Tire size comparison
185/55R15 vs 185/65R15
185/65R15 is +6.3% taller than 185/55R15: 24.5 in vs 23.0 in (+1.46 in), and the same width. With 185/65R15 on a vehicle set up for 185/55R15, the speedometer shows 60 mph at an actual 63.8 mph. That is more than the ±3% difference many shops use as a rule of thumb, so check carefully before switching.
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). If your current tires aren't the original size, the placard shows what the vehicle came with. Digits are enough here: 2657017 or 33125020.
Size you're considering
185/55R15 vs 185/65R15: full comparison
| 185/55R15 | 185/65R15 | Difference | |
|---|---|---|---|
| Overall diameter | 23.01 in | 24.47 in | +1.46 in (+6.3%) |
| Width | 7.28 in | 7.28 in | ±0.00 in (±0.0%) |
| Sidewall height | 4.01 in | 4.73 in | +0.73 in |
| Circumference | 72.3 in | 76.9 in | +4.6 in |
| Revs per mile (nominal) | 876 | 824 | −52 |
| Wheel diameter | 15 in | 15 in | ±0 in |
| Diameter in mm | 584 | 622 | +37 mm |
Speedometer difference
Your speedometer is calibrated for 185/55R15. Fitted with 185/65R15, the actual speed is the reading × 24.47 ÷ 23.01.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 21.3 mph | +1.3 mph |
| 30 mph | 31.9 mph | +1.9 mph |
| 45 mph | 47.8 mph | +2.8 mph |
| 55 mph | 58.5 mph | +3.5 mph |
| 60 mph | 63.8 mph | +3.8 mph |
| 70 mph | 74.4 mph | +4.4 mph |
| 80 mph | 85.1 mph | +5.1 mph |
Real tires: published diameters
185/55R15: 22.9–23.1 in published by 4 makers; 185/65R15: 24.3–24.5 in published by 4 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 185/55R15 · 185/65R15
Related comparisons
FAQ
Which is bigger, 185/55R15 or 185/65R15?
185/65R15 is +6.3% taller than 185/55R15: 24.5 in vs 23.0 in (+1.46 in).
How much will my speedometer be off with 185/65R15 instead of 185/55R15?
The speedometer is calibrated for 185/55R15. With 185/65R15 it shows 60 mph when you are doing 63.8 mph, and 70 mph at 74.4 mph (+6.3%). Your speedometer also has its own factory tolerance.
Can I replace 185/55R15 with 185/65R15?
The diameter difference is +6.3%. 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.