How to read and compare tire sizes

What the numbers on the sidewall mean

Take a common size, P225/60R16. Read from left to right and each piece answers a different question. The P marks a passenger-vehicle tire; LT marks a light-truck tire, and a size with no leading letter is a metric size. The 225 is the tread width in millimeters. The 60 is the aspect ratio, meaning the sidewall height is 60 percent of the width. The R means radial construction. The 16 is the diameter of the wheel the tire fits, in inches.

Notice the unit mix: width and sidewall in millimeters, rim in inches. That mix is the reason the diameter formula needs a conversion, and it is where most hand calculations go wrong. NHTSA's TireWise guidance adds that an aspect ratio of 50 or lower indicates a short sidewall, which improves steering response and handling.

After the size you will often see a two- or three-digit number followed by a letter, such as 98H. That is the load index and speed rating, covered below.

The overall diameter math

A tire's overall diameter is the wheel plus a sidewall on each side. Start with the sidewall height in millimeters: width x aspect ratio / 100. Convert to inches by dividing by 25.4. Double it, since there is sidewall above and below the wheel, and add the rim diameter.

Worked example, 225/60R16. Sidewall = 225 x 60 / 100 = 135 mm, which is 5.31 inches. Overall diameter = (2 x 135 / 25.4) + 16 = 10.63 + 16 = 26.63 inches. That is the same arithmetic the Tire Size Calculator uses.

Second example, 215/65R16. Sidewall = 215 x 65 / 100 = 139.75 mm. Overall diameter = (2 x 139.75 / 25.4) + 16 = 27.00 inches. Circumference is diameter x pi, which is 84.84 inches, so this tire turns about 63,360 / 84.84 = 746.9 revolutions per mile (63,360 is the number of inches in a mile).

Notice what drives the result: the rim diameter is fixed, so every change in the overall diameter comes from the sidewall, and the sidewall depends on both width and aspect ratio. A wider tire at the same aspect ratio is also a taller tire.

Comparing two sizes and the speedometer error

To compare, find both diameters and take the percentage change: (new - old) / old x 100. Your speedometer and odometer count wheel rotations and assume the original tire's circumference. A larger tire travels farther per rotation, so the dash under-reports. The true speed is the displayed speed multiplied by new diameter over old diameter.

Worked example: stock 215/65R16 (27.00 in) to 225/60R17 (27.63 in). The change is (27.63 - 27.00) / 27.00 = 2.32 percent. When the speedometer shows 60 mph you are actually going 60 x 27.63 / 27.00 = 61.4 mph, and at 70 it is about 71.6. The odometer drifts the same way: after the display shows 10,000 miles you have really driven about 10,232.

The engine side moves the other way. The new tire needs about 729.9 revolutions per mile instead of 746.9, so at the same road speed the drivetrain turns about 2.3 percent slower. That subtly changes effective gearing, which is why a large diameter change can alter how a car feels and can nudge fuel economy.

The larger tire is also only taller by half the diameter change on the ground side. A 0.63 inch gain in diameter is about 0.31 inch of extra radius, which matters for wheel-well clearance and ground clearance. Anti-lock brakes, traction control, and stability control are also calibrated around the original tire's rotation, so a large mismatch can affect them as well as the speedometer.

Plus-sizing: bigger wheel, same overall diameter

Plus-sizing means fitting a larger-diameter wheel and lowering the aspect ratio so the overall tire diameter barely moves. The aspect ratio has to fall in step with the rim. Start with 225/60R16 (26.63 in) and keep the width at 225:

• Plus one, 225/55R17: sidewall 123.75 mm, diameter 26.74 in, a change of +0.43 percent. At 60 mph on the dash you are doing 60.3.
• Plus two, 225/50R18: diameter 26.86 in, a change of +0.86 percent.
• The wrong way, 225/60R17: keeping the 60 aspect ratio while raising the rim gives 27.63 in, a change of +3.76 percent, with a speedometer reading 60 when you are doing 62.3.
• Exact math: for a 17-inch wheel at width 225, a perfect match would need an aspect ratio of 54.36, so the nearest standard 55 is the right choice.

Where the comparison breaks down

The calculator gives a nominal diameter from the printed numbers. Real tires differ a little: manufacturers build the same size slightly differently, wheel width changes the tire's shape, tread wears down over time, and pressure and load change the rolling radius. Treat the result as a very good estimate and not an exact dimension. For critical fitment, compare the manufacturer's published overall diameter for the specific tire models.

Percent changes also get larger on bigger tires. For a truck, 265/70R17 is 31.61 inches. Widening only to 285/70R17 gives 32.71 inches, a +3.49 percent change, and at a displayed 60 mph you are doing 62.1. Stepping to 285/75R17 reaches 33.83 inches, +7.04 percent, and the same 60 on the dash is really 64.2.

A common rule of thumb, and the one the calculator uses to flag a result, is to stay within about 3 percent of stock. Look at what that rule does to a sport setup: 225/45R17 (24.97 in) to 235/40R18 (25.40 in) is +1.72 percent, but jumping to 245/35R19 (25.75 in) is +3.12 percent, just over the line. Width alone does it too: 225/60R16 to 245/60R16 is +3.55 percent, with 60 on the dash meaning 62.1.

Going smaller deserves equal care. 225/60R16 to 205/60R16 is -3.55 percent, so a displayed 60 mph is really 57.9, and the odometer over-counts. A speedometer reading slow is not a safety win, since the error reverses direction and a low-reading dash can hide speeding.

Finally, the dash may already be off before you change anything, and diameter math cannot detect that. A GPS speed reading is a useful reality check before and after a tire change.

Load index, speed rating, and pressure at a high level

The load index is the number after the size, and it tells you how much weight each tire can carry. It is a code, not the weight itself, so you look it up on a load chart. The speed rating is the letter after it, which corresponds to the top speed the tire is built to sustain. Both are part of what the manufacturer engineered the tire to handle.

The rule for replacement is simple: do not go below the load index and speed rating the vehicle manufacturer specified, which are on the door placard and in the owner's manual. A tire that fits the wheel but has a lower load index can be unsafe on a heavy vehicle. A higher rating than required is generally fine.

Pressure is not on the tire. NHTSA is explicit that the correct pressure is what the vehicle manufacturer lists on the Tire and Loading Information label on the driver's door edge or post, or in the owner's manual, not the maximum pressure printed on the sidewall.

Two more sidewall facts. The Uniform Tire Quality Grading ratings for treadwear, traction, and temperature are printed on passenger tires sold in the US, and they help compare tires. And NHTSA notes that a 10 percent reduction in rolling resistance is estimated to improve vehicle fuel economy by 1 to 2 percent, so tire choice can nudge fuel costs too.

Common mistakes

Most tire-size errors are unit slips or assumptions about which number matters. These are the ones that cost people the most.

• Forgetting that width is millimeters and rim is inches: dividing the sidewall by 25.4 is what puts everything in the same unit.
• Forgetting to double the sidewall: the tire has a sidewall on both the top and bottom of the wheel.
• Raising the rim without lowering the aspect ratio: 225/60R16 to 225/60R17 adds 3.76 percent.
• Assuming only the aspect ratio changes the diameter: width at the same aspect ratio changes sidewall height too.
• Matching diameter and ignoring load index and speed rating: a size that fits can still be under-rated for the vehicle.
• Reading maximum pressure off the sidewall instead of the vehicle's placard.
• Treating a smaller tire as harmless: a tire that is too small causes the same speedometer and system calibration issues in the opposite direction.