TreadSize

Computed tire swap

305/30R18 vs 315/30R18

Diameter +0.94% · Speedo at 60 mph reads 60.6 mph actual · Ground clearance +3.0 mm

Methodology: this page compares ETRTO-derived tire dimensions, circumference, speedometer error, and the TreadSize 3% diameter rule. Last reviewed June 2026 by the TreadSize editorial team.

Generally safe — within the 3% tolerance

The diameter difference is ≤3%, which is the widely-accepted fitment tolerance for speedometer accuracy and ABS/traction-control calibration. Verify clearance and load rating with a tire professional before installing.

Physical fitment still needs inspection: this comparison checks diameter, circumference, speedometer error, and ground clearance. It does not verify fender clearance, brake clearance, rim width compatibility, load index, or speed rating.

Visual diameter overlay

305/30R18 width305 mm
315/30R18 width315 mm

Fitment risk beyond diameter

Low fitment risk

  • Diameter is within the common ±3% first-gate tolerance.
  • New tire is 10 mm wider; check inner strut clearance, fender clearance, and wheel offset.
  • Always compare against the vehicle door-placard OE size, load index, and speed rating before buying.

Speedometer effect

When the vehicle's speedometer (calibrated to 305/30R18) reads 60 mph, the actual speed with 315/30R18 mounted is:

Indicated
60 mph
Actual
60.6 mph

Speedometer error: +0.93%  ·  Odometer reads high by 0.93%

Full comparison

Measurement 305/30R18 315/30R18 Delta
Overall diameter 25.20" 25.44" +0.94%
Section width 305 mm 315 mm +10 mm
Sidewall height 91.5 mm 94.5 mm +3.00 mm
Rim diameter 18" 18" +0"
Circumference 2011 mm 2030 mm +19 mm
Revs / mile 800 793 -7
Ground clearance change +3.0 mm

Questions about this swap

What is the diameter difference between 305/30R18 and 315/30R18?

315/30R18 is 0.94% larger in overall diameter than 305/30R18 — a difference of 6.0 mm (0.24 inches).

How does 315/30R18 affect the speedometer if the car was calibrated for 305/30R18?

When the speedometer reads 60 mph (calibrated for 305/30R18), the actual speed with 315/30R18 is 60.6 mph — an error of +0.93%.

Is it safe to replace 305/30R18 with 315/30R18?

Generally safe — within the 3% tolerance. The diameter difference is ≤3%, which is the widely-accepted fitment tolerance for speedometer accuracy and ABS/traction-control calibration. Verify clearance and load rating with a tire professional before installing.

All calculations use standard ETRTO formulas. Confirm fitment with a qualified tire professional and your vehicle's door-placard before purchasing.