# Tire Size & Gear Ratio Calculator Enter a tire size (285/70R17), differential/final-drive ratio, and transmission gear ratios to compute the true overall tire diameter, circumference, revolutions per mile/km, an RPM × gear speed table, effective final drive after a tire change, and speedometer error. Metric or imperial units, with optional before/after tire comparison. > Canonical page: https://elysiatools.com/en/tools/tire-gear-ratio-calculator - **Category:** Calculator - **Keywords:** tire size, gear ratio, final drive, differential, rpm, speedometer, transmission, 4x4, off-road, wheel, p-metric, revs per mile, effective gear ratio, regear ## Overview A drivetrain calculator for car, truck, and 4×4 modifiers: 1. **Tire size** — enter a P-metric spec like `285/70R17`. Computes sidewall, diameter, circumference, revolutions per mile/km. 2. **Final drive** — your differential / axle ratio (e.g. `4.10`). 3. **Transmission gears** — one ratio per line/comma; prefix `R` for reverse. 4. **RPM table** — a matrix of speed in every gear at every RPM. 5. **Tire comparison** — diameter change, effective final drive, speedometer error, actual speed. All math runs locally. ## Inputs - **Tire size (e.g. 285/70R17)** (text): Full P-metric tire size, e.g. 285/70R17, 265/75R16, 245/45R18. - **Differential (final drive) ratio** (number): 4.10 - **Transmission gears (one ratio per line, e.g. 3.6,2.1,1.4,1,0.8,R3.8)** (textarea): Gear ratios in order; prefix with R for reverse (e.g. R3.8). Comma or newline separated. - **Units** (select) - **Reference speed** (number): Speed used for the cruise-RPM and speedometer-error calculations (in your selected unit). - **Engine RPM for table** (text): Comma-separated RPM values for the speed matrix columns. - **Original tire (for comparison, optional)** (text): Optional original tire size for before/after comparison. ## When to use - When upgrading to larger off-road tires and planning a differential regear to restore factory performance. - When comparing engine RPMs at highway cruising speeds across different transmission gear ratios. - When correcting speedometer error and calculating the effective final drive ratio after changing tire sizes. ## How it works - Enter your current or target tire size in P-metric format (e.g., 285/70R17) to calculate overall diameter, circumference, and revolutions per mile or kilometer. - Input your differential final drive ratio and transmission gear ratios, separating each gear ratio with a comma or newline. - Specify your target engine RPM range and reference speed to generate a complete speed-to-RPM matrix and calculate speedometer deviation. ## Use cases - Calculating engine RPM at a specific cruising speed to optimize fuel economy on the highway. - Determining the correct differential gear ratio (regearing) needed to compensate for larger 4x4 tires. - Generating a comprehensive speed-in-gear matrix for track or off-road driving preparation. ## Frequently asked questions ### How does changing tire size affect my speedometer? Larger tires have a greater circumference, meaning they travel further per revolution, which causes your speedometer to read slower than your actual speed. ### What is the effective final drive ratio? It is the adjusted gear ratio resulting from a tire size change, showing how the new tire diameter alters your vehicle's mechanical leverage compared to the original setup. ### Can I input reverse gear ratios in the calculator? Yes, you can include your reverse gear ratio by prefixing the value with the letter 'R' (e.g., R3.8). ### Does this calculator support both metric and imperial units? Yes, you can toggle between metric units (km/h, mm) and imperial units (mph, inches) for all inputs and outputs. ### Are my vehicle specifications sent to a server? 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