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Tire Size Calculator

Compare two tire sizes from the sidewall code (225/45R17 style): overall diameter change, speedometer error, circumference, and revs per mile — with the ±3% fitment rule explained.

Tire Size Calculator

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Diameter Change

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Enter your details and press “Calculate” to see your results.

The sidewall code decodes as width (mm) / aspect ratio (sidewall height as % of width) R rim (inches). Overall diameter = rim + two sidewalls, converted to inches — and the diameter difference is what matters for fitment: it changes your speedometer (which counts wheel revolutions calibrated to the original size), odometer, effective gearing, and clearance. The widely used rule of thumb keeps replacements within ±3% of the original diameter; the calculator flags anything beyond it.

About Tire Sizes

225/45R17 reads like a serial number but it's really three measurements — and when you change wheels or tires, keeping the OVERALL diameter close to stock is the thing that keeps your speedometer honest and your fenders unscratched. Plus-sizing (bigger rim, shorter sidewall) done right holds diameter nearly constant.

Enter your current size and the candidate size for the diameter change in percent, the speedometer error at 60 mph, and the new tire's circumference and revolutions per mile — the numbers a tire shop checks before nodding at your upgrade.

The aspect-ratio idea itself — one number describing a shape — has a general tool: the Aspect Ratio Calculator.

Decoding the Sidewall

Three numbers, one diameter:

Sidewall (mm) = width × aspect% Diameter (in) = rim + 2 × sidewall ÷ 25.4 Speedo error: actual speed = indicated × (new dia ÷ old dia)

Worked example: 225/45R17 stands 24.97″ tall; a 235/40R18 stands 25.40″ — +1.72%, so an indicated 60 mph is really 61.0. That's a textbook plus-one upgrade: bigger rim, shorter sidewall, diameter within the safe band.

Common Upgrades Compared

Typical swaps from a 225/45R17 baseline (24.97″) — computed by this calculator:

New sizeDiameterChangeVerdict
235/40R1825.40″+1.72%Classic plus-one — fine
245/35R1925.75″+3.1%At the rule's edge
215/50R1725.46″+2.0%Comfort upsize — fine
225/40R1724.09″−3.5%Speedo reads fast — flag
265/70R1731.61″+26.6%Truck territory — lift kit math

Within ±3%, most cars run happily with no recalibration; beyond it, speedometer, ABS tuning, and clearance all deserve attention.

What a Diameter Change Touches

A taller tire makes the speedometer read SLOW (you're traveling faster than indicated — ticket risk) and the odometer under-count; shorter reads fast and over-counts, quietly aging your lease mileage. Effective gearing shifts too: taller tires lengthen gearing (lazier acceleration, lower cruising RPM), shorter do the reverse — the same math off-roaders exploit deliberately with regears.

Beyond the math: load index and speed rating on the new tire must meet or beat the original (numbers printed right after the size), width changes affect rim compatibility and rubbing at full lock, and AWD systems can be genuinely damaged by mismatched diameters ACROSS axles — many manufacturers spec all four tires within a few 32nds of tread. When the calculator flags ±3%, the tire shop conversation isn't optional.

Frequently Asked Questions

What do tire size numbers mean?

225/45R17: 225 mm of tread width, sidewall height 45% of that width (101 mm), on a 17-inch rim — R for radial construction. Overall diameter works out to about 25 inches, which is the number swaps need to preserve.

Can I use a different tire size on my car?

Within about ±3% of the original overall diameter, usually yes — with matching load/speed ratings and adequate clearance. The calculator does the diameter math; door-jamb placard sizes and your tire shop settle the rest.

What is plus-sizing?

Bigger rim + proportionally shorter sidewall, keeping diameter nearly constant: 225/45R17 → 235/40R18 (+1.7%) is a classic plus-one. Looks and handling improve; ride comfort and pothole tolerance pay for it.

How does tire size affect my speedometer?

The speedometer counts wheel revolutions assuming the factory diameter. Taller tires cover more ground per rev — indicated 60 becomes actual 61–62 on a +2–3% tire. The calculator states the exact error for your swap.

Do bigger tires change fuel economy?

Modestly, both ways: taller gearing can help highway cruising, but heavier/wider tires with more rolling resistance usually cost more than the gearing saves — and the odometer under-counting makes MPG LOOK worse than it is. Track fuel per GPS miles for the truth.

Why do AWD cars care so much about matching tires?

Different diameters across axles force the center differential/clutch pack to work constantly — heat and wear on hardware that expected near-identical rotation. Many AWD makers spec replacing all four together or keeping tread depths within tight tolerance; the ±3% rule is NOT sufficient across axles.

Methodology. This calculator uses standard, peer-reviewed mathematical formulas. It is reviewed and maintained by the Vast Calculators editorial team.

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Results are estimates for general use; verify critical figures independently.

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