What tire-size difference is acceptable?

Many comparison tools use about 3% diameter difference as a quick screening threshold. It is not a universal approval; always check the vehicle and tire requirements.

Why does tire size affect the speedometer?

The speedometer is calibrated around wheel rotation. A larger rolling circumference means the car travels farther per revolution, so real speed can be higher than indicated.

Does this calculator confirm a tire will fit?

No. It checks geometry only. Fitment also depends on wheel width, offset, brake clearance, fender clearance, load index, speed rating and vehicle manufacturer guidance.

Tire Size Calculator

Compare two tire sizes with diameter, sidewall, circumference, revolutions per mile, speedometer change and a practical fitment warning.

Stock tire size

Stock section width, mm

Stock aspect ratio, %

Stock rim diameter, in

New tire size

New section width, mm

New aspect ratio, %

New rim diameter, in

Stock diameter

24.88 in

632 mm

New diameter

25.46 in

647 mm

Diameter difference

+2.36%

+15 mm

Circumference

80 in

new tire

Revs per mile

792

new tire estimate

Speedometer at 60 mph

61.4 mph

error +1.4 mph

Ride height

+0.29 in

about half the diameter change

Verdict: Within the common 3% diameter guideline. Still confirm the size in the vehicle manual and tire placard.

This is a geometric estimate. It does not verify wheel width, load and speed ratings, fender clearance, actual rolling radius, or the sizes approved by the vehicle manufacturer.

Compare tire sizes before changing fitment

Tire-size search intent is comparison-focused: drivers want to know how a new size changes diameter, circumference, clearance and speedometer reading compared with the stock size.

Enter the stock tire width, aspect ratio and rim diameter.
Enter the new tire size you want to compare.
Review diameter difference, circumference, revs per mile and real speed at a 60 mph indicated speed.
Check the vehicle placard, owner's manual, wheel width, load index, speed rating and physical clearance before buying.

Fitment is more than diameter

The calculator does not verify wheel offset, brake clearance, fender clearance, load rating, speed rating or manufacturer-approved sizes.

Tire size formulas

S i d e w a l l = S ec t i o n w i d t h \times \frac{A s p ec t r a t i o}{100}

D iam e t er = R im d iam e t er \times 25.4 + 2 \times S i d e w a l l

C i r c u m f er e n ce = π \times D iam e t er

R e v s p er mi l e = \frac{63360}{C i r c u m f er e n ce in in c h es}

Frequently Asked Questions

Sources and References

Calculations are based on the listed reference sources. Links open in a new tab.

Updated: June 2, 2026

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