I can't take credit, this was posted by someone on here several, several years ago, quite extensive description of fitting tires:
First, some quick facts:
Factory 20x8.0 Wheel Offset = +19.05 mm - Back Space = 5.25" Inches
Lug Pattern = 5 x 139.7
* Fourth gen 20" rims can be either 8" or 9" wide. To the best of my knowledge the chrome clads are 9" and the painted rims are 8".
* Here is a good site to visit if you don't know for certain how wide your rims are. Simply pick your model year and match the photo.
*275/60/20 is a 33" tall, 10.8" wide tire.
* As stated above, stock is a 33" tire. All third and fourth gen trucks can run a 34" tall tire stock and a 35" tire with a proper level. This is height only...for info on width, see below.
* 17" diameter rims are the smallest that will clear the brakes on third and fourth gen trucks!
Sizing, fitment, and other info by model year:
* The general rule is that 285 is the widest that will fit on stock rims and without spacers.
* Exception for fourth gens with 8" wide stock rims: There is an exception to the above general rule for fourth gen trucks with the stock 8" wide rims. The narrower rim pulls the sidewall in more than the wider rim and allows fitment of a wider tire. Members with the 8" rims have confirmed no rubbing with 295/60/20 and 295/65/20.
Fitment and other info by tire size:
All of these are compared to stock 275/60/20 (33x10.8x20).
* 275/55/20 (32x10.8x20) = Same width as stock, 1" shorter than stock, great selection, slightly cheaper or same price as stock.
* 275/65/20 (34x10.8x20) = Same width as stock, 1" taller than stock, large selection and not overly expensive compared to stock.
* 285/50/20 (31.2x11.2x20) = 0.4" wider than stock, 1.8" shorter than stock, great selection, slightly cheaper or same price as stock, used a lot on lowered trucks with stock rims.
* 285/55/20 (32.3x11.2x20) = 0.4" wider than stock, 0.7" shorter than stock, somewhat limited selection, fairly expensive.
* 285/60/20 (33.5x11.2x20) = 0.4" wider than stock, 0.5" taller than stock, very limited selection and expensive.
* 285/65/20 (34.6x11.2x20) = 0.4" wider than stock, 1.6" taller than stock, very limited selection and expensive.
* 295/60/20 (34x11.6x20) = 0.8" wider than stock, 1" taller than stock, limited to only the Nitto Terra Grappler all-terrain that I am aware of, expensive but a good blend of wider and taller.
* 295/65/20 (35x11.6x20) = 0.8" wider than stock, 2" taller than stock, limited to only the Nitto Trail Grappler mud-terrain that I am aware of, expensive.
* 305/50/20 (32x12.00x20) = 1.2" wider than stock, 1" shorter than stock, pretty good selection, priced comparable to stock size.
* 305/55/20 (33.2x12.00x20) = 1.2" wider than stock, basically same height as stock, great selection, somewhat expensive.
Or if you want to convert it yourself:
1. take the first number (the width in millimeters) and divide by 25.4, this give you the width in inches
2. multiply be the second number (aspect ratio) divide by 100 (percentage), this gives you the sidewall height
3. multiply by 2, the add the 3rd number (wheel diameter)
Example: 275/60 R20 (factory 20" Ram tires)
1. 275/25.4= 10.82677 inches
2. 10.82677*60/100 or 10.82677/.6 = 6.496 inches
3. 6.496*2 = 12.992 + 20 = 32.992 = 33" tire height.
Thanks.
