Nominal size comparison
245/40R18 vs 255/35R18
Diameter -2.68% · At indicated 60 mph: 58.39 mph estimated · Nominal radius change -8.8 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 245/40R18) reads 60 mph, the estimated speed with 255/35R18 mounted is:
Estimated speed change relative to indicated: -2.68% · Odometer reads high by 2.75% relative to actual distance. Per 100 actual miles, it records 102.75 miles (unchanged calibration).
Full comparison
| Measurement | 245/40R18 | 255/35R18 | Delta |
|---|---|---|---|
| Overall diameter | 25.72" | 25.03" | -2.68% |
| Section width | 245 mm | 255 mm | +10 mm |
| Sidewall height | 98 mm | 89.25 mm | -8.75 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2052 mm | 1997 mm | -55 mm |
| Revs / mile | 784 | 806 | 22 |
| Nominal radius change | -8.8 mm | ||
Questions about this swap
What is the diameter difference between 245/40R18 and 255/35R18?
255/35R18 is 2.68% smaller in overall diameter than 245/40R18 — a difference of 17.5 mm (0.69 inches).
How does 255/35R18 affect the speedometer if the car was calibrated for 245/40R18?
When the speedometer reads 60 mph (calibrated for 245/40R18), the estimated speed with 255/35R18 is 58.39 mph — an error of -2.68%.
Is it safe to replace 245/40R18 with 255/35R18?
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.