Quick Example
RAM's 2025 owner's manual lists 275/65R18 (32.07" calculated diameter) and 275/55R20 (31.91") for most 1500 trims, a difference of -0.5%. For the 2500 it lists LT275/70R18E (33.16") and LT285/60R20E (33.46"), +0.9% apart, and LT285/70R17D (32.71") on the 4x4 Tradesman and Power Wagon. RAM's manuals do not list lift or leveling-kit requirements for other sizes, so use the calculator for the arithmetic and confirm other sizes with a RAM dealer.
Calculated diameter: 31.91"
Diameter: 32.34"
Diameter Difference
+1.36%
275/55R20 285/55R20 (+0.43")
Speedo Error
+1.36%
Listed Revs/Mile
632
New Revs/Mile
624
Common Guideline
±3%
Get a plain-English explanation of your results and what they mean for your vehicle.
RAM Tire Guide
Tire sizes from RAM owner's manuals for the 1500, 2500 and 3500.
RAM Factory Size Comparisons
| Trim | Size A | Size B | Diff % |
|---|---|---|---|
| 1500 (2020–2025) | 275/65R18 (32.07") | 275/55R20 (31.91") | -0.5% |
| 2500 4x4 Tradesman (2025) | LT245/70R17E (30.50") | LT285/70R17D (32.71") | +7.2% |
| 2500 4x4 Power Wagon (2025) | LT285/70R17D (32.71") | LT275/70R18E (33.16") | +1.4% |
| 2500 Big Horn / Laramie / Limited (2025) | LT275/70R18E (33.16") | LT285/60R20E (33.46") | +0.9% |
Both sizes in each row are listed in RAM owner's manuals. Diameters are calculated: Rim × 25.4 + 2 × (Width × Aspect ÷ 100). Always verify against your tire placard.
How to Use This Calculator
- Select your RAM trim to load a tire size from the RAM owner's manuals.
- Enter the target tire size you're considering.
- Review the diameter difference, speedometer error and size-change rating. A taller tire also changes gearing, which the effective gear ratio calculator shows, and the load range letters on a truck tire are explained on their own page.