Tire Comparison Calculator

Overall diameter difference (B vs A) —

Tire A diameter (in): —
Tire B diameter (in): —
Tire A sidewall (mm): —
Tire B sidewall (mm): —
Tire A revolutions per mile: —
Tire B revolutions per mile: —
True speed on tire B: —

Overall diameter of a metric tire is the rim size plus twice the sidewall, where sidewall = width × aspect ratio. A 225/45R17 measures 24.97 in and a 245/40R18 measures 25.72 in, so the 18 in size is 2.98% taller and an indicated 60 becomes a true 61.8.

About this tool

Changing tire size, for a plus-one wheel upgrade, a winter set on smaller rims or simply because the original size is out of stock, shifts the rolling diameter. That moves the speedometer, the odometer, gearing and the clearance in the wheel well. Enter the current size and the candidate size from the sidewall code (for example 225/45R17) and the calculator gives each tire's sidewall height in millimetres, overall diameter in inches, revolutions per mile, the percentage difference between them, and the true road speed when the speedometer shows a chosen figure. It works on nominal dimensions: the real tire on a real rim can differ by a few millimetres between brands and with inflation and load, so treat a result close to the 3% guideline as borderline.

How to use it

Enter the current tire
Type the width in mm, aspect ratio in percent and rim diameter in inches from the sidewall of the tire now fitted.
Enter the new tire
Fill the same three numbers for the size you are considering.
Set a speedometer reading
Optionally change the indicated speed (60 by default) to see what the car really does on tire B.
Read the difference
The headline is the diameter change in percent; the stats and working list sidewalls, diameters and revolutions per mile.

Examples

205/55R16 vs 225/50R16

Width: 205
Aspect ratio: 55
Rim diameter: 16
Width: 225
Aspect ratio: 50
Rim diameter: 16
Speedometer reading: 60

Result Overall diameter difference (B vs A): -0.08%
Tire A diameter (in): 24.88
Tire B diameter (in): 24.86
Tire A sidewall (mm): 112.8
Tire B sidewall (mm): 112.5
Tire A revolutions per mile: 810.7
Tire B revolutions per mile: 811.3
True speed on tire B: 60

Tire A sidewall = 205 × 55% = 112.75 mm; diameter = 16 + 2 × 112.75 ÷ 25.4 = 24.878 in
Tire B sidewall = 225 × 50% = 112.5 mm; diameter = 16 + 2 × 112.5 ÷ 25.4 = 24.858 in
Difference = (24.858 − 24.878) ÷ 24.878 × 100 = -0.08%
Revolutions per mile = 63,360 in ÷ (π × diameter): A 810.7, B 811.3
Speedometer reading 60 → true speed on tire B = 60 × 24.858 ÷ 24.878 = 60

The wider 225 tire has a lower 50% profile, so its overall height lands within 0.1% of the original and the speedometer stays true.

225/45R17 to 245/40R18

Width: 225
Aspect ratio: 45
Rim diameter: 17
Width: 245
Aspect ratio: 40
Rim diameter: 18
Speedometer reading: 60

Result Overall diameter difference (B vs A): 2.98%
Tire A diameter (in): 24.97
Tire B diameter (in): 25.72
Tire A sidewall (mm): 101.3
Tire B sidewall (mm): 98
Tire A revolutions per mile: 807.6
Tire B revolutions per mile: 784.2
True speed on tire B: 61.8

Tire A sidewall = 225 × 45% = 101.25 mm; diameter = 17 + 2 × 101.25 ÷ 25.4 = 24.972 in
Tire B sidewall = 245 × 40% = 98 mm; diameter = 18 + 2 × 98 ÷ 25.4 = 25.717 in
Difference = (25.717 − 24.972) ÷ 24.972 × 100 = 2.98%
Revolutions per mile = 63,360 in ÷ (π × diameter): A 807.6, B 784.2
Speedometer reading 60 → true speed on tire B = 60 × 25.717 ÷ 24.972 = 61.8

A plus-one move to an 18 in rim adds 0.74 in of height: at an indicated 60 the car is really doing about 61.8.

215/60R16 vs 225/45R17

Width: 215
Aspect ratio: 60
Rim diameter: 16
Width: 225
Aspect ratio: 45
Rim diameter: 17
Speedometer reading: 60

Result Overall diameter difference (B vs A): -4.53%
Tire A diameter (in): 26.16
Tire B diameter (in): 24.97
Tire A sidewall (mm): 129
Tire B sidewall (mm): 101.3
Tire A revolutions per mile: 771
Tire B revolutions per mile: 807.6
True speed on tire B: 57.3

Tire A sidewall = 215 × 60% = 129 mm; diameter = 16 + 2 × 129 ÷ 25.4 = 26.157 in
Tire B sidewall = 225 × 45% = 101.25 mm; diameter = 17 + 2 × 101.25 ÷ 25.4 = 24.972 in
Difference = (24.972 − 26.157) ÷ 26.157 × 100 = -4.53%
Revolutions per mile = 63,360 in ÷ (π × diameter): A 771, B 807.6
Speedometer reading 60 → true speed on tire B = 60 × 24.972 ÷ 26.157 = 57.3

The low-profile tire is 4.5% shorter, past the usual 3% guideline, so the speedometer reads about 2.7 high at 60.

How it is calculated

D = R + 2 × (W × A ÷ 100) ÷ 25.4; difference = (D_B − D_A) ÷ D_A × 100; true speed = indicated × D_B ÷ D_A

W: section width in millimetres
A: aspect ratio, sidewall height as a percent of width
R: rim diameter in inches
D: overall tire diameter in inches

The sidewall code mixes units: width is in millimetres, the aspect ratio is a percentage of that width, and the rim is in inches. The sidewall height in millimetres is converted with 25.4 mm per inch and added twice, once above and once below the rim. The speedometer counts wheel revolutions calibrated for the original tire, so a taller tire covers more ground per turn and the true speed scales with the diameter ratio. Revolutions per mile are 63,360 inches divided by the circumference, π × D.

When not to use it

Flotation and LT sizes written in inches, such as 33×12.50R15, use a different code.
It cannot confirm load index, speed rating or offset compatibility for a vehicle.
Exact clearance checks need the manufacturer's measured dimensions, not nominal ones.

Common mistakes

Reading the aspect ratio as millimetres; 45 means the sidewall is 45% of the width.
Comparing only the rim size and ignoring that a lower profile offsets a bigger wheel.
Assuming a speedometer reading high is harmless; the odometer drifts by the same percent.

Frequently asked questions

How much diameter difference is acceptable between tire sizes?

A common guideline in fitment guides is to stay within about 3% of the original overall diameter. Beyond that, speedometer error grows, ABS and stability systems may see mismatched wheel speeds, and a taller tire can rub on the arch or suspension at full lock.

Why does a bigger tire make my speedometer read low?

The speedometer infers speed from how fast the wheels or gearbox output turn. A taller tire travels farther per revolution, so at the same rotation rate the car moves faster than the dial shows. A 3% taller tire at an indicated 60 means about 61.8 on the road.

What do the three numbers in 225/45R17 mean?

225 is the section width in millimetres, 45 is the aspect ratio (the sidewall is 45% of 225 mm, or 101.25 mm), R marks radial construction, and 17 is the wheel diameter in inches the tire fits.

Can I mix tire sizes on the front and rear axles?

Some vehicles ship with staggered sizes, but on all-wheel-drive cars many manufacturers limit the diameter difference between axles to a small tolerance to protect the centre differential. Check the owner's manual before mixing sizes.

Does the odometer change with tire size too?

Yes. The odometer uses the same revolution count, so a tire that is 2% taller under-records distance by roughly 2%, which affects service intervals and fuel-economy figures you work out from it.

Why do two tires with the same size code measure slightly differently?

The code gives nominal dimensions. Tread depth, construction, rim width and inflation all change the real height by a few millimetres, so brands publish their own measured diameters that may differ from the formula.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Tire Comparison Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/automotive/tyres-and-wheels/tire-comparison-calculator/

MLA

Tools Directory Editorial Team. “Tire Comparison Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/automotive/tyres-and-wheels/tire-comparison-calculator/. Accessed 28 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Tire Comparison Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 28, 2026. https://tools.milunir.com/automotive/tyres-and-wheels/tire-comparison-calculator/.

Link

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