# Speedometer Calculator Speedometer calculator for tire-size changes and axle gear swaps. Get actual speed, speedometer error percent, and the corrected MPH reading. Speedometer gear calculator mode included. ## What this calculates When you change tire size or regear the axle, your speedometer reads wrong. This speedometer calculator takes the original and new tire diameters (or the old and new axle ratios for a speedometer gear calculator workflow) and returns your true ground speed, the error percentage, and the corrected speedometer reading. Works for trucks with larger off-road tires, race cars with shorter final drives, or any vehicle where someone has asked is my speedometer accurate after a change. ## Inputs - **Change Type** — options: Tire size change, Axle gear ratio change — Tire mode uses tire diameters. Gear mode uses old and new axle ratios (speedometer gear calculator). - **Original Tire Diameter** (in) — min 0 — Factory tire diameter. For a 225/70R16, enter 28.4 inches, or switch to tire-size mode. - **New Tire Diameter** (in) — min 0 — Diameter of the tire you are putting on (or have on) the vehicle. - **Speedometer Reading** (mph) — min 0 — Speed your speedometer is showing right now. - **Original Axle Ratio** — min 1, max 6 — Factory rear axle (or transmission speedometer calibration) ratio. Used in gear-change mode. - **New Axle Ratio** — min 1, max 6 — New rear axle ratio after a regear. Used in gear-change mode. ## Outputs - **Actual Speed** (mph) — Real ground speed with the new tires or gears. - **Speedometer Error** (%) — Positive = speedo reads slow (you are faster than it says). Negative = reads fast. - **Corrected Indicated Speed** (mph) — What the speedometer should read (if recalibrated) to match the displayed number on a stock setup. - **Actual Speed** (km/h) — Actual speed in kilometers per hour. - **Diameter or Gear Ratio** — new / original ratio applied to the indicated speed. ## Details Tire-size speedometer error A speedometer measures how fast the driveshaft (or output shaft) is turning. The vehicle computer converts that to MPH assuming the stock tire diameter. Put a taller tire on and each revolution covers more ground, but the speedometer still thinks it covered the stock distance. Result: speedometer reads low, you are actually going faster. The formula is actual speed = indicated speed x (new tire diameter / original tire diameter). For a truck with 28 inch factory tires swapped to 31 inch off-road tires, at an indicated 60 MPH: actual = 60 x (31 / 28) = 66.43 MPH. The speedometer is reading 6.43 MPH (about 10 percent) slow. Gear-change (regear) speedometer error If you swap a 3.73 rear axle for a 4.10 to improve low-end torque, the driveshaft spins faster for the same ground speed. A gear-sensing speedometer now reads high. The formula flips: actual speed = indicated speed x (original ratio / new ratio). At an indicated 60 MPH with a 4.10 swap from 3.73: actual = 60 x (3.73 / 4.10) = 54.58 MPH. The speedometer is reading 5.4 MPH high. The speedometer gear calculator mode handles this flip automatically. For a transmission speedometer calibration (the old driven-gear kind in a manual Ford or Jeep), the same math applies: new driven gear teeth over old driven gear teeth. Why does my car speedometer not accurate anymore? Three common reasons. First, you changed tire size (the biggest offender). Second, you regeared the axle. Third, the tire has worn or is underinflated, reducing effective diameter by up to 3 percent. Even a stock vehicle reads 1 to 3 percent high from the factory because regulators in most countries allow a positive bias (speedometer reads slightly high to reduce speeding tickets), but not a negative bias. Speedometer calibration tool options Once you know the error percent, you have three ways to fix it. One: a tuner or handheld programmer (SCT, Bully Dog, Hypertech) updates the tire size in the ECU. Two: a dedicated inline speedometer calibration tool like the Yellow Box or TrueSpeed. Three: for older vehicles, swap the transmission speedometer driven gear to a different tooth count. Search for speedometer calibration near me if you need a shop to do it. ## Frequently Asked Questions **Q: How does a speedometer calculator work?** A: It compares new tire diameter to original tire diameter. Actual speed = indicated speed x (new diameter / original diameter). Larger tires make the speedometer read low, so actual is higher than indicated. For gear changes, the ratio flips: actual = indicated x (original gear / new gear). **Q: Is my speedometer accurate after bigger tires?** A: No, not without recalibration. A 3 inch diameter increase on a 28 inch stock tire is about 10 percent error. Most drivers notice it first as an odometer that racks up too few miles. Plug your original and new tire diameter into this speedometer calculator to see the exact error percentage. **Q: How do I use the speedometer gear calculator mode?** A: Switch mode to axle gear change, enter the original and new axle ratio. The ratio new/original drives error in the opposite direction from tires: a numerically-higher gear (4.10 from 3.73) makes the speedometer read high, not low. Same math, opposite sign. **Q: My car speedometer is not accurate. Can I fix it at home?** A: Sometimes. Modern OBD-II vehicles can be recalibrated with a tuner or handheld programmer that lets you set tire size. Older vehicles with cable-drive speedometers use a speedometer calibration tool or a swapped driven gear in the transmission speedometer housing. Worst case, search speedometer calibration near me to find a shop that does it. **Q: Why do even stock speedometers read slightly high?** A: Regulators in most countries (US, UK, EU, Australia) let manufacturers set the speedometer 1-3 percent high, but forbid it reading low. Automakers take the positive bias for liability reasons, which is why a tire-size speedometer calculator often shows a stock vehicle reading 2-3 percent high before you modify anything. **Q: Does tire wear affect speedometer accuracy?** A: Yes. A worn tire can be 0.5 to 1 inch smaller in diameter than new, which makes the speedometer read high by 2-4 percent. Underinflated tires have a similar effect because the loaded radius drops. Check with a tire size speedometer calculator using the actual measured diameter, not the sidewall spec. --- Source: https://vastcalc.com/calculators/physics/speedometer Category: Physics Last updated: 2026-04-08