Nominal size comparison
305/30R18 vs 315/30R18
Diameter +0.94% · At indicated 60 mph: 60.56 mph estimated · Nominal radius change +3.0 mm
Calculated from nominal size labels. Instrument estimates assume unchanged calibration to the original size. Actual rolling circumference, clearance and tire specifications are not measured.
Within ±3% nominal diameter
The diameter difference is within this calculator's ±3% comparison band. This geometry does not verify vehicle fitment, wheel compatibility, load capacity, or calibration. Check vehicle and tire manufacturer specifications before fitting.
Visual diameter overlay
Installation still unverified
Small geometry change
- Diameter is within the ±3% geometry comparison band; vehicle fitment is not verified.
- New tire is 10 mm wider; check inner strut clearance, fender clearance, and wheel offset.
- Check the vehicle placard and owner's manual for size and pressure, and confirm load and speed requirements with the vehicle and tire manufacturer. These are geometry estimates only.
Speedometer effect
When the vehicle's speedometer (calibrated to 305/30R18) reads 60 mph, the estimated speed with 315/30R18 mounted is:
Estimated speed change relative to indicated: +0.94% · Odometer reads low by 0.93% relative to actual distance. Per 100 actual miles, it records 99.07 miles (unchanged calibration).
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 |
| Nominal radius 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 estimated speed with 315/30R18 is 60.56 mph — an error of +0.94%.
Is it safe to replace 305/30R18 with 315/30R18?
Within ±3% nominal diameter. The diameter difference is within this calculator's ±3% comparison band. This geometry does not verify vehicle fitment, wheel compatibility, load capacity, or calibration. Check vehicle and tire manufacturer specifications before fitting.
All calculations use standard ETRTO formulas. Confirm fitment with a qualified tire professional and your vehicle's door-placard before purchasing.