On my way home from Reno NV to CA when going down hill if I was coasting around 75 to 80 MPH and had to slow down a little the car would shift into 5th gear (had the dash info on gear) it would not go back into 6th until I gave it a pretty good shot of gas, speeding up not just holding speed. I put it into S mode and manually up shifted to 6th and it would stay there. Engine braking on its own is ok I guess but having to gas it pretty hard to up shift seemed not the best for gas mileage.
Just my observation on my new car. Mileage for the ride home was 25.9 avg and 27.8 for the tank so far.

 

Mine does the same thing, especially down donner pass, if you keep coasting until you need to gas it will eventually shift up without giving it the lead foot. Also leaving it in fifth won't hurt the mpgs really as the Rpm difference is about 1000 depending on speed (I was just using 70mph as reference).

 

I just put it on neutral. (I believe it is illegal though).

 

Yes it is illegal, and it isn't very good for newer cars, especially rwd. Unless your driving a manual which wouldn't really apply here as manuals don't come equipped with paddle shifters.

 

just out of curiosity, why is it bad for RWD cars?

 

Fluid looses pressure in N. Planetaries and plates start to dry-up. That is not a good place to be.

 

What he said ^^^^^