Joined: Jul 2002
Posts: 40,863
Likes: 3,115
From: Sacramento, California, USA
Bikes: Specialized Tarmac, Canyon Exceed, Specialized Transition, Ellsworth Roots, Ridley Excalibur
39 x 27 = 38.0 gear inches
38 x 25 = 39.9 gear inches
Since you'll eventually wear out and need to replace your cassette (and you'll rarely need to replace a chainwheel), it makes more sense to get a new cassette with a bigger cog than to get a newer, smaller chainwheel.