I agree the 200-4R will need better internals, being a factory non-turbo V6 application. Unfortunately, the donor motorhome is a 3-speed, likely a TH-475 that I don't believe will fit the Regal without cutting the tunnel. I do, however, think the 6.2 could achieve 30 or better in this car for three reasons. One, we have a member attesting to a 4.3 Olds diesel getting 43 highway. Two, in my experience, larger engines, to a point, will actually burn less fuel because they don't work as hard. Three, gearing is everything. My uncle claims 28 mpg with a 5000-lb 2500 series GMC that probably has 3.73 or lower gears. Maybe it's my "noob" status showing, but these three factors make me believe the 6.2 could very well top 30 mpg in a 3200lb mid-size sedan.
