I only use D5.

D5 - Use this position for
normal driving. The transmission
automatically selects a suitable gear
for your speed and acceleration. You
may notice the transmission shifting
up at higher speeds when the engine
is cold. This helps the engine warm
up faster.

D4, D3 - These positions are similar
to D5 , except when you
select the D4 position, only the first
four gears are selected. When you
select D3 , only the first three gears
are selected. D4 can also keep the
transmission from cycling between
fourth and fifth gears in stop-and-go
driving, and D3 can keep the
transmission from cycling between
third and fourth gears.


Use D3 to provide engine braking
when going down a steep hill. D
gives you more power and increased
engine braking.

 

I use D5 on the highway, and use D4 for in-town (stop and go) driving.

 

I'm with Midnight on this one. For 99% of my driving, I use D5 solely which I consider my default. When I'm in the mountains (I have a wicked hill near a vacation home) I use D3. For long downhills I'll often use D4 to keep speed in check without having to brake. I emphasise downhill only, as I leave it in D5 for ascending hills because the transmission will automatically shift to a lower gear when needed.

When I feel the need to shift, I jump into the RSX!