I have an 86 with the auto tranny. It seems like when i accelerate in D, the tranny shifts differently. It seems like it is slipping more and holding the gears longer. I never use it because i think it lowers my MPG.
Does anyone know what, if any, differences exists between the two settings? And which is better for daily commuting, and which is better for launches, i.e. dragstrip?

 

D, 3rd gear, is 1-1 drive direct drive, this is what 3 or D on the old TH350 and TH400's is. OD ie overdrive is a .7 overdriven ratio that makes your car more efficient at highway speeds and ultimately faster on top end, it allows for lower RPM at a given speed. Too steep of an overdrive actually can take more HP to turn. Aroud town, stop light to stop light, I put my car in D. This prevents it hunting up and down in the 40 mph range in traffic. If I'm going around 50 or faster then I used OD. Line pressures, trans temp, et..., can all effect shift feel. Lee Willis here is probably much more capable than I at explaining.

 

I understand that D disables the overdrive gear. I overheard some guys talking about a run at the drag strip, and he asked him if he took off in D or OD (he also had a c4). So thats why i ask if the two have different shifting characteristics.

 

you are going to drag in D .....OD is for highway it will save your gas milage. as for the slipping could be some problems inside your tranny.[]

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Your 86 has a TH700R transmission in it. Have you tried adjusting your detent cable? I have a post in which I describe how to adjust the cable. On the side of the cable where it snaps in the bracket behind your throttle lever, there should be a metal half moon piece inside the plastic housing of the cable. If you can, depress the metal and take the end of the cable housing and push it in all the way til it stops, then release the metal. Now, sit in the car and depress the accelerator all the way to the floor, (you will hear a crunching noise from the cable, but pay it no mind) then the cable will be adjusted properly. This might help, but there really shouldn't be a difference in the way your transmission shifts unless you have valvew body problems.