Whats the difference between the D and the D with a circle around it in a Cadillac?

The D WITHOUT the circle around it is your 3rd gear, the D WITH the circle around it is your OVERDRIVE gear. The overdrive allows you to get better gas mileage and causes less wear on the engine. But there are times when you should NOT use overdrive, like if you are pulling a trailer up a steady hill, or some snow and icey conditions. If you want to notice the difference, put the car in D WITHOUT the cirlce around it, start out and drive at 50......then as you are going along change it to the D WITH the circle around it and you will feel and hear the difference. The engine revs will come down. It's also the same if you are in Overdrive and you press the throttle completely to the floor, your car will go into a lower gear , the D without the circle, to give you added power and then when you let off the throttle, then transmission will once again shift into Overdrive and the revs will come down and this happens without you changing the gear selector at all. D = Drive: standard drive for transmition engaged, allows for faster transition for a kick-down/passing gear... (D)= Over-Drive is for freeway travel lets the transmition stay in a higher gear for fuel economy (less RPM) the D with out the circle is normal drive wich cause more engine torque. the D with the circle is drive with overdrive wich gives less torque because it allows the transmission to shift to another higher gear.