Nominal size comparison
235/45R18 vs 235/50R18
Diameter +3.51% · At indicated 60 mph: 62.11 mph estimated · Nominal radius change +11.8 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.
3–5% nominal diameter difference
The diameter difference is outside this calculator's ±3% comparison band. Estimated speed and distance changes are larger; fitment and calibration are not verified. Check vehicle and tire manufacturer specifications before fitting.
Visual diameter overlay
Installation still unverified
Diameter outside ±3%
- Diameter exceeds the ±3% comparison band; review vehicle-specific speedometer, ABS, and stability-control requirements.
- Nominal section width is unchanged; actual width depends on the tire model and measuring rim, so clearance still needs checking.
- 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 235/45R18) reads 60 mph, the estimated speed with 235/50R18 mounted is:
Estimated speed change relative to indicated: +3.51% · Odometer reads low by 3.40% relative to actual distance. Per 100 actual miles, it records 96.61 miles (unchanged calibration).
Full comparison
| Measurement | 235/45R18 | 235/50R18 | Delta |
|---|---|---|---|
| Overall diameter | 26.33" | 27.25" | +3.51% |
| Section width | 235 mm | 235 mm | +0 mm |
| Sidewall height | 105.75 mm | 117.5 mm | +11.75 mm |
| Rim diameter | 18" | 18" | +0" |
| Circumference | 2101 mm | 2175 mm | +74 mm |
| Revs / mile | 766 | 740 | -26 |
| Nominal radius change | +11.8 mm | ||
Questions about this swap
What is the diameter difference between 235/45R18 and 235/50R18?
235/50R18 is 3.51% larger in overall diameter than 235/45R18 — a difference of 23.5 mm (0.93 inches).
How does 235/50R18 affect the speedometer if the car was calibrated for 235/45R18?
When the speedometer reads 60 mph (calibrated for 235/45R18), the estimated speed with 235/50R18 is 62.11 mph — an error of +3.51%.
Is it safe to replace 235/45R18 with 235/50R18?
3–5% nominal diameter difference. The diameter difference is outside this calculator's ±3% comparison band. Estimated speed and distance changes are larger; fitment and calibration are not verified. 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.