Tire Diameter Speedometer Error Calculator
How a tire size change affects speedometer accuracy.
Enter your values
Enter the original tire diameter in in.
Enter the new tire diameter in in.
Enter the indicated speed in mph.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Speedometer error
- 5.96%
- Actual speed when showing 70 mph
- 74.2 mph
- Direction of error
- Speedometer reads slow
- Diameter change
- 1.7 in
Quick answer
Speedometer error percentage equals the percentage change in overall tire diameter between old and new tires.
error % = (new diameter - old diameter) / old diameter x 100
At a glance
| What it does | How a tire size change affects speedometer accuracy. |
|---|---|
| Category | Automotive |
| Inputs needed | Original tire diameter, New tire diameter, Indicated speed |
| Main output | Speedometer error |
| Formula | error % = (new diameter - old diameter) / old diameter x 100 |
| Cost | Free — no sign-up, no download |
How to use the Tire Diameter Speedometer Error Calculator
- 1Enter the original tire's overall diameter.
- 2Enter the new tire's overall diameter.
- 3The error percentage shows how far off your speedometer and odometer will be.
Inputs explained
- Original tire diameter(in)
- Enter the original tire diameter in in.
- New tire diameter(in)
- Enter the new tire diameter in in.
- Indicated speed(mph)
- Enter the indicated speed in mph.
Worked example
Using the values the calculator loads with:
Inputs
- Original tire diameter28.5 in
- New tire diameter30.2 in
- Indicated speed70 mph
Results
- Speedometer error5.96%
- Actual speed when showing 70 mph74.2 mph
- Direction of errorSpeedometer reads slow
- Diameter change1.7 in
Frequently asked questions
Why does bigger diameter make the speedometer read low?
A larger tire covers more distance per revolution, so the speedometer (calibrated for the old size) underreports actual speed.
Can this be recalibrated?
Many modern vehicles allow speedometer recalibration through the ECU or a gear ratio adjustment.
What do I need to enter into the Tire Diameter Speedometer Error Calculator?
Just 3 values: original tire diameter, new tire diameter, indicated speed. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Tire Diameter Speedometer Error Calculator work out the answer?
Speedometer error percentage equals the percentage change in overall tire diameter between old and new tires. It applies the formula error % = (new diameter - old diameter) / old diameter x 100 and shows the working so you can check each step by hand.
Is the Tire Diameter Speedometer Error Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.