185/65R15 vs 215/40R18 tire size comparison
215/40R18 is 0.3 in (1.24%) taller than 185/65R15 and 1.2 in wider. They fit different wheels: 185/65R15 a 15-inch wheel and 215/40R18 an 18-inch wheel.
185/65R15 and 215/40R18 side by side
| Measurement | 185/65R15 | 215/40R18 | Difference | Difference % |
|---|---|---|---|---|
| Section width | 7.3 in (185 mm) | 8.5 in (215 mm) | +1.2 in (+30 mm) | +16.22% |
| Sidewall height | 4.7 in (120 mm) | 3.4 in (86 mm) | -1.3 in (-34 mm) | -28.48% |
| Wheel (rim) diameter | 15.0 in (381 mm) | 18.0 in (457 mm) | +3.0 in (+76 mm) | +20.00% |
| Overall diameter | 24.5 in (622 mm) | 24.8 in (629 mm) | +0.3 in (+7 mm) | +1.24% |
| Circumference | 76.9 in (1952 mm) | 77.8 in (1977 mm) | +0.9 in (+25 mm) | +1.24% |
| Revolutions per mile | 824 | 814 | -10 | -1.22% |
Differences are 215/40R18 minus 185/65R15; percentages are relative to 185/65R15. These are nominal design dimensions. Real tires vary by brand, tread depth, wheel width, load and pressure.
The overall diameter differs by 1.24%, which is within the common 3% rule of thumb.
The rule of thumb keeps a replacement size within about 3% of the original overall diameter. It covers diameter only. It is not a fit, load or safety check, because width, wheel diameter, load capacity and clearance matter too. Confirm any size change with your vehicle's owner's manual or a tire professional.
Speedometer difference
When the speedometer reads 60 mph on a vehicle built for 185/65R15, the actual speed on 215/40R18 is about 60.7 mph. When it reads 60 mph on a vehicle built for 215/40R18, the actual speed on 185/65R15 is about 59.3 mph. These figures assume the speedometer is accurate on the original size and use nominal dimensions, so treat them as estimates.
| Speedometer reads | Actual on 215/40R18 (built for 185/65R15) | Actual on 185/65R15 (built for 215/40R18) |
|---|---|---|
| 20 mph | 20.2 mph | 19.8 mph |
| 30 mph | 30.4 mph | 29.6 mph |
| 40 mph | 40.5 mph | 39.5 mph |
| 50 mph | 50.6 mph | 49.4 mph |
| 60 mph | 60.7 mph | 59.3 mph |
| 70 mph | 70.9 mph | 69.1 mph |
| 80 mph | 81.0 mph | 79.0 mph |
A common rule of thumb is to keep a replacement size within about 3% of the original overall diameter. Check the tire placard on the driver's door jamb and your owner's manual for the sizes your vehicle was built for. Confirm any size change with your vehicle's owner's manual or a tire professional.
Which vehicles use each size?
These vehicles list the size among their verified sizes, by trim. Open one to see all of its sizes.
Check it in the calculator
The calculator takes the same two sizes and shows the full comparison, or lets you swap either one for another size.
Open this comparison in the calculatorShop for 185/65R15 or 215/40R18 tires
Confirm the size on your tire sidewall or door placard first. These links open each store's own search; prices and stock are on their sites.
185/65R15
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Other comparisons with 185/65R15
Side-by-side dimensions and speedometer difference for sizes close to 185/65R15 in overall diameter.
- 185/65R15 vs 195/55R16-0.10% shorter than 185/65R15
- 185/65R15 vs 215/45R17+0.61% taller than 185/65R15
- 185/65R15 vs 205/45R17-0.84% shorter than 185/65R15
- 185/65R15 vs 205/55R16+1.67% taller than 185/65R15
- 185/55R16 vs 185/65R15-1.87% shorter than 185/65R15
Other comparisons with 215/40R18
Side-by-side dimensions and speedometer difference for sizes close to 215/40R18 in overall diameter.
- 205/55R16 vs 215/40R18+0.43% taller than 215/40R18
- 215/40R18 vs 215/45R17-0.62% shorter than 215/40R18
- 215/40R18 vs 225/45R17+0.81% taller than 215/40R18
- 195/65R15 vs 215/40R18+0.84% taller than 215/40R18
- 205/50R17 vs 215/40R18+1.21% taller than 215/40R18
You might also need
- Digital tire pressure gauge (Amazon search, opens in a new tab)
Check your tire pressure against the number on your door placard.
- Portable tire inflator (Amazon search, opens in a new tab)
Top up a low tire at home without a trip to the gas station.
- Tire tread depth gauge (Amazon search, opens in a new tab)
Measure how much tread is left before you decide to replace.
- Lug wrench or torque wrench (Amazon search, opens in a new tab)
Useful for rotations and tire changes; follow the torque spec in your owner's manual.
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