18-inch rim · tyre size comparison
275/70 R18 vs 325/65 R18
325/65 R18 is 37.5 mm (1.48 in) taller than 275/70 R18, a +4.45% difference in overall diameter. That is outside the ±3% limit commonly used to screen a tire size change, so expect a noticeable speedometer error; it is a nominal calculation, not a fitment approval.
- Diameter difference
- +37.5 mm
- +1.48 in · +4.45%
- Indicated 70 mph
- 73.1 mph
- Actual speed on 325/65 R18, if the speedometer was accurate on 275/70 R18.
- Sidewall height
- +18.75 mm
- 192.5 mm to 211.25 mm.
Dimensions side by side
| Measurement | 275/70 R18 | 325/65 R18 | Difference |
|---|---|---|---|
| Overall diameter | 842.2 mm33.16 in | 879.7 mm34.63 in | +37.5 mm (+4.45%)+1.48 in |
| Section width | 275 mm10.83 in | 325 mm12.8 in | +50 mm+1.97 in |
| Sidewall height | 192.5 mm7.58 in | 211.25 mm8.32 in | +18.75 mm+0.74 in |
| Circumference | 2,645.85 mm104.17 in | 2,763.66 mm108.81 in | +117.81 mm+4.64 in |
| Rim diameter | 457.2 mm18 in | 457.2 mm18 in | 0 mm0 in |
| Revolutions per mile | 608.25 | 582.32 | -25.93 |
| Revolutions per km | 377.95 | 361.84 | -16.11 |
Calculated from the nominal size codes, without load or tread wear. Real tyres vary by model, rim width and pressure, so the exact tyre's published dimensions take priority.
Speedometer with 325/65 R18
If the speedometer was accurate on 275/70 R18, fitting 325/65 R18 means the car is travelling 4.45% faster than it shows, and the odometer under-records distance by about the same amount.
| Speedometer shows | Actual on 325/65 R18 | Difference |
|---|---|---|
| 30 mph | 31.34 mph | +1.34 mph |
| 50 mph | 52.23 mph | +2.23 mph |
| 60 mph | 62.67 mph | +2.67 mph |
| 70 mph | 73.12 mph | +3.12 mph |
| 50 km/h | 52.23 km/h | +2.23 km/h |
| 100 km/h | 104.45 km/h | +4.45 km/h |
UN ECE Regulation 39, used in the UK and EU, does not allow a speedometer to show less than the true speed, so many read slightly high from the factory. A taller tyre uses up some of that margin. Calibration varies by car; these figures are the tyre-size effect only.
What changes from 275/70 R18 to 325/65 R18
Wheels
Both fit 18-inch wheels. Our rim-width starting estimate is 7.5 to 8.5 inches for 275/70 R18 and 9 to 10 inches for 325/65 R18, so the two may not suit the same wheel width. The exact tyre's approved rim range takes priority.
Sidewall and ride comfort
325/65 R18 has 18.75 mm more sidewall (211.25 vs 192.5 mm). A taller sidewall usually rides noticeably softer and gives the rim more protection from potholes and kerbs, at the cost of some steering sharpness.
Contact width
325/65 R18 is 50 mm wider in nominal section width (18.2%). A wider tyre usually puts a wider, shorter contact patch on the road, which can help dry grip, but it may follow ruts more, clear standing water less readily and add some rolling resistance.
Wheel well clearance
Ride height changes by about +18.8 mm, and the top of the tyre moves the same distance closer to the arch. On the same wheel width and offset, each sidewall moves about 25 mm further out towards the arch lip and further in towards the suspension. Check clearance at full steering lock and full suspension compression, especially on a lowered car.
Gearing
The effective final drive changes by -4.26%: 3,000 rpm at a given road speed becomes about 2,870 rpm. Gearing gets noticeably longer, so cruising revs drop and acceleration in each gear softens.
ABS, stability control and AWD
These systems compare wheel speeds. Fit the same size on both ends of an axle; with a 4.5% diameter gap, never mix 275/70 R18 and 325/65 R18 on an AWD car, and follow the car maker's rolling-circumference limits, which can be much tighter than 3%.
General tendencies from the nominal numbers, not a test of any tyre. Load index, speed rating, tyre construction and the car's approved sizes matter more than these rules of thumb.
Explained in our guides
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- Staggered vs square wheels: tyres, rotation and AWDCompare square and staggered wheel setups, calculate front-to-rear tyre diameter, and understand rotation limits and all-wheel-drive requirements.
- How to read a tyre size, load index and sidewall markingsDecode tyre width, aspect ratio, rim diameter, load and speed markings, then calculate sidewall height and diameter using a 225/45 R17 example.
Cars in our guides using these sizes
Factory trims and popular community setups from our fitment guides. A popular setup is not a factory approval; each guide explains its trims, axles and clearance limits.
275/70 R18
- Chevrolet Silverado 1500 4th gen / T1 (2019–present)Factory trim
- Ford F-150 14th gen (2021–present)Factory and popular setup
- Nissan Patrol Y62 (2010–2026)Popular setup
- Ram 1500 DT / 5th gen (2019–present)Factory trim
325/65 R18
- Ram 1500 DT / 5th gen (2019–present)Factory trim
More comparisons
275/70 R18 compared
Questions
Can I use 325/65 R18 instead of 275/70 R18?
Both fit 18-inch wheels. The diameter difference is +4.45%, outside the ±3% screening limit, so most fitters would treat it as a different size class rather than a direct replacement. That does not approve fitment: check the car maker's approved sizes, load index, speed rating, rim width and clearance first.
How much bigger is 325/65 R18 than 275/70 R18?
Overall diameter is 37.5 mm (1.48 in) larger: 879.7 vs 842.2 mm. Section width is 50 mm wider, and the sidewall is 18.75 mm taller. These are nominal figures; real tyres vary by model and rim width.
Will 325/65 R18 change my speedometer reading?
Yes, by 4.45%. If the speedometer was accurate on 275/70 R18, an indicated 60 mph would be about 62.7 mph on 325/65 R18, and an indicated 100 km/h about 104.5 km/h. You would be going faster than the speedometer shows.
Which is better, 275/70 R18 or 325/65 R18?
Neither in general. 325/65 R18 has the taller sidewall, which usually rides more comfortably; 275/70 R18 usually steers more sharply. 325/65 R18 is wider, which can help dry grip; the narrower size is usually better in standing water and snow. The size listed for your car's trim, on its placard or in the handbook, is the safe default.
Check the exact tyre
Size codes give nominal dimensions. Before buying, confirm the size is approved for your car and check the exact tyre's load and speed ratings and published dimensions.