Tire Size Calculator: Diameter, Comparison and Speedometer Error

1. Read your current size off the sidewall. It looks like 225/45R17: width 225 mm, aspect ratio 45, wheel 17 inches. Type those three numbers into the original row.
2. To compare a different tire, enter the new size in the second row the same way. Leave the new row blank to study one tire on its own.
3. Read the overall diameter, sidewall, circumference and revolutions per mile for each size, plus the diameter difference and the speedometer reading at 60 mph. The result updates as you type.

The three numbers in a metric size are width in millimetres, aspect ratio as a percent, and wheel diameter in inches. This tool reads a P-metric or ISO-metric size, the kind printed on almost every passenger-car tire.

A metric tire size is the code molded on the sidewall, such as 225/45R17. Each part describes one dimension, so the three numbers fully define the shape of the tire once it is mounted on its wheel.

The first number, 225, is the section width in millimetres, measured across the widest part of the tire. The second number, 45, is the aspect ratio: the sidewall height as a percent of the width. The letter R means radial construction, and 17 is the wheel diameter in inches that the tire fits.

Because width is in millimetres and the wheel is in inches, the overall diameter mixes both units. That is why a calculator helps: it does the unit conversion and the arithmetic in one step, and it shows the sidewall height that the aspect ratio hides.

Fitment is about more than size. A heavier car also needs the right load index and speed rating on the sidewall, and the tire must clear the suspension and bodywork. For the mileage side of running costs, see the mileage calculator.

Two short formulas do all the work. The sidewall is a percent of the width, and the overall diameter is the wheel plus a sidewall on the top and the bottom.

1. Sidewall: 225 x 45 / 100 = 101.25 mm.
2. Diameter: 17 x 25.4 + 2 x 101.25 = 431.8 + 202.5 = 634.3 mm, which is 634.3 / 25.4 = 24.97 in.
3. Circumference: 24.97 x pi = 78.45 in, so revolutions per mile = 63360 / 78.45 = 807.6.

The number 25.4 converts inches to millimetres exactly, and 63360 is the number of inches in a mile (5280 x 12). Every output on this page comes from these three lines, so the geometry is exact. The one real-world gap is that a loaded tire squashes slightly, so the rolling diameter is a touch smaller than the free diameter here.

The original, 225/45R17. Sidewall 101.25 mm, overall diameter 634.3 mm (24.97 in), about 807.6 revolutions per mile. This is a common sporty-sedan size.

The new size, 245/40R18. Sidewall 245 x 40 / 100 = 98 mm, so diameter = 18 x 25.4 + 2 x 98 = 457.2 + 196 = 653.2 mm (25.72 in). It is wider and on a bigger wheel, yet the shorter sidewall keeps the height close.

The comparison. The new tire is (653.2 / 634.3 - 1) x 100 = about +3.0 percent taller. Because it is larger, the speedometer reads slow: when the dial shows 60 mph the true speed is 60 x 653.2 / 634.3 = about 61.8 mph. That is right at the edge of the usual plus-or-minus 3 percent comfort zone.

This is a classic plus-sizing move: a wider tread and a larger wheel while holding the diameter nearly constant so gearing and the speedometer barely change. Driving further afield on the new rubber? Estimate the trip with the commute cost calculator.

The formula is fixed, but a few things decide whether a size swap is a good idea.

Aspect Ratio

A lower aspect ratio means a shorter sidewall, so going from 45 to 40 can cancel out a larger wheel. This is how plus-sizing keeps the diameter steady.

Wheel Diameter

Each inch of wheel adds 25.4 mm to the diameter before any sidewall change. Jumping two wheel sizes without dropping the aspect ratio makes a much taller tire.

Width

Width changes the tread and, at a fixed aspect ratio, the sidewall height too, since the sidewall is a percent of the width. A wider tire at the same aspect ratio is slightly taller.

Load and Inflation

The tool uses the free, unloaded diameter. In the real world the tire flattens where it meets the road, so the rolling radius under load is a little smaller and changes with inflation pressure.

A speedometer counts wheel turns and assumes a fixed tire size, so changing the diameter shifts every reading. A larger tire travels further per turn, so the car is actually going faster than the dial shows. A smaller tire does the opposite.

Many fitters aim to keep a new tire within about plus-or-minus 3 percent of the original diameter. Inside that band the speedometer error, the odometer drift and the effect on gearing all stay small, and clearance is usually fine.

Odometer and trip distances drift by the same percentage. A tire 3 percent larger makes the odometer read about 3 percent low over the life of the tires.

Plus-sizing

Fit a larger wheel with a lower-profile tire while holding the overall diameter. It sharpens steering response and looks sportier, but the shorter sidewall rides firmer and is easier to damage on potholes.

Downsizing

Fit a smaller wheel with a taller sidewall, often for winter tires. It cushions the ride and cuts the chance of rim damage, and a narrower tire can bite better through snow.

Keeping the Diameter

Whichever way you go, aim to match the original overall diameter within a few percent. That keeps the speedometer honest, the anti-lock brakes and stability systems calibrated, and the gearing unchanged.

Watching Running Costs

Wider or taller tires can nudge rolling resistance and fuel use. To see the fuel side in your own units, use the fuel economy converter.

Before Buying a New Set

Check that a size the shop suggests, or one you found cheaper, is close to your factory diameter before you commit.

Plus-sizing or Fitting Winter Wheels

Compare a staggered or winter package against stock so the speedometer and traction systems stay accurate.

After a Speedometer Feels Off

If your dial seems to disagree with GPS, a past tire size change may be the cause. The percentage here explains the gap.

Reading a Used-car Listing

Decode an unfamiliar size and see how it compares with the original fitment for that model.

1. Reading the Aspect Ratio as Millimetres

The 45 in 225/45R17 is a percent, not 45 mm. The sidewall is 225 x 45 / 100 = 101.25 mm.

2. Forgetting There Are Two Sidewalls

Overall diameter adds a sidewall at the top and the bottom, so multiply by two before adding the wheel.

3. Mixing the Wheel Unit

The wheel is in inches while the width is in millimetres. Convert the wheel with 25.4 before adding it to the sidewalls.

4. Ignoring Load and Speed Ratings

Two tires can share a size yet carry different load indexes or speed ratings. Match or exceed the originals.

5. Chasing Looks Past 3 Percent

A much taller tire can rub, confuse the speedometer and dull acceleration. Keep the diameter close to stock.

The geometry on this page is exact for a nominal metric size. The limits are about the real tire, not the math.

Treat the numbers as a precise nominal estimate. Always confirm real fitment against the vehicle placard and a qualified fitter.

It is a metric tire size: 225 mm section width, a sidewall that is 45 percent of the width, radial construction (R), on a 17-inch wheel. That works out to a 634.3 mm overall diameter.

Multiply the width by the aspect ratio divided by 100 to get the sidewall, then add two sidewalls to the wheel diameter in millimetres. For 225/45R17: 17 x 25.4 + 2 x 101.25 = 634.3 mm.

The sidewall is 225 x 45 / 100 = 101.25 mm, about 4 inches. The aspect ratio of 45 means the sidewall is 45 percent of the 225 mm width.

Most fitters keep a new tire within about plus-or-minus 3 percent of the original overall diameter. Inside that range the speedometer error, gearing change and clearance stay small.

A larger tire makes the speedometer read slow, so your true speed is higher than the dial. A smaller tire reads fast. The error equals the diameter change in percent.

The new tire is about 3 percent larger, so at an indicated 60 mph your actual speed is about 61.8 mph. The speedometer reads a little slow.

It is how many times a tire turns to cover one mile, found from 63360 inches per mile divided by the circumference in inches. A 634.3 mm tire turns about 807.6 times per mile.

Often yes, within a range, but a wider tire at the same aspect ratio is slightly taller and may need more clearance. Check the wheel width range and the vehicle placard.

Yes. The odometer drifts by the same percentage as the diameter. A tire 3 percent larger makes the odometer read about 3 percent low, so real mileage is higher.

No. The math runs in your browser. Anything you Save stays in your own browser storage until you clear it.

• Tyre markings and metric sizing explained (TyreSafe, UK tyre safety body).
• How to read a tire sidewall and size code (US NHTSA, tire safety basics).
• Units: inch to millimetre (25.4 exact) and miles (NIST SP 811, Appendix B.9).
• Tire and rim dimensional standards (US Tire and Rim Association).