diameter (or parse from a tire size), current axle ratio, transmission overdrive ratio, and your typical cruise speed. The calculator shows your current RPM and recommends a new gear ratio to hit your target RPM range.
Tip: For daily driving, 2,000โ2,400 RPM at highway speed is the sweet spot. For towing, aim for 2,500โ2,800 RPM to keep the engine in its power band.
๐ RPM vs Speed Curve
Engine RPM across the speed range for your current setup vs. recommended gearing.
๐ Saved Calculations
| Tire | Gear | RPM @ 70 | Recommended | Speedo Error | Action |
|---|
