3.55 open, but with TC, not that big of a problem. If you are getting 20s, get the 3.92. The engine will turn just less than 200 more rpm and the mpg is the same really. On the highway, you might see .2-.3 more miles per gallon. In town or pulling a heavy trailer, you will get better mileage with the 3.92. I have the 3.55 because that is std on the sport model. I would much rather have the 3.92, just could not find one at the time and the demo I bought had the 3.55. I will put it to you this way. If you get the 3.55, you may have wished for the 3.92. If you get the 3.92, you will never wish that you had the 3.55. If you are getting the hemi with 20s, the hemi is smoother at 70 mph turning the slightly higher rpm of the 3.92.