Tire-Size Speedometer and Odometer Difference Calculator

The short answer: bigger tires read slow, smaller tires read fast

Changing tire size does affect your speedometer and odometer whenever the new tire's overall diameter differs from the size your vehicle was calibrated for. A taller tire makes your speedometer read lower than your actual speed and makes your odometer count fewer miles than you really drove. A shorter tire does the opposite. Your speedometer reads higher than your actual speed, and your odometer adds miles faster than you cover them.

The size of the error is set by one number, the percentage change in overall tire diameter. If the new tire is 3% taller, your true speed is about 3% higher than the reading, and every indicated mile is about 3% longer than the odometer shows. If you change wheel size but keep the overall tire diameter essentially the same, which is the idea behind plus-sizing, the speedometer is effectively unaffected.

Why tire diameter changes the reading

A vehicle does not measure distance directly. It counts rotations, either of a transmission output shaft or of the wheels through wheel-speed sensors, and converts those rotations into speed and distance. The conversion assumes a fixed number of tire revolutions per mile, based on the original tire size.

A tire's circumference is the distance it travels in one revolution. A taller tire has a larger circumference, so it covers more ground per turn and needs fewer revolutions to travel a mile. The vehicle still applies the old conversion, so it underestimates both speed and distance. A shorter tire needs more revolutions per mile, so the vehicle overestimates both.

How to read the tire size numbers

Most passenger and light-truck tires use metric sizing, such as 265/70R17. The first number is the section width in millimeters. The second number is the aspect ratio, which is the sidewall height expressed as a percentage of the width. The R means radial construction. The last number is the wheel (rim) diameter in inches. To get the overall diameter, you add the height of two sidewalls (top and bottom) to the wheel diameter.

Some off-road and flotation tires are sized differently, for example 33x12.50R15. In that format, the first number is already the approximate overall diameter in inches, so you can use it directly.

The calculator formulas

Sidewall height (in)    = width_mm × (aspect_ratio ÷ 100) ÷ 25.4
Overall diameter (in)   = rim_diameter_in + (2 × sidewall height)
Circumference (in)      = π × overall diameter
Revolutions per mile    = 63,360 ÷ circumference

Diameter change (%)     = (new_diameter ÷ old_diameter − 1) × 100
Actual speed            = indicated speed × (new_diameter ÷ old_diameter)
Actual distance         = odometer distance × (new_diameter ÷ old_diameter)
Indicated at a true speed = true speed × (old_diameter ÷ new_diameter)

The figure 63,360 is the number of inches in a mile, and 25.4 converts millimeters to inches. The ratio of new diameter to old diameter is the only correction factor you need. The same factor applies to speed and to distance.

Step-by-step: calculating your true speed

• 1. Find the original tire size. It is usually printed on the tire placard in the driver's door jamb or listed in the owner's manual.
• 2. Note the new tire size from the sidewall or the product listing.
• 3. Calculate the overall diameter of each tire using the formulas above.
• 4. Divide the new diameter by the old diameter to get the correction factor.
• 5. Multiply any indicated speed or odometer distance by that factor to get the actual value.
• 6. If you want to know what the speedometer will show at a given true speed, such as a posted limit, divide that speed by the correction factor.

Worked example: going to a larger tire

In this illustrative example, a truck originally fitted with 265/70R17 tires moves to 285/70R17.

• Original 265/70R17: sidewall = 265 × 0.70 ÷ 25.4 ≈ 7.30 in; diameter ≈ 17 + 14.61 = 31.61 in; about 638 revolutions per mile.
• New 285/70R17: sidewall = 285 × 0.70 ÷ 25.4 ≈ 7.85 in; diameter ≈ 17 + 15.71 = 32.71 in; about 617 revolutions per mile.
• Correction factor: 32.71 ÷ 31.61 ≈ 1.035, so the new tire is about 3.5% taller.
• At an indicated 60 mph, actual speed ≈ 60 × 1.035 ≈ 62.1 mph.
• After 10,000 indicated miles, actual distance ≈ 10,349 miles.

In this case the speedometer reads slow. At a true 65 mph, it would display about 62.8 mph.

Worked example: going to a smaller tire

In this illustrative example, a car originally on 225/50R17 tires switches to 205/50R17.

• Original 225/50R17: sidewall ≈ 4.43 in; diameter ≈ 25.86 in.
• New 205/50R17: sidewall ≈ 4.04 in; diameter ≈ 25.07 in.
• Correction factor: 25.07 ÷ 25.86 ≈ 0.970, so the new tire is about 3% shorter.
• At an indicated 60 mph, actual speed ≈ 58.2 mph.
• After 10,000 indicated miles, actual distance ≈ 9,696 miles.

Here the speedometer reads fast and the odometer adds mileage faster than the car actually travels.

Worked example: bigger wheels, same overall diameter

A larger wheel affects the speedometer only if the overall tire diameter changes. In this illustrative plus-one example, 205/55R16 (about 24.88 in tall) is replaced by 225/45R17 (about 24.97 in tall). The lower-profile sidewall offsets the larger wheel, and the overall diameter grows by only about 0.4%. At an indicated 60 mph, the actual speed is roughly 60.2 mph, a difference too small to notice in normal driving.

Comparison table for the examples

Original size	New size	Diameter change	Indicated 60 mph = actual	10,000 odometer miles = actual
265/70R17	285/70R17	+3.5%	≈62.1 mph	≈10,349 mi
225/50R17	205/50R17	−3.0%	≈58.2 mph	≈9,696 mi
205/55R16	225/45R17	+0.4%	≈60.2 mph	≈10,037 mi

Why the calculated figure is an estimate

The formulas use nominal, unloaded dimensions. On the road, the tire's effective rolling diameter is somewhat smaller because the tire flattens under the vehicle's weight. That rolling diameter also shifts with inflation pressure, load and tread wear, and two tires with the same size marking from different manufacturers can measure slightly differently. A worn tire is also shorter than a new one, so even the original size drifts over the life of the tread. The percentage comparison between two sizes is still a reliable guide to the direction and approximate size of the change. To check your actual speed, compare the speedometer with a GPS speed reading at a steady speed on a flat road.

What else a diameter change can affect

Effects beyond the speedometer depend on the vehicle, its model year and its equipment. On many modern vehicles, the systems that rely on wheel-speed data can include anti-lock brakes, stability and traction control, transmission shift timing and all-wheel-drive or four-wheel-drive logic. A different tire diameter also changes effective gearing. Taller tires tend to reduce acceleration feel and lower engine speed at a given road speed, and shorter tires tend to do the opposite.

Odometer error affects anything tracked by mileage, including service intervals, warranty coverage, lease mileage limits and fuel-economy calculations. If your speedometer reads slow, a fuel economy figure based on odometer miles will look worse than it really is. If it reads fast, the figure will look better than it really is.

Correcting the reading

The calculator tells you how far off the reading is. It does not confirm that a tire size fits your vehicle or is appropriate for it. If the difference matters to you, the correction method depends on how your vehicle measures speed. On many vehicles with electronic speedometers, a dealer or a compatible programming tool can update the stored tire size. Some older vehicles with mechanical speedometer drives are corrected by changing a drive gear. If you cannot recalibrate, use the correction factor to know what the speedometer will show at the speeds you actually want to drive.