Tire size comparison
255/40R18 vs 265/35R18
265/35R18 is −2.8% shorter than 255/40R18: 25.3 in vs 26.0 in (−0.73 in), and 0.4 in wider. With 265/35R18 on a vehicle set up for 255/40R18, the speedometer shows 60 mph at an actual 58.3 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 (265/35R18 instead of 255/40R18) · 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
255/40R18 vs 265/35R18: full comparison
| 255/40R18 | 265/35R18 | Difference | |
|---|---|---|---|
| Overall diameter | 26.03 in | 25.30 in | −0.73 in (−2.8%) |
| Width | 10.04 in | 10.43 in | +0.39 in (+3.9%) |
| Sidewall height | 4.02 in | 3.65 in | −0.36 in |
| Circumference | 81.8 in | 79.5 in | −2.3 in |
| Revs per mile (nominal) | 775 | 797 | +22 |
| Wheel diameter | 18 in | 18 in | ±0 in |
| Diameter in mm | 661 | 643 | −18 mm |
Speedometer difference
Your speedometer is calibrated for 255/40R18. Fitted with 265/35R18, the actual speed is the reading × 25.30 ÷ 26.03.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 19.4 mph | −0.6 mph |
| 30 mph | 29.2 mph | −0.8 mph |
| 45 mph | 43.7 mph | −1.3 mph |
| 55 mph | 53.5 mph | −1.5 mph |
| 60 mph | 58.3 mph | −1.7 mph |
| 70 mph | 68.0 mph | −2.0 mph |
| 80 mph | 77.8 mph | −2.2 mph |
Real tires: published diameters
255/40R18: 25.8–26.2 in published by 5 makers; 265/35R18: 25.1–25.4 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: 255/40R18 · 265/35R18
FAQ
Which is bigger, 255/40R18 or 265/35R18?
265/35R18 is −2.8% shorter than 255/40R18: 25.3 in vs 26.0 in (−0.73 in).
How much will my speedometer be off with 265/35R18 instead of 255/40R18?
The speedometer is calibrated for 255/40R18. With 265/35R18 it shows 60 mph when you are doing 58.3 mph, and 70 mph at 68.0 mph (−2.8%). Your speedometer also has its own factory tolerance.
Can I replace 255/40R18 with 265/35R18?
The diameter difference is −2.8%. 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.