Tire size comparison
275/35R18 vs 275/40R18
275/40R18 is +4.2% taller than 275/35R18: 26.7 in vs 25.6 in (+1.08 in), and the same width. With 275/40R18 on a vehicle set up for 275/35R18, the speedometer shows 60 mph at an actual 62.5 mph. That is more than the ±3% difference many shops use as a rule of thumb, so check carefully before switching.
Set up for: going bigger (275/40R18 instead of 275/35R18) · 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
275/35R18 vs 275/40R18: full comparison
| 275/35R18 | 275/40R18 | Difference | |
|---|---|---|---|
| Overall diameter | 25.58 in | 26.66 in | +1.08 in (+4.2%) |
| Width | 10.83 in | 10.83 in | ±0.00 in (±0.0%) |
| Sidewall height | 3.79 in | 4.33 in | +0.54 in |
| Circumference | 80.4 in | 83.8 in | +3.4 in |
| Revs per mile (nominal) | 788 | 756 | −32 |
| Wheel diameter | 18 in | 18 in | ±0 in |
| Diameter in mm | 650 | 677 | +28 mm |
Speedometer difference
Your speedometer is calibrated for 275/35R18. Fitted with 275/40R18, the actual speed is the reading × 26.66 ÷ 25.58.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.8 mph | +0.8 mph |
| 30 mph | 31.3 mph | +1.3 mph |
| 45 mph | 46.9 mph | +1.9 mph |
| 55 mph | 57.3 mph | +2.3 mph |
| 60 mph | 62.5 mph | +2.5 mph |
| 70 mph | 73.0 mph | +3.0 mph |
| 80 mph | 83.4 mph | +3.4 mph |
Real tires: published diameters
275/35R18: 25.3–25.7 in published by 7 makers; 275/40R18: 26.4–26.7 in published by 6 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 275/35R18 · 275/40R18
FAQ
Which is bigger, 275/35R18 or 275/40R18?
275/40R18 is +4.2% taller than 275/35R18: 26.7 in vs 25.6 in (+1.08 in).
How much will my speedometer be off with 275/40R18 instead of 275/35R18?
The speedometer is calibrated for 275/35R18. With 275/40R18 it shows 60 mph when you are doing 62.5 mph, and 70 mph at 73.0 mph (+4.2%). Your speedometer also has its own factory tolerance.
Can I replace 275/35R18 with 275/40R18?
The diameter difference is +4.2%. 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.