Why does changing tire size affect the speedometer?

Your speedometer measures how fast the wheels rotate, then converts that to speed assuming the stock tire diameter. A larger tire travels farther per revolution, so the speedometer under-reads your true speed. A smaller tire does the opposite.

How is speedometer error calculated?

Error% = ((new diameter / stock diameter) - 1) × 100. If the new tire is 3% larger in diameter, the speedometer reads about 3% lower than true speed. At a GPS-measured 60 mph, the dash would show roughly 58.3 mph.

What is an acceptable level of speedometer error?

Most manufacturers and regulations consider ±3% acceptable. Beyond that, speedometer or ECU recalibration is recommended, especially to keep odometer readings accurate for warranty and lease purposes.

Does tire size affect the odometer too?

Yes. The odometer uses the same rotation count as the speedometer. A 3% larger tire means the odometer logs only 97 miles for every 100 miles actually driven — a meaningful difference over a vehicle's lifetime.

Which tire size goes where — stock or new?

Stock (OEM) is the original size printed in your door jamb or owner's manual. New is the size you have installed or are considering. The calculator always shows how the new size deviates from stock.

Can I fix speedometer error without replacing tires?

Yes. A speedometer recalibration tool, aftermarket gear set, or ECU tune can correct the reading. Some vehicles also support software corrections via OBD-II programmers. This calculator gives you the exact correction factor to provide to a tuner.

Speedometer Error Calculator

Name: Speedometer Error Calculator
Availability: InStock
Author: Nham Vu

Enter your stock and new tire sizes to instantly see how much your speedometer over- or under-reads, and find your true speed at any indicated speed.

Tire Sizes

Stock (OEM) Tire Size from your door jamb

Width (mm)

Aspect %

Rim (in)

New (Installed) Tire Size

Width (mm)

Aspect %

Rim (in)

Indicated Speed Lookup

Enter the speed your dashboard shows to find your actual speed.

Enter stock and new tire sizes, then click Calculate.

Speedometer Error Summary

Stock Diameter

—in

New Diameter

—in

Error

—

Odometer Impact

Actual Distance	Odometer Reads	Difference

Speed Reference Table

Indicated Speed	True Speed	Difference

Summary

Enter your stock and new tire sizes to instantly see how much your speedometer over- or under-reads, and find your true speed at any indicated speed.

How it works

Enter your stock (OEM) tire size in P-metric format, e.g. 225/45R17.
Enter your new (installed) tire size in the same format.
The calculator computes the overall diameter of each tire from its section width, aspect ratio, and rim diameter.
Speedometer error is derived from the ratio of new to stock circumference — a bigger tire makes the speedometer read low.
Enter an indicated speed to see your true speed and the difference in mph or km/h.
Review the odometer impact table to understand cumulative distance error.

Use cases

Verify how much a tire upsizing affects your speedometer before buying.
Find your real speed when driving with aftermarket or plus-size tires.
Assess odometer error for lease-end mileage calculations.
Decide whether recalibrating your speedometer or ECU is necessary.
Compare multiple candidate tire sizes to find the one with the smallest error.
Confirm that a wheel-and-tire package keeps speedometer error within legal limits.
Calculate true speed when towing with a spare of a different size.
Understand why a radar speed sign reads differently from your dashboard.

Frequently Asked Questions

Related tools

Tire Size Calculator Wheel Offset Calculator Gas Mileage Calculator

Last updated: 2026-05-29 · Reviewed by Nham Vu