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Tire Size Calculator

Compare tire sizes and calculate speedometer differences

Original Tire Size

Compare With (Optional)

Speedometer Accuracy Check

Tire Size Calculator Guide

A tire size calculator compares overall diameter, circumference, revs per mile, and speedometer error between tire sizes (e.g., 225/45R17 vs 235/40R18). It helps you pick replacements that fit without rubbing and stay close to stock gearing.

What is Tire Size Calculator?

The tire size calculator parses metric sizes W/A R D (width, aspect, rim in inches) and LT flotation sizes where applicable, computes overall diameter, and shows how changes affect ride height and speedometer accuracy.

How to Use the Tire Size Calculator

  1. Enter current tire size and new size.
  2. Calculate overall diameter, circumference, revs/mile (or km), and speedometer difference.
  3. Check percent change; many aim for <= 3% diameter difference from stock.
  4. Verify rim width compatibility and load/speed ratings.
  5. Inspect wheel well clearance, especially at full lock and compression.

Formulas & Methods

  • Overall diameter (metric):
    D = (2 * width_mm * (aspect/100) / 25.4) + rim_in (inches).
  • Circumference: C = pi * D.
  • Revs per mile: RPM = 63360 / C (inches per mile).
  • Speedometer: Speed_new = Speed_actual * (D_new / D_old); percent error from ratio.
  • Clearance: compare D and section width impact for fender/strut clearance.

Assumptions & limitations

  • Actual tire dimensions vary by manufacturer; use published measured dimensions for precision.
  • Load index, speed rating, and offset/scrub radius are outside this calculator; verify before purchase.
  • Large size changes may require reprogramming the vehicle's speedometer.

Examples

Example A — Size change
Old 205/55R16: D_old ~ (2*205*0.55/25.4) + 16 = 24.9 in.
New 225/45R17: D_new ~ (2*225*0.45/25.4) + 17 = 25.0 in.
Difference ~ 0.4% → speedometer nearly unchanged.

Example B — Larger tire
Old 225/45R17 vs new 245/45R18:

  • Check percent diameter change and revs/mile
  • Ensure <3% difference
  • Verify rim width compatibility

| Metric | Old | New | |---|---:|---:| | Diameter (in) | 24.9 | 25.0 | | Circumf (in) | 78.2 | 78.5 | | Revs/mile | 810 | 807 |

Pro Tips & Best Practices

  • Keep diameter change <= 3% to maintain speedometer and ABS/TC behavior.
  • Verify load index and speed rating meet or exceed OEM.
  • Consider offset and suspension travel for clearance.
  • Reprogram speedometer if change is significant and your vehicle supports it.
  • Check insurance/warranty implications of non-stock sizes.

Related Calculators

FAQ

Q: How do I read tire size like 225/45R17?

A: 225 = tread width (mm), 45 = aspect ratio (% of width), R = radial, 17 = wheel diameter (inches).

Q: How does tire size affect speedometer?

A: Larger overall diameter reads slower; smaller reads faster. The calculator shows the percent difference.

Q: What is the impact on clearance and gearing?

A: Bigger tires can rub and effectively increase gearing; smaller do the opposite. Check clearance and load ratings.

Q: Can I put any width on my rim?

A: Rim width range must match tire width; consult tire fitment guides.

Q: What about load index and speed rating?

A: Match or exceed OEM specs for safety and legal compliance.

Call to Action

Compare your current and new tire sizes to see diameter, revs/mile, and speedometer impact—choose a size that fits and stays within safe limits.