Tire size comparison
225/50R18 vs 235/45R18
235/45R18 is −2.0% shorter than 225/50R18: 26.3 in vs 26.9 in (−0.53 in), and 0.4 in wider. With 235/45R18 on a vehicle set up for 225/50R18, the speedometer shows 60 mph at an actual 58.8 mph. That is within the ±3% difference many shops use as a rule of thumb (not a manufacturer or legal limit).
Set up for: checking a replacement size (235/45R18 instead of 225/50R18) · Different situation?
Your current size
e.g. 265/70R17, 2657017 or 33x12.50R20
Where do I find my size?
It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. Digits are enough here: 2657017 or 33125020.
Size you're considering
225/50R18 vs 235/45R18: full comparison
| 225/50R18 | 235/45R18 | Difference | |
|---|---|---|---|
| Overall diameter | 26.86 in | 26.33 in | −0.53 in (−2.0%) |
| Width | 8.86 in | 9.25 in | +0.39 in (+4.4%) |
| Sidewall height | 4.43 in | 4.16 in | −0.27 in |
| Circumference | 84.4 in | 82.7 in | −1.7 in |
| Revs per mile (nominal) | 751 | 766 | +15 |
| Wheel diameter | 18 in | 18 in | ±0 in |
| Diameter in mm | 682 | 669 | −14 mm |
Speedometer difference
Your speedometer is calibrated for 225/50R18. Fitted with 235/45R18, the actual speed is the reading × 26.33 ÷ 26.86.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 19.6 mph | −0.4 mph |
| 30 mph | 29.4 mph | −0.6 mph |
| 45 mph | 44.1 mph | −0.9 mph |
| 55 mph | 53.9 mph | −1.1 mph |
| 60 mph | 58.8 mph | −1.2 mph |
| 70 mph | 68.6 mph | −1.4 mph |
| 80 mph | 78.4 mph | −1.6 mph |
Real tires: published diameters
225/50R18: 26.7–27.2 in published by 7 makers; 235/45R18: 26.1–26.6 in published by 7 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 225/50R18 · 235/45R18
Related comparisons
FAQ
Which is bigger, 225/50R18 or 235/45R18?
235/45R18 is −2.0% shorter than 225/50R18: 26.3 in vs 26.9 in (−0.53 in).
How much will my speedometer be off with 235/45R18 instead of 225/50R18?
The speedometer is calibrated for 225/50R18. With 235/45R18 it shows 60 mph when you are doing 58.8 mph, and 70 mph at 68.6 mph (−2.0%). Your speedometer also has its own factory tolerance.
Can I replace 225/50R18 with 235/45R18?
The diameter difference is −2.0%. Whether it fits depends on things this calculation can't see: wheel width and offset, clearance at full lock and over bumps, load index and speed rating, and your vehicle maker's approval. Check your door-jamb placard and owner's manual and ask a tire professional.