Speedometer Error Calculator
True speed after a tire or gearing change.
Enter your values
Enter the stock tire diameter in in.
Enter the current 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.
- True speed
- 73.7 mph
- Error
- 5.26% reading low
- Odometer per 100 indicated miles
- 105.3 real miles
- Speed at indicated 30 mph
- 31.6 mph
Quick answer
Speedometer error is the ratio of current tire diameter to stock diameter. Taller tires make the needle read low; shorter tires make it read high.
True speed = indicated × (new diameter ÷ stock diameter)
At a glance
| What it does | True speed after a tire or gearing change. |
|---|---|
| Category | Auto |
| Inputs needed | Stock tire diameter, Current tire diameter, Indicated speed |
| Main output | True speed |
| Formula | True speed = indicated × (new diameter ÷ stock diameter) |
| Cost | Free — no sign-up, no download |
How to use the Speedometer Error Calculator
- 1Enter the stock tire diameter from the door placard size.
- 2Enter the diameter of the tire fitted now.
- 3Enter any indicated speed to see the true speed.
Inputs explained
- Stock tire diameter(in)
- Enter the stock tire diameter in in.
- Current tire diameter(in)
- Enter the current tire diameter in in.
- Indicated speed(mph)
- Enter the indicated speed in mph.
Worked example
Using the values the calculator loads with:
Inputs
- Stock tire diameter28.5 in
- Current tire diameter30.0 in
- Indicated speed70 mph
Results
- True speed73.7 mph
- Error5.26% reading low
- Odometer per 100 indicated miles105.3 real miles
- Speed at indicated 30 mph31.6 mph
Frequently asked questions
Can I recalibrate the speedometer?
Most modern vehicles allow a tire-size recalibration with a scan tool or dealer software; older vehicles may need a speedometer gear or an inline calibrator.
Is a speedometer allowed to read high?
In most regions a speedometer may over-read but never under-read, which is why factory needles usually sit a few percent optimistic.
What do I need to enter into the Speedometer Error Calculator?
Just 3 values: stock tire diameter, current 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 Speedometer Error Calculator work out the answer?
Speedometer error is the ratio of current tire diameter to stock diameter. Taller tires make the needle read low; shorter tires make it read high. It applies the formula True speed = indicated × (new diameter ÷ stock diameter) and shows the working so you can check each step by hand.
Is the 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.