Tire size comparison
175/65R15 vs 185/65R15
185/65R15 is +2.1% taller than 175/65R15: 24.5 in vs 24.0 in (+0.51 in), and 0.4 in wider. With 185/65R15 on a vehicle set up for 175/65R15, the speedometer shows 60 mph at an actual 61.3 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 (185/65R15 instead of 175/65R15) · 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
175/65R15 vs 185/65R15: full comparison
| 175/65R15 | 185/65R15 | Difference | |
|---|---|---|---|
| Overall diameter | 23.96 in | 24.47 in | +0.51 in (+2.1%) |
| Width | 6.89 in | 7.28 in | +0.39 in (+5.7%) |
| Sidewall height | 4.48 in | 4.73 in | +0.26 in |
| Circumference | 75.3 in | 76.9 in | +1.6 in |
| Revs per mile (nominal) | 842 | 824 | −18 |
| Wheel diameter | 15 in | 15 in | ±0 in |
| Diameter in mm | 608 | 622 | +13 mm |
Speedometer difference
Your speedometer is calibrated for 175/65R15. Fitted with 185/65R15, the actual speed is the reading × 24.47 ÷ 23.96.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.4 mph | +0.4 mph |
| 30 mph | 30.6 mph | +0.6 mph |
| 45 mph | 46.0 mph | +1.0 mph |
| 55 mph | 56.2 mph | +1.2 mph |
| 60 mph | 61.3 mph | +1.3 mph |
| 70 mph | 71.5 mph | +1.5 mph |
| 80 mph | 81.7 mph | +1.7 mph |
Real tires: published diameters
175/65R15: 23.9–24.1 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: 175/65R15 · 185/65R15
Related comparisons
FAQ
Which is bigger, 175/65R15 or 185/65R15?
185/65R15 is +2.1% taller than 175/65R15: 24.5 in vs 24.0 in (+0.51 in).
How much will my speedometer be off with 185/65R15 instead of 175/65R15?
The speedometer is calibrated for 175/65R15. With 185/65R15 it shows 60 mph when you are doing 61.3 mph, and 70 mph at 71.5 mph (+2.1%). Your speedometer also has its own factory tolerance.
Can I replace 175/65R15 with 185/65R15?
The diameter difference is +2.1%. 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.