I have had my 08 SLT for a few days now. This is my first diesel and so far I am very happy with it. So there's this old man I know with a 07 dura, and he was telling me that the 6th gear was only usable in manual mode. I assured him that was not true. I decided to do a test to see why he would have thought this. I was going 50mph and put it into manual and it showed M5. So I put it into 6th and within about 7 miles my average mpg went up 1.2 mpg. So I'm thinking this is cool, but I don't want to have to manual shift all the time. turns out if you use manual mode and just bump it to 6th, you can drive normally. Runs just like it's in drive, but will go into the 6th gear at lower speeds. Even went over a bridge and the rpm's never moved. Just thought I would share this. Maybe most know this already. Hope it helps others improve city/slower driving mpg.