Enter sizes like 225/45R17 or use the fields. Everything runs locally in your browser.
Quick answer
Use this tire size calculator to compare your original tire size with a new size before changing wheels or tires.
The most important number is the overall diameter difference, because it affects speedometer accuracy,
ride height, gearing, and clearance.
Larger diameter: actual speed is higher than the speedometer shows.
Smaller diameter: actual speed is lower than the speedometer shows.
Common guideline: many drivers try to stay within about ±3% of the original diameter.
Important: this tool does not check wheel width, offset, fender clearance, brake clearance, load rating, or local regulations.
Inputs
You can also fill the fields below.
Examples: 205/55R16, 195/65R15, 275/40R19
You can also fill the fields below.
Supports R or dash: 235/40-18, 255 35 19, etc.
Adds inner-clearance and outer-poke estimates.
Stock wheel
New wheel
Used for speedometer error
Results
Results will appear here.
How the calculations work
Tire sizes are given as Width/Aspect R Rim, e.g., 225/45R17.
Sidewall Height (mm) = Width × (Aspect ÷ 100)
Rim Diameter (mm) = Rim(in) × 25.4
Overall Diameter (mm) = 2 × Sidewall + Rim(mm)
Circumference = π × Diameter
Revs per mile = 63,360 ÷ Circumference(in)
Revs per km = 1,000,000 ÷ Circumference(mm)
Speedometer: Actual = Indicated × (New Diameter ÷ Stock Diameter)
Added a Quick answer section above the calculator with speedometer, diameter, and clearance guidance.
Added popular comparison and alternative-size links that preload shareable calculator results.
Added static result snippets for common comparisons, including diameter difference, ride-height change, and actual speed at 60 mph.
Added equivalent-size guidance plus tire size charts for common 15, 16, 17, 18, 19, and 20 inch wheel diameters.
v1.1(February 10, 2026)
Added wheel specs mode with optional stock/new wheel width and offset inputs.
Added fitment delta outputs for inner clearance, outer poke, and risk band guidance.
Added speed correction table, odometer correction, and fuel economy factor.
Added equivalent-size suggestion engine by target rim within ±1%, ±2%, and ±3% diameter.
Added visual tire comparison and share/export actions.
Note: This calculator compares tire sizes only. Wheel width, offset, brake and fender clearances, and load/speed ratings are not assessed.
Popular tire size comparisons
These links open the calculator with common stock and replacement tire sizes prefilled. Use them as
starting points, then adjust the sizes, speed, wheel width, or offset for your vehicle.
225/45R17 vs 235/40R18:
the new tire is about 1.7% larger in diameter, raises ride height by roughly 0.21 in, and reads about 61.0 mph actual at 60 mph indicated.
205/55R16 vs 225/45R17:
the new tire is about 0.4% larger in diameter, raises ride height by roughly 0.05 in, and reads about 60.2 mph actual at 60 mph indicated.
265/70R17 vs 275/60R20:
the new tire is about 4.4% larger in diameter, raises ride height by roughly 0.69 in, and reads about 62.6 mph actual at 60 mph indicated.
245/40R19 vs 255/35R20:
the new tire is about 1.2% larger in diameter, raises ride height by roughly 0.16 in, and reads about 60.7 mph actual at 60 mph indicated.
Popular tire size alternatives
Alternative tire sizes are useful for plus sizing, winter tire downsizing, wheel upgrades, or replacing
an uncommon OEM size. These links preload the original size and a common candidate so you can compare
diameter, speedometer error, and equivalent-size suggestions.
Equivalent tire sizes are alternative sizes with a similar overall diameter to the original tire.
They are often used when changing wheel diameter, choosing winter tires, or finding a more available
replacement size. Start with your stock size, choose a target rim diameter, and compare candidates
against the original diameter.
Plus sizing: increase wheel diameter while lowering aspect ratio so overall diameter stays close.
Minus sizing: reduce wheel diameter and use a taller sidewall, often for winter tires or ride comfort.
Diameter tolerance: many drivers target about ±3%, but tighter is better for speedometer and gearing accuracy.
Final fitment: equivalent diameter does not prove that the tire clears the fender, strut, brakes, or suspension.
Tire size chart by wheel diameter
These static tire size charts list common sizes by rim diameter. Use them to find candidate sizes,
then run the calculator to compare diameter, sidewall, revs per mile, and speedometer error.
15 inch tire size chart
185/65R15
195/60R15
195/65R15
205/60R15
205/65R15
215/70R15
16 inch tire size chart
195/55R16
205/55R16
215/55R16
215/60R16
225/60R16
255/70R16
17 inch tire size chart
215/55R17
225/45R17
225/50R17
225/60R17
235/65R17
265/70R17
18 inch tire size chart
225/40R18
235/40R18
235/55R18
245/45R18
255/55R18
265/60R18
19 inch tire size chart
235/45R19
245/40R19
255/40R19
255/45R19
275/40R19
285/35R19
20 inch tire size chart
245/35R20
255/35R20
255/45R20
275/40R20
275/55R20
285/50R20
Fitment and safety notes
Formula used
Overall diameter is calculated as 2 × (width × aspect ÷ 100) + rim × 25.4.
Speedometer correction is actual speed = indicated speed × new diameter ÷ stock diameter.
What this calculator does not check
Load index or speed rating
Brake caliper clearance
Fender, liner, strut, or suspension clearance
Vehicle legality, insurance rules, or TPMS calibration
When to ask a tire professional
Ask a tire shop or fitment specialist when changing wheel width, offset, load rating, tire class,
or overall diameter by more than a small amount, especially on AWD vehicles or vehicles with driver-assist systems.
Last reviewed / updated
Last reviewed: May 18, 2026. Version v1.2 added quick decision guidance, popular comparison links,
alternative-size coverage, tire size charts, fitment notes, and updated schema metadata.
Editorial review: Starlight Tools engineering team. This page is a geometry calculator, not a fitment certification.
How to read a tire size
Modern tire codes look cryptic at first glance—something like 225/45R17—but each part
describes a dimension that affects how your car accelerates, brakes, corners, and reports speed.
The first number is the section width in millimetres (225 mm here). The second is the
aspect ratio, a percentage of the width (45% of 225 mm) that determines sidewall height.
The letter R indicates radial construction, and the last number is the rim diameter in inches.
From these, you can derive overall diameter, circumference, and revolutions per distance—exactly what this calculator shows.
Key formulas at a glance
Sidewall height = width × (aspect ÷ 100)
Rim diameter (mm) = rim(in) × 25.4
Overall diameter = (2 × sidewall) + rim(mm)
Circumference = π × diameter
Revs per mile = 63,360 ÷ circumference(in)
Revs per km = 1,000,000 ÷ circumference(mm)
Actual speed = indicated × (new diameter ÷ stock diameter)
How close is “close enough”?
A common rule of thumb is to keep overall diameter within roughly ±3% of the original size to
preserve speedometer accuracy, gearing feel, and fender/brake clearance. Larger diameters reduce
revs per mile (taller gearing), often showing a lower indicated speed than actual. Smaller
diameters do the reverse. Our results call out diameter deltas and speedometer error so you can
judge suitability quickly.
Real-world considerations beyond diameter
Wheel width & offset: Even if diameter matches, an overly wide tire can rub struts, liners, or fenders—offset and wheel width determine stance and clearance.
Load index & speed rating: Match or exceed OEM specs to maintain safety, handling, and insurance compliance.
TPMS & calibration: Significant size changes can affect ABS/TC logic and TPMS thresholds; some vehicles allow speedometer/odometer recalibration.
Season & tread type: Winter tires often run narrower for traction and slush evacuation; ultra-high-performance summer tires may square up wider on the same wheel.
Unsprung mass & inertia: Bigger wheels/tires can add weight, affecting ride and acceleration; lighter assemblies improve response.
Gear ratio feel: Fewer revs per mile = taller gearing (lower engine rpm at a given road speed), which can slightly soften launch but reduce cruise rpm.
Worked example
Compare 225/45R17 to 235/40R18. The 18-inch option typically has a shorter sidewall to
offset the larger rim, keeping the overall diameter similar. If the calculator shows a diameter
change near zero, speedometer and gearing feel remain close to stock; if it shows ~2–3% larger,
expect a small actual-speed increase versus indicated and slightly taller gearing.
Tips for choosing an alternative size
Start from OEM size and target ±3% diameter or less.
Check wheel manufacturer fitment guides for width/offset compatibility.
Verify load index/speed rating; do not down-spec for cost.
Inspect clearances at full lock, full bump (suspension compressed), and with chains if applicable.
Disclaimer: This tool estimates geometry only. It does not guarantee fitment or legal compliance.
Always confirm clearances on-vehicle and follow manufacturer specifications.
Frequently Asked Questions
Which formats can I enter?
Try 225/45R17, 225/45-17, or 225 45 17. Width and aspect are metric; rim is inches.
What’s a safe diameter change?
Many guides suggest keeping overall diameter within ~3% of stock for speedometer accuracy and clearance. This is not a guarantee—always check clearances.
Why do revs per mile matter?
They relate to gearing and speedometer calibration. Fewer revs/mile typically means a larger diameter tire.
Is my data private?
Yes. Everything runs locally in your browser.
🛞 5 Fun Facts about Tire Sizes
1
Speedos lie when you resize
A 3% larger diameter makes your speedometer under-read by the same 3%—an indicated 60 mph is ~61.8 mph actual.
Sneaky mph
2
Sidewall = flex & comfort
Taller sidewalls act like extra suspension. Dropping from 55 to 35 series can stiffen ride noticeably, even at the same diameter.
