D4 advantage?

[cbird]

My 2005 Thunderbird has the D4 and D5 options on the gear selector. I always use D5 and don't understand why the D4 option is even available. I think I may be missing something, so please educate me - is there ever any reason to have the transmission in D4 rather than D5 and, if so, why?

[George]

cbird:
I believe your year has a thing called "select shift". Personally, I don't know what that is, but your manual should help you if you have one.

[cursorofold]

There are good reasons for limiting an automatic transmissions to specific gears. The two most useful features are disabling overdrive for towing applications, and forcing it into 2nd gear on takeoff when roads are slippery. The T-Bird's first gear is really easy to spin on slick roads. This is one of the more useful features of select shift.

Towing is the most common example where D4 would be useful. Frequent shifts can wear out the transmission. But the T-Bird is not a towing vehicle. It has a strong motor and doesn't have a frame capable of pulling the kind of weight where D4 would be useful.

High Performance may be another example. The car tops out in 4th gear. There may be some small advantage in not allowing the car into 5th gear on a track.

What I would like to hear someone tell me is that the computer tunes the transmission shift points differently in D4 than in D5. I have tried it and I don't notice a difference.

A mechanic once told me the transmission on the T-Bird is mostly the same as the Ford Explorer (or the Ranger). The Explorer would have need of a towing setting (D4). So it is likely the T-Bird has a feature it doesn't really need.

[yrbamje]

The following is copied out of the user's manual. Enjoy
Jim
Drive 5 (Overdrive)
The normal driving position for the best fuel economy. Transmission
operates in gears one through five.
Drive 4 (Overdrive cancelled)
Activated when the transmission shift lever is moved to the D4 position.
• This position allows for all forward gears 1–4, except overdrive.
• Provides engine braking.
• Use when driving conditions cause excessive shifting from O/D to
other gears. Examples: city traffic, hilly terrain, heavy loads and when
engine braking is required.
• To return to D5 (overdrive mode), move the transmission shift lever
into the D5 position.
• Select D4 at higher speeds will cause the transmission to downshift
into fourth gear.