Depends on the model year. I found the following info on a diesel forum, which I can't vouch for the accuracy of, but sounds correct:
"03/04 are leaf spring front ends, so they used a wider offset to push the tires away from the springs. 05+ are coil springs and can use less of an offset. So 03/04 wheels fit all years, and 05+ stockers you may find rubbing issues when used on 03/04 trucks."
Since I believe those model years both had 18" rims stock or available as an option, I'd find out the specs of the OEM 18" rims, which should give you the 18" sizing, and then adjust for 20" rims, by comparing width, backspacing, offset against the OEM 18" rims for that model year. Part also depends on preference & whether you want (and have room for) wheels that tuck, wheels that are flush with the fenders, or wheels that stick out a little.
"03/04 are leaf spring front ends, so they used a wider offset to push the tires away from the springs. 05+ are coil springs and can use less of an offset. So 03/04 wheels fit all years, and 05+ stockers you may find rubbing issues when used on 03/04 trucks."
Since I believe those model years both had 18" rims stock or available as an option, I'd find out the specs of the OEM 18" rims, which should give you the 18" sizing, and then adjust for 20" rims, by comparing width, backspacing, offset against the OEM 18" rims for that model year. Part also depends on preference & whether you want (and have room for) wheels that tuck, wheels that are flush with the fenders, or wheels that stick out a little.
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