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Tire Size Calculator

Compare two tire sizes by diameter, sidewall, width and revolutions per mile, and see how a new size changes your speedometer reading.

Level: Grades 6–7

  • Common Core standard 7.G.B.4Know the formulas for the area and circumference of a circle and use them to solve problems; give an informal derivation of the relationship between the circumference and area of a circle.
  • Common Core standard 6.RP.A.3.dUse ratio reasoning to convert measurement units; manipulate and transform units appropriately when multiplying or dividing quantities.
Printable worksheet

Tire Size Calculator

Know the answer you need? Pick an input to work backward.

Inputs

As printed on the sidewall, for example P225/65R17, LT245/75R16 or 33x12.50R15.

Works in mph or km/h. The true speed comes out in the same unit.

Result

True speed at an indicated 60

61.23

The new tire is 2.06% taller, so at an indicated 60 you would really be doing about 61.23.

Diameter difference
2.06%
Speedometer reads low by
2.02%
Current diameter
28.52 in
New diameter
29.1 in
Change in ride height (half the diameter change)
0.29 in
Current revolutions per mile
707.3
New revolutions per mile
693

See the step-by-step solution ↓

225/65R17 compared with 235/60R18
225/65R17 compared with 235/60R18
MeasureCurrentNewDifference
Overall diameter (in)28.5229.10.59
Section width (in)8.869.250.39
Section width (mm)22523510
Sidewall height (in)5.765.55-0.21
Rim diameter (in)17181
Circumference (in)89.5891.431.84
Revolutions per mile707.26693.01-14.26

Step-by-step solution

  1. Sidewall of the current tire: width × aspect ratio, converted from mm to inches

    225 mm×0.6525.4=5.7579 in

  2. Overall diameter of the current tire: rim + 2 × sidewall

    17+2×5.7579=28.5157 in

  3. Sidewall of the new tire: width × aspect ratio, converted from mm to inches

    235 mm×0.625.4=5.5512 in

  4. Overall diameter of the new tire: rim + 2 × sidewall

    18+2×5.5512=29.1024 in

  5. Diameter difference, as a percent of the current tire

    (29.102428.5157−1)×100=2.0572%

  6. True speed when the speedometer reads 60: the reading × new diameter ÷ current diameter

    60×29.102428.5157=61.2343

  7. Revolutions per mile of the new tire: 63,360 inches in a mile ÷ circumference

    63,360π×29.1024=693.01

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

Please note: Results are estimates for planning and comparison. Lenders, banks, and tax authorities may round differently or include fees not modeled here, so confirm figures with your provider before deciding.

What this calculator does

See how a new tire size compares with your current one, and what it does to your speedometer. The size molded into a tire's sidewall describes its shape completely. Take a metric size such as 225/65R17. 225 is the width in millimeters. 65 is the aspect ratio: the sidewall is 65% as tall as the tire is wide. 17 is the wheel diameter in inches.

A P in front marks a passenger tire, and LT a light-truck tire. Those letters change the load rules, not the shape. Off-road flotation sizes such as 33x12.50R15 give the overall diameter and width in inches directly.

The calculator turns two sizes into overall diameter, sidewall height, width, circumference and revolutions per mile. Then it compares them. A speedometer counts wheel turns. So a taller tire covers more ground per turn, and the speedometer reads low. A shorter tire makes it read high. Enter the speed your speedometer shows, and you'll see the speed you'd really be going on the new size.

Where it's used

Why this math matters outside the classroom:

  • Choosing new tires. A driver finds that 235/60R18 tires are 29.10 in tall against 28.52 in for the originals, about 2.06% bigger.
  • Speedometer correction. With tires 10% taller, an indicated 60 mph is really 66 mph, because each turn of the wheel covers 10% more road.
  • Odometer checks. Fleet managers divide 63,360 inches by a 91.4 in circumference to get about 693 turns of the wheel per mile.
  • Unit conversion practice. Students convert a 146.25 mm sidewall to inches by dividing by 25.4, which gives about 5.76 in.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 7.G.B.4, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Type the size on your current tires exactly as printed on the sidewall, for example P225/65R17 or LT245/75R16.
  2. Type the size you're considering. You can mix metric and flotation sizes.
  3. Enter a speedometer reading, in mph or km/h.
  4. Read the true speed, the diameter difference and the side-by-side table of both sizes.

What the inputs mean

Current tire size
Anything after the wheel size is ignored, such as the load index and speed rating in 225/65R17 102H. R means radial construction. ZR is a radial rated for high speeds.
New tire size
Many drivers keep a new size within about 3% of the original diameter. That keeps the speedometer, gearing and anti-lock brake settings close to how they were set up. Check your vehicle maker's guidance and wheel clearance before changing size.
Speedometer reading
The number on the speedometer. The answer is in the same unit, so this works in mph or km/h.

Formula and method

sidewall (in) = width (mm) × aspect ratio ÷ 100 ÷ 25.4
overall diameter = rim diameter + 2 × sidewall
flotation sizes: sidewall = (overall diameter − rim) ÷ 2
circumference = π × diameter
revolutions per mile = 63,360 ÷ circumference (in)
true speed = speedometer reading × new diameter ÷ current diameter

A metric size doesn't state the sidewall height directly. Multiply the width by the aspect ratio to get the sidewall in millimeters. Then divide by exactly 25.4 to turn millimeters into inches. The tire's overall diameter is the wheel plus one sidewall above it and one below it.

The circumference is the distance once around the tire, and π (pi) is the fixed number that links a circle's width to its distance around. Every turn of the wheel moves the car one circumference. A mile is 63,360 inches, so dividing gives the turns per mile. The speedometer is set up for the original turns per mile. So the true speed grows or shrinks in step with the ratio of the two diameters.

Worked example

You want to replace 225/65R17 tires with 235/60R18 and wonder what the speedometer will show at a true highway speed.

  1. Current sidewall: 225 × 0.65 ÷ 25.4 = 5.7579 in, so the diameter is 17 + 2 × 5.7579 = 28.5157 in.
  2. New sidewall: 235 × 0.60 ÷ 25.4 = 5.5512 in, so the diameter is 18 + 2 × 5.5512 = 29.1024 in.
  3. Diameter difference: (29.1024 ÷ 28.5157 − 1) × 100 = 2.06% larger.
  4. True speed at an indicated 60: 60 × 29.1024 ÷ 28.5157 = 61.23.
  5. Revolutions per mile fall from 707.3 to 693.0.

Result: The 235/60R18 is about 2.06% taller. When the speedometer shows 60 mph you would really be doing about 61.23 mph, so the speedometer reads about 2.02% low.

Understanding the result

A positive diameter difference means the new tire is taller. You travel faster than the speedometer shows, and the odometer counts fewer miles than you drive. A negative difference means the opposite. The ride height changes by half the diameter change, because only the bottom half of the tire lifts the axle.

The width and sidewall rows help with fit and feel. A wider tire may rub on suspension parts or fenders. A shorter sidewall usually feels firmer over bumps.

Assumptions and limitations

  • Dimensions are the design sizes printed on the sidewall, for a new tire with no weight on it.
  • The speedometer was accurate on the current tires.
  • Revolutions per mile use the diameter with no weight on the tire. Under a car's weight, a tire flattens slightly, so the measured figure is a little higher.
  • Real tires of the same size vary by brand, tread depth and inflation, often by a few tenths of an inch.
  • The calculator doesn't check load index, speed rating, wheel width or clearance. For fit and safety, follow the vehicle maker's guidance or ask a tire professional.
  • Older letter-and-number sizes (such as FR78-15) and motorcycle sizes aren't supported.

Frequently asked questions

What do the numbers in 225/65R17 mean?

225 is the width in millimeters. 65 is the aspect ratio, the sidewall height as a percentage of the width. R means radial construction, and 17 is the wheel diameter in inches. The sidewall is 225 × 0.65 = 146.25 mm, about 5.76 inches tall.

Is a P-metric tire the same size as an LT tire with the same numbers?

The shape is the same, so the diameter and speedometer effect are identical. The difference is in how they're built and the load rules. LT tires are built for heavier loads and usually need higher pressures. Choose the type your vehicle calls for.

How much bigger can I go?

A common rule of thumb is to stay within about 3% of the original overall diameter. That keeps the speedometer and other systems close to how they were set up. Bigger changes may mean resetting those systems and can affect clearance. Check with the vehicle maker or a tire shop.

Why does my speedometer read low with bigger tires?

The speedometer counts how fast the wheels turn. A taller tire travels farther per turn. So at the same turning speed, the car is moving faster than the speedometer assumes.

How this calculator is checked

Method: Standard sidewall geometry for metric (P, LT, ST, C) and flotation tire size designations; circumference π × d; exact 25.4 mm per inch and 63,360 in per mile. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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