What does the circle d on the shift column mean?
Overdrive. The transmission contains a final section that causes the output to turn at a faster rate than the input. When this happens it is refered to as "overdriving". Very common in new vehicles as once at speed the overdrive kicks in and the rpms become lower to keep the car at the same speed, hence increasing gas mileage. Press lightly on the accelerator and you can feel the overdrive gear kick out and the car then begins to accelerate. The circle d refers to overdrive gear in your transmission , which is less than a 1:1 or direct drive gear ratio. This gearing lowers your engine rpm for fuel economy but effects pulling power of your vehicle.In a truck there is normally an overdrive disconnect switch on the end of the shift lever.