Tire Size Calculator – Compare Tire Dimensions and Speedometer Error
Compare two tire sizes side by side: overall diameter in millimeters and inches, sidewall height, revolutions per mile, the diameter difference percent — and exactly how far off your speedometer will read.
Enter two tire sizes
Read the size from your tire sidewall — for example 205/55R16 means width 205, aspect 55, rim 16.
| Speedo reads | Actual speed | Difference |
|---|
—
- Read the size off the sidewall: 205/55R16 means width 205 mm, aspect 55 — the sidewall height as a percentage of the width — R for radial construction, and a 16-inch rim.
- The default swap, 205/55R16 → 225/45R17, is just +0.38% in diameter (634.3 mm vs 631.9 mm) — well inside the safe ±3% window most fitters treat as the limit.
- A taller tire rolls farther per revolution, so the speedometer reads low: at +0.38%, a 60 mph speedometer reading is an actual 60.2 mph.
Decode a single tire size
No second size to compare? Type one size exactly as it is written on the sidewall and get its full spec. Include the service description — the number and letter after the size, like the 91V in 205/55R16 91V — and the tool decodes its load index and speed rating too.
Equivalent tire sizes for your wheel
Type your current size and see which other sizes fit the same wheel while keeping the overall diameter inside the ±3% window most fitters treat as safe — the same window this page uses for its swap verdict.
205/55R16 vs 225/45R17, drawn to scale
The new tire is just 0.38% taller — visually almost identical, and well inside the safe ±3% window.
Both circles are drawn from the same scale, so the 2.4 mm difference is honest: a 205/55R16 tire stands 631.9 mm tall while the 225/45R17 stands 634.3 mm — the difference is thinner than a coin.
The inner circles are the rims (16 in vs 17 in). Even though the new wheel is a full inch bigger, its shorter sidewall (101.3 mm vs 112.8 mm) keeps the overall diameter nearly identical — that is the whole trick of plus-sizing.
More auto tools to try
Handy calculators for car costs and conversions.
Where to find your tire size, and what the letters after it mean
The calculator cannot look a size up from your car, your registration, or your VIN. It needs the size typed in exactly as written. Here are the three places it is written, in the order to check them.
1. The tire sidewall. Every tire carries its size moulded into the sidewall, in the form 205/55R16, sometimes followed by the service description, such as 91V. This is the authoritative source for the tire that is actually fitted, and it is the size the decode tool above reads. Check all four tires: a car can carry a different size on one axle, and a spare is often a different size again.
2. The door jamb placard. On the driver-side door jamb, or sometimes inside the fuel-filler door or glove box, the maker lists the recommended tire size and pressures for the car as sold. This is the size to compare a proposed swap against, and the size to use if the sidewall is worn or unreadable. If the placard and the sidewall disagree, the car may have been fitted with non-standard tires, and a tire professional should confirm what fits before you buy.
3. The owner’s manual. The manual lists the approved sizes for the model, including any optional sizes for different wheel packages. It does not know what is fitted to your car today, so it settles which sizes the maker approved, not which tire is on the car. Between the three, the sidewall says what is fitted, the placard says what the maker recommends, and the manual says what else was approved.
The service description after the size is two ratings in one: a load index number and a speed letter. The load index is the maximum load one tire can carry, and the speed letter is the maximum sustained speed the tire is rated for. The decode tool above reads both from the same chart as the table below. Never fit a replacement with a lower load index than the placard calls for, and do not fit a lower speed rating than the original tire carried.
| Load index | Max load per tire | Load index | Max load per tire | Speed letter | Max speed |
|---|---|---|---|---|---|
| 84 | 500 kg | 98 | 750 kg | Q | 160 km/h |
| 87 | 545 kg | 100 | 800 kg | S | 180 km/h |
| 91 | 615 kg | 102 | 850 kg | T | 190 km/h |
| 94 | 670 kg | 104 | 900 kg | H | 210 km/h |
| 95 | 690 kg | 107 | 975 kg | V | 240 km/h |
| 97 | 730 kg | 110 | 1,060 kg | W | 270 km/h |
Three preset swaps, checked against the 3% window
The comparison tool above ships with three preset swaps. All three diameters come from the same formula, rim in inches converted at 25.4 mm, plus two sidewalls. Only the first two stay inside the window most fitters treat as safe. The third is shown so the failure is visible, not hidden: a 7% taller tire is a different tire, not a swap.
| Swap | Current diameter | New diameter | Difference | Speedometer at 60 mph | Verdict |
|---|---|---|---|---|---|
| 205/55R16 to 225/45R17 | 631.9 mm | 634.3 mm | +0.38% | 60.2 mph actual | Inside the window |
| 195/65R15 to 205/55R16 | 634.5 mm | 631.9 mm | -0.41% | 59.8 mph actual | Inside the window |
| 265/70R17 to 285/75R17 | 802.8 mm | 859.3 mm | +7.04% | 64.2 mph actual | Outside the window |
Diameter is only the first fitment check. Before any swap, confirm clearance at full steering lock and full suspension travel, confirm the rim width suits the new tire width, and confirm the load index and speed rating do not drop below the placard and the original tire. The equivalent-sizes tool above lists sizes for the same wheel inside the window, but it cannot check clearance or rim width, so treat its list as candidates and have a tire professional confirm the fit.
How to compare tire sizes
Three steps, instant answers.
Width, aspect ratio and rim for each tire.
Diameter percent decides if the swap is safe.
See your real speed at any reading.
⚙️How tire sizes are compared
Overall diameter comes from the rim plus two sidewalls. Because the rim is in inches and the sidewall is in millimeters, the rim is converted first (1 in = 25.4 mm):
diameter = rim_in × 25.4 + 2 × sidewall
Worked example: 205/55R16 → sidewall = 205 × 0.55 = 112.75 mm; diameter = 16 × 25.4 + 2 × 112.75 = 631.9 mm (24.88 in). 225/45R17 → sidewall = 101.25 mm; diameter = 17 × 25.4 + 2 × 101.25 = 634.3 mm (24.97 in). Difference: (634.3 − 631.9) ÷ 631.9 = +0.38%.
Rim: the “R16” is the wheel diameter in inches — 16 × 25.4 = 406.4 mm of the total height. Sidewall: “55” means the sidewall is 55% of the 205 mm width, or 112.75 mm — and there is one above and one below the rim, so it counts twice. Circumference: π × diameter tells you how far the tire rolls per revolution; divide a mile (1,609,344 mm) by it for revs per mile — the number your odometer and speedometer were calibrated against.
Your speedometer assumes the original tire’s revolutions per mile. Actual speed = speedo reading × (new diameter ÷ old diameter). A taller new tire covers more ground per revolution, so the speedometer under-reads — with +0.38%, a 60 mph reading is really 60.2 mph. A shorter tire makes it over-read. The table above shows both, so you can check any speed at a glance.
Frequently asked questions
How do I read a tire size like 205/55R16?
205 is the tread width in millimeters, 55 is the aspect ratio — the sidewall height as a percentage of the width — R means radial construction, and 16 is the wheel (rim) diameter in inches. The number and letter that can follow — the 91V in 205/55R16 91V — are the service description: 91 is the load index, the maximum load one tire can carry (615 kg for a 91), and V is the speed rating (240 km/h). Type them into the decode tool above and it reads those too.
What does the diameter difference percent mean?
It is how much taller (or shorter) the new tire is than the current one: (new − old) ÷ old × 100. Most fitters treat ±3% as the safe window — beyond that, speedometer, gearing and clearance can suffer. The 205/55R16 → 225/45R17 swap at +0.38% is an easy pass.
How does a bigger tire affect my speedometer?
A taller tire rolls farther per revolution, so your car thinks it is going slower than it is. The error scales with the diameter change: at +0.38%, a 60 mph speedometer reading is an actual 60.2 mph. Shorter tires cause the opposite — the speedo reads high.
What is revs per mile?
The number of full rotations the tire makes in one mile: 1,609,344 mm divided by the tire’s circumference. Your car’s speedometer, odometer and transmission shift points were calibrated to the original tire’s number — matching it closely keeps all three honest.
Does sidewall height change ride comfort?
Yes. A taller sidewall (higher aspect ratio) has more rubber to flex, so it absorbs bumps better and protects rims from potholes. A shorter sidewall sharpens steering response but transmits more road harshness. Compare the sidewall stat above before you switch.
Can I just change the rim size and keep everything else?
Only if you lower the aspect ratio to compensate — that is plus-sizing. Going from a 16-inch to a 17-inch rim while keeping the same diameter means a shorter sidewall, which is exactly what the 225/45R17 example does (101.25 mm vs 112.75 mm of sidewall).
Where do I find my tire size if the sidewall is worn?
Check the driver-side door jamb placard first: it lists the recommended tire size and pressures for the car as sold. The owner’s manual lists the approved sizes for the model, including optional wheel packages. Neither tells you what is actually fitted today, so if the placard and the tires disagree, a tire professional should confirm the fit before you buy. This calculator cannot look a size up from your car or VIN: the size has to be typed in as written.
Can I fit a tire with a lower load index than the original?
No, not safely. The load index is the maximum load one tire can carry: a 91 carries 615 kg, a 94 carries 670 kg, and a 97 carries 730 kg. Fitting a lower index than the placard calls for overloads the tire even when the car looks fine standing still, because braking, cornering and bumps all add load. The same rule holds for the speed letter: do not fit a lower rating than the original tire carried. The decode tool above reads both ratings from the service description, such as 91V.
Related guides
Reading a size, and comparing one fairly.
Related Auto Calculators
Explore more useful car and money tools.
Car loan payments.
Monthly EMI schedule.
Loan payments, any kind.
Trip distances.
mm, inches and more.