Speedometer Gear Calculator
See how a tire size or gear ratio change shifts your speedometer reading and your true speed.
🔧 What is the Speedometer Gear Calculator?
The speedometer gear calculator tells you your true speed after you change your tires or axle gear ratio, and how far off the speedometer now reads. You enter the speed shown on the dial along with the old and new tire diameters, and optionally the old and new axle ratios, and it returns your actual speed and the percentage error.
This matters to anyone who modifies a vehicle. Off-road and truck owners who fit larger tires suddenly find they are driving faster than the gauge shows, because the bigger tire rolls further per revolution. People who re-gear the differential for towing or performance shift the error the other way. Even everyday drivers see a small effect as tires wear down. Knowing the true speed helps avoid unintentional speeding, keeps trip and fuel estimates accurate, and guides whether to recalibrate.
The physics is straightforward. A speedometer is calibrated to count wheel or driveshaft rotations and assume a fixed rolling distance per turn. Fit a larger tire and each rotation covers more ground, so the true speed is higher than indicated and the gauge under-reads. Change to a numerically higher axle ratio and the driveshaft spins faster for the same road speed, so the gauge over-reads. This calculator combines both effects: true speed equals the indicated speed times the new-over-old tire diameter ratio times the old-over-new gear ratio.
The tool is useful because it turns a modification into a concrete correction. Instead of guessing, you get your real speed for any dial reading and a clear percentage error, with the working shown so you understand which change pushed the reading which way.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Larger tires, 28 to 31 inches
Example 2 - Re-geared to a higher ratio, 3.55 to 4.10
Example 3 - Bigger tires, 27 to 30 inches
❓ Frequently Asked Questions
🔗 Related Calculators
How do bigger tires affect the speedometer?
Larger tires cover more ground per revolution, so the wheels turn fewer times at any given speed and the speedometer reads lower than your true speed. Going from a 28-inch to a 31-inch tire makes you about 11 percent faster than the dial shows, so an indicated 60 is really about 66.
How do you calculate speedometer error from tire size?
Multiply the indicated speed by the new tire diameter divided by the old diameter. For a change from 28 to 31 inches at an indicated 60, true speed = 60 x (31 / 28) = 66.4. The error is (66.4 - 60) / 60 = about 10.7 percent.
Does changing the axle gear ratio affect the speedometer?
Yes, on vehicles whose speedometer is driven from the transmission or driveshaft. A numerically higher axle ratio makes the driveshaft spin faster for a given road speed, so the speedometer reads high. This calculator lets you enter both the old and new ratios to include that effect.
Does a larger tire make the speedometer read high or low?
Low. Because a bigger tire travels further per turn, the car is actually going faster than the speedometer indicates. The dial under-reads, so you could be speeding without the gauge showing it. Smaller tires do the reverse and make the speedometer read high.
What tire measurement should I use, width or diameter?
Use the overall diameter, not the width or the printed size code. The speedometer error depends only on how far the tire rolls per revolution, which is set by its diameter. You can find the overall diameter on the tire manufacturer's specs or a tire size chart.
Is it illegal to have an inaccurate speedometer?
Rules vary by country, but many regulations require that a speedometer never reads lower than the true speed, since an under-reading gauge can lead to unintentional speeding. After fitting larger tires it is wise to recalibrate or at least know your error, which this calculator provides.
How do I fix speedometer error after changing tires?
Options include recalibrating the vehicle's computer with a dealer tool or aftermarket programmer, fitting a correction gear on older mechanical speedometers, or using a calibration device. If recalibration is not possible, you can still work out your true speed for any dial reading from the tire and gear ratio change.
Does tire wear change my speedometer reading?
Slightly. A worn tire has a smaller diameter than a new one of the same size, which makes the speedometer read a touch high. The effect is small, usually under 3 percent between new and fully worn, but it adds to any error from a tire size change.