a preferred embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings . as shown in fig1 the learning system according to the embodiment of this invention includes a vehicle driving - state detecting unit 2 for detecting a plurality of factors related to the vehicle driving - state , a transmission control unit ( referred to as “ tcu ” hereinafter ) 20 for controlling a speed - shift based on the factors detected by the vehicle driving - state detecting unit 2 , and an actuator unit 18 for performing the speed - shift under the control of the tcu 20 . tcu 20 may include a memory and cpu as generally known in the art such that it may be programmed to contain and execute instructions for controlling actuator unit 18 according to the present invention as described herein . the vehicle driving - state detecting unit 2 includes a throttle valve open - angle detector 4 , a vehicle speed detector 6 , a select lever position detector 8 , a hydraulic fluid temperature detector 10 , an engine - speed detector 12 , and pulse generators - a 14 and - b 16 . the throttle valve open - angle detector 4 detects an open - angle of a throttle valve , the select lever position detector 8 detects the position of a select lever such as neutral “ n ”, drive “ d ”, second “ 2 ”, and low “ l ” ranges . the pulse generators 14 and 16 respectively detect a turbine - speed of a turbine disposed in a torque converter of the automatic transmission , and a revolution speed of - an output - shaft of the automatic transmission . the actuator unit 18 , preferably disposed in the automatic transmission , includes a solenoid valve for controlling hydraulic pressure for a shift , and the tcu 20 controls shifting by sending a hydraulic duty signal to the solenoid valve . [ 0020 ] fig2 illustrates how the hydraulic duty is learned by the tcu 20 . when a down - shift occurs during an up - shift at step s 100 , the tcu 20 detects the revolution speed of the engine , or more preferably , the revolution speed of the transmission turbine at step s 110 . the tcu 20 then calculates an amount of overrun , produced during the down - shift , at the detected revolution speed at step s 120 . accordingly , the tcu 20 determines whether the amount of overrun is larger than a predetermined level , for example , 30 rpm , at step s 130 . when the amount of overrun is determined to be larger than 30 rpm at step s 130 , the tcu 20 calculates a correction value | α | of the hydraulic duty pr , and adds the correction value | α | to the current hydraulic duty pr at step s 140 . when the amount of overrun is determined not to be larger than 30 rpm at step s 130 , the tcu 20 calculates a correction value | α | of the hydraulic duty pr , and subtracts the correction value | α | from the current hydraulic duty pr at step s 150 . the hydraulic duty pr is a hydraulic duty for controlling hydraulic pressure supplied to an engaging element of the down - shift . if the correcting steps s 140 and s 150 are initially performed , current hydraulic duty pr will be the same as an originally set value . the hydraulic duty pr corrected at either step of s 140 and s 150 is used for a hydraulic duty for controlling a next down - shift . the correction value | α | depends on how much overrun has been detected at step s 110 , and fig3 shows the dependency . as shown in fig3 an exemplary correction rate for the correction value | α | increases as a difference between the amount of overrun and the predetermined level increases . as learning steps s 100 - s 150 are performed repeatedly , the amount of overrun will converge to the predetermined level of 30 rpm , because the hydraulic duty pr is repeatedly learned . the actual correction value and correction rate will vary based on size or capacity of the engine or transmission as determined by a person of ordinary skill in the art in view of the teachings of the present invention . preferably , the predetermined overrun speed ( 30 rpm in the example above ) can be set based on the measured output torque profile of the transmission . larger values , such as 50 or 100 rpm , may be selected , but factors such as a lower pressure to secure an improved shift feel must be balanced with the unacceptability of large overrun speeds . for these reasons , smaller predetermined overruns speeds are generally preferred ; however , a person skilled in the art may calibrate this value in the event that the transmission torque profile is unacceptable , i . e ., an undesirable level of shift shock occurs . for example , using the correction rate as illustrated in fig3 at an overrun of about 200 rpm , the correction value | α | is about 1 %. adjusting the duty control by + 1 % as in step s 140 may lead to a reduced overrun of about 80 rpm in a particularly sized engine . in this case , in a second iteration , a correction value of about 0 . 3 % would be added to approach convergence with the predetermined overrun value of 30 rpm . in the event that the overrun correction caused a reduction in overrun to under 30 rpm , for example to 20 rpm , then a value of about − 0 . 1 % would be added in step s 150 to approach convergence . a further example , with a different capacity transmission or engine is illustrated in fig4 . in this example , when the amount of overrun is initially about 200 rpm , hydraulic duty is corrected by + 2 . 5 % and resultantly the amount of overrun is reduced to 80 rpm in a second performance of a down - shift . during the second performance of the down - shift , the hydraulic duty is learned still more , by an additional 1 . 0 %, such that the hydraulic duty is corrected by a total of 3 . 5 %. resultantly , the amount of overrun is reduced to near the predetermined level , 30 rpm . even if no overrun is detected at step s 110 , the hydraulic duty pr is learned such that eventually the amount of overrun will converge to the predetermined level , which decreases the possibility of tie - up of the transmission , and reduces shift - shock . as shown above , this invention enables a more rapid shift and prevents a tie - up because a hydraulic duty for an engaging element in a down - shift is learned on the basis of an overrun , which enhances shift - feel and increases durability of an automatic transmission of a vehicle . while this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment , it is to be understood that the invention is not limited to the disclosed embodiments , but , on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims .