How Much Speedometer Error Do New Tires Cause? Plus Japan's Inspection Standards

Why a different outer diameter throws off the speedometer

A car's speedometer doesn't measure speed directly. It counts the revolutions of the tire (or the transmission's output shaft) and displays a speed calculated as original tire circumference × revolutions. Fit a tire with a different outer diameter and the distance covered per revolution changes, so the display and the actual speed drift apart.

• A tire with a larger diameter travels farther per revolution, so actual speed is higher than the speedometer shows
• A tire with a smaller diameter travels less per revolution, so actual speed is lower than the speedometer shows
• The odometer and trip meter are off by the same proportion
• Fuel economy displays also work from distance traveled, so a larger diameter makes economy read worse than it is

Chart of diameter difference and speedometer error

Assuming the original speedometer is accurate (40 km/h on the speedometer is really 40 km/h on original tires), this is how diameter difference relates to actual speed.

Even at +3%, 100 km/h on the speedometer is really 103 km/h, so on the expressway you can be going faster than you realize. For how to choose sizes with a small diameter difference, see How to Choose a Plus Size.

The speedometer standard at vehicle inspection

At inspection, a speedometer tester (a machine that spins the tires on rollers) measures the actual speed when the speedometer reads 40 km/h. For cars made on or after January 1, 2007, the standard is as follows.

This standard is generous toward speedometers that read high and strict toward those that read low (actual speed being higher). With an accurate speedometer, a diameter about 6.4% larger puts actual speed above 42.5 km/h, which fails. In the other direction, a tire has to be 20% or more smaller to reach the lower limit, so the practical problem comes when the diameter gets larger.

Cars made on or before December 31, 2006 have a different standard. For older cars, check with the inspection station or a repair shop, including how the manufacture date is treated and not only the first registration date on the vehicle certificate.

Add the original speedometer error too

Most cars' speedometers are built to read slightly high even on original tires. For instance, the actual speed may be 38 km/h when the speedometer shows 40 km/h. In that case, a larger diameter takes longer to reach the inspection limit, thanks to that original margin.

You can check your original speedometer error while driving at a steady speed, using a GPS speedometer app on your smartphone or a speed display board on the expressway. Enter it in the tool's "Actual speed at an indicated 40 km/h on stock tires" field, and the actual-speed table and inspection guide are recalculated with that error included.

Other inspection points besides the speedometer

• The load index (LI) is equal to or above the original (How to Read Tire Sizes)
• Tires and wheels don't protrude past the fender beyond what the standard allows (Calculating Flush Fitment and Protrusion)
• Tires don't hit the body or suspension when steering or going over bumps
• If a larger diameter changes the ride height, minimum ground clearance and overall height stay within the registered range
• Tread depth is at least 1.6 mm (no wear bars showing)

Enter the original and new sizes in the Tire Size Diameter Calculator to check the diameter difference, ride height change and actual speed at 40 km/h on the speedometer all at once. The calculation of the outer diameter itself is explained in How to Calculate Tire Outer Diameter, with a Chart.

FAQ

What happens to the speedometer with a larger tire diameter?

Actual speed becomes higher than the speedometer shows. With a diameter 3% larger, 100 km/h on the speedometer is really about 103 km/h. The odometer also records less distance than was actually driven.

What is the speedometer standard at vehicle inspection?

For cars made on or after January 1, 2007, the car passes if actual speed is at least 30.9 km/h and at most 42.5 km/h when the speedometer shows 40 km/h.

How much larger can the diameter get before failing inspection?

With an accurate speedometer, a diameter about 6.4% larger puts actual speed above 42.5 km/h. But original speedometers often read high, so the real limit varies by car. For safety, it's best to keep the difference within ±3%.

Is a smaller diameter a problem at inspection?

In the speedometer test, the lower limit (30.9 km/h) isn't reached unless the diameter is 20% or more smaller, so normal size changes rarely cause trouble. Do check minimum ground clearance as the ride height drops, and that the LI doesn't fall below the original.

How can I find my car's speedometer error?

Use a GPS speedometer app on your smartphone, and compare the speedometer reading with the GPS speed while driving at a steady speed. Once you know the GPS speed at 40 km/h on the speedometer, enter it in the tool to correct the actual-speed table.