Tires and Wheels
Tire Size Comparison Calculator
Compare overall diameter and width between two tire sizes. Fitment also depends on wheel width, offset, suspension, brakes, and body clearance.
Values used in the formula
Enter current information for Tire Size Comparison and leave unrelated adjustments outside the form.
Scope of the estimate
Compare overall diameter and width between two tire sizes — a calculated clearance or dimension still requires a physical check on the vehicle.
Fitment also depends on wheel width, offset, suspension, brakes, and body clearance — that condition defines when diameter change is comparable with another result.
Collecting the data in practice
Measure the installed component and the vehicle reference points rather than relying only on a catalog dimension — this workflow must also account for the fact that fitment also depends on wheel width, offset, suspension, brakes, and body clearance.
- Record Original tire width as width of the original tire — measure from the stated reference points and note whether the vehicle or component is loaded.
- Record Original aspect ratio as original sidewall ratio — use a measurement or specification from the exact component and operating condition being evaluated.
- Record Original wheel diameter as original wheel size — measure from the stated reference points and note whether the vehicle or component is loaded.
- Record New tire width as width of the replacement tire — measure from the stated reference points and note whether the vehicle or component is loaded.
- Record New aspect ratio as replacement sidewall ratio — use a measurement or specification from the exact component and operating condition being evaluated.
Collecting compatible values
Document Original tire width as width of the original tire — this means you should measure from the stated reference points and note whether the vehicle or component is loaded.
Original aspect ratio. Original sidewall ratio — for this measurement, use a measurement or specification from the exact component and operating condition being evaluated.
Original wheel diameter: Original wheel size — a compatible entry should measure from the stated reference points and note whether the vehicle or component is loaded.
The New tire width entry represents width of the replacement tire — before calculating, measure from the stated reference points and note whether the vehicle or component is loaded.
New aspect ratio is defined here as replacement sidewall ratio — keeping that definition intact requires you to use a measurement or specification from the exact component and operating condition being evaluated.
For New wheel diameter, use the quantity described as replacement wheel size — in the vehicle record, measure from the stated reference points and note whether the vehicle or component is loaded.
Formula used on this page
In “diameter change = new overall diameter ÷ original overall diameter − 1,” the equation links original tire width, original aspect ratio, and original wheel diameter to diameter change.
No term beyond original tire width, original aspect ratio, original wheel diameter, new tire width, new aspect ratio, and new wheel diameter is introduced in “diameter change = new overall diameter ÷ original overall diameter − 1.”
Example using the displayed inputs
The sample vehicle case enters Original tire width = 225 mm, Original aspect ratio = 50%, Original wheel diameter = 17 in, New tire width = 245 mm, New aspect ratio = 45%, and New wheel diameter = 18 in.
That specific combination leads to Diameter change = 3.18%, Original diameter = 25.86 in, New diameter = 26.68 in, and Width change = 20 mm.
Interpreting the headline value
Diameter change answers “Compare overall diameter and width between two tire sizes.” The additional displays, Original diameter, New diameter, and Width change, are a different view of the same entered measurements.
A small diameter match does not guarantee safe fitment — when that condition changes, compare separate calculator runs instead of blending the inputs.
Because fitment also depends on wheel width, offset, suspension, brakes, and body clearance, a disagreement between diameter change and an outside reference should trigger a review of original tire width and new wheel diameter.
Suspension travel, steering angle, tire growth, body tolerances, and alignment can reveal interference that a static dimension misses — for original tire width, the page specifically expects width of the original tire.
This page does not attempt to compare a proposed larger wheel and lower-profile tire with the original size — the Plus-Sizing Tire provides that calculation.
Questions about the formula and inputs
What measurement source fits Original tire width when it represents width of the original tire?
Because original tire width represents width of the original tire, use a source tied to the exact vehicle, component, and operating period described by the other fields.
How does the warning “Fitment also depends on wheel width, offset, suspension, brakes, and body clearance” affect Diameter change?
The condition “Fitment also depends on wheel width, offset, suspension, brakes, and body clearance” is not corrected automatically by the numeric inputs, so create a separate tire size comparison case when it changes.
What assumption is expressed by “diameter change = new overall diameter ÷ original overall diameter − 1”?
In “diameter change = new overall diameter ÷ original overall diameter − 1,” original tire width and original aspect ratio are treated as parts of one vehicle case.