Rim diameter does nothing to determine the proper rear gear. It is the overall tire diameter. And for that, the differnce is negligible as seen in the diameters I got of the Tire Rack website for the stock 18s and 20s...
235/55 R 18s....28.4 inches or 731 revolutions per mile
245/45 R 20s....28.7 inches or 729 revs per mile
Not enough of a difference to require a gear change from 3.73 to 3.92.
The lower (numerically higher) 3.92:1 gears are for performance only (and just maybe to move the extra heft of the heavier wheels).
But traction is a limiting factor when using the lower 3.92s. The wheels will accelerate faster which could lead to more spin and slower times if the driver is not careful.
ORIGINAL: 10secDodge
The different gears in the 6 speed are due to the tire diameter. As far as the acceleration between the two, it will be almost identical. I have ran 3.91, 4.10, and 4.30 gears in my race car. The only thing that changes is the trap speed (slightly) and the RPM's. The ET is virtually the same.
Lastly... I would not pay any attention to the road tests you read in magazines. There are many people that are getting better times out of their stock cars than the rags. You best bet is to test drive one of each.