fig2 shows one embodiment of the present invention . a voltage at the neutral point of an ac generator is applied to an input terminal 1 . the ac generator may be mounted on a diesel engine of a truck and driven by the engine . a pulse - voltage converter 1a converts ac pulse signals at the input terminal 1 to a voltage level signal . a characteristic converter 2 converts an input voltage v 1 , which has output from the pulse - voltage converter 1a , to an output voltage v 2 as shown in fig3 . a voltage signal proportional to a travelling amount of an accelerator pedal with the diesel engine is applied to a further input terminal 3 . leads 2b and 3a are respectively connected to first and second inputs of a comparator 4 . an output lead 4a of the comparator 4 is connected to a green lamp 5 through an amplifier 5a and is also connected to a red lamp 6 through an inverter 6a and an amplifier 6b . the operation of the construction as described above will now be described . the ac generator of the engine outputs a plurality of pulse signals proportional to the engine speed , which are applied to the input terminal 1 , and the pulse signals are converted to a voltage level proportional to the number thereof . thus , the voltage level on the lead 2a corresponds to the engine speed . the characteristic converter 2 produces the relationship between the input voltage v 1 on the lead 2a and the output voltage v 2 on the lead 2b , as shown in fig2 . the converted voltage value is compared with a voltage value corresponding to an accelerator pedal travelling amount , which is inputted through the lead 3a , by the comparator 4 which outputs a voltage signal on a lead 4a when the voltage value on the lead 2b becomes larger than that on the lead 3a . in the operation of the comparator 4 , the output voltage v 2 presents a constant value to the extent of which the input voltage v 1 corresponds to a low engine speed characteristic a 2 as shown in fig3 . in this characteristic a 2 , when the voltage on the lead 3a corresponding to an accelerator pedal travelling amount is smaller than the output voltage v 2 corresponding to the characteristic a 2 , the comparator 4 generates a voltage at the output thereof . this state presents that , in fig1 the operating points of the engine exist in the area below the border line a . similarly , a characteristic b 2 in fig3 corresponds to the border line b in fig1 and , in the extent of b 2 , the comparator 4 outputs a voltage signal on the line 4a when the voltage value on the lead 2a in fig2 becomes larger than that on the lead 3a . this state represents that , in fig1 the operating points are below the border line b . also , a characteristic c 2 in fig3 corresponds to the border line c in fig1 and this shows that the voltage on the lead 2b always becomes a zero value when the engine speed n exceeds no . accordingly , while the accelerator pedal is pushed down at the engine speed n exceeding no , the voltage value on the lead 3a is above zero on the lead 2b and , therefore , there is no voltage signal at the output of the comparator 4 . according to the operation above , when the voltage signal is outputted to the lead 4a , the signal is amplified by the amplifier 5a and then the amplified signal illuminates the lamp 5 . however , the lamp 6 is not energized because of the inverter 6a which is inserted between the lamp 6 and the comparator 4 . alternatively , when the voltage does not appear on the lead 4a , the lamp 5 is not energized while the signal is applied to the amplifier 6b because of the inverter 6a and then the amplified signal illuminates the lamp 6 . in this way , the fuel economy running indicator as shown in fig2 is constructed such that the green lamp 5 is illuminated when the operating points of the engine are within the scope of an economic fuel consumption state defined by the border lines a , b and c in fig1 and the red lamp 6 is illuminated when the operating points of the engine exist in the outside of the border lines a , b , and c . the vehicle will be operated by using the fuel economy running indicator in fig2 in the following manner . as described above , in general , the driver of an automobile must continuously accelerate and decelerate , while watching obstacles in the forward direction . illustrating the decelerating through accelerating states in relation to fig1 the operating point of the engine is a point h on the running resistance characteristic i when the vehicle speed is in a normal state after deceleration . if the accelerator pedal is further pushed down at the normal state , the engine is accelerated and , therefore , the vehicle is also accelerated . when an accelerator pedal travelling amount reaches the border line a , the red lamp 6 illuminates to warn the operator , as described in relation to fig2 . if the operator releases slightly the accelerator pedal in obedience to the warning indication , the red lamp 6 turns off and the green lamp 5 turns on . subsequently , if this pedal position is maintained as it is , the engine speed n increases gradually along the border line a and then exceeds the border line b to thereby turn on the red lamp 6 . thus , the operator again releases the accelerator pedal by degrees until the red lamp 6 turns off . this state is continued until the operating point of the engine follows the course from an arrow d 1 to d and reaches a normal operating state point j on the running resistance characteristic i . in the illustration above , since the weight of both the lead and the body itself of the truck is relatively heavy , the arrows d 1 and d change greatly in comparison with these of an automobile for riding . the operator , therefore , may have enough scope to control the travelling of the accelerator pedal , while paying attention to the indication of the lamps 5 and 6 . this , of course , can be applied not only to a truck but also to a vehicle , such as a bus , used for carrying a number of passengers , and may also be applied to a general automobile for riding . in contrast to the accelerating operation as described above , the running resistance characteristic in the running of climbing of the vehicle is upwardly shifted in fig1 . the acceleration at climbing requires a change gear having a relatively low gear ratio to the over - top gear ratio as noted above and , in response to the selection of such a gear ratio , the operating scope is changed . however , since any operating point is indicated by the illumination of either the lamp 5 or 6 , it is possible that the operator selects an efficient gear ratio as desired . the characteristic converter 2 in fig2 converts the input voltage v 1 corresponding to the engine speed to the output voltage v 2 , as shown in fig3 . the comparator 4 compares the output voltage v 2 with the voltage value corresponding to a travelling amount of the accelerator pedal . in this operation , the characteristic of the border lines a , b and c in fig1 is presented as a function of the engine speed n and the presented reference output voltage is compared with a function of the accelerator pedal travelling amount . however , it is apparent that the border lines a , b and c may be presented as a function of the travelling amount and the presented reference output voltage is compared with a function of the true engine speed n . in other words , if the characteristic of fig3 is presented by the true engine speed ( corresponding to v 1 ) and the accelerator pedal travelling amount ( corresponding to v 2 ), the relationship , θi = f ( n ), presents a characteristic , as shown in fig4 which is analogous to that of fig3 . the characteristic of fig4 means that the characteristic of the border lines a , b and c in fig1 is presented as a function of both the accelerator pedal travelling amount θi and the true engine speed n , and furthermore the travelling amount θi is presented as a function , f ( n ), of the engine speed n . alternatively , this means that an engine speed ni ( reference value ) on the border lines a , b and c in fig1 may be presented as a function , f ( θ ), of the accelerator pedal travelling amount , the characteristic being such as that shown in fig5 as understood from fig1 and the border lines a 1 , b 1 and c 1 in fig4 and the lines a 2 , b 2 and c 2 in fig5 respectively correspond to the lines a , b and c . in this way , regarding the characteristic conversion as shown in fig5 one detected signal , θ , is inputted to the terminal 1 , in fig2 and another detected signal , the engine speed n , is inputted to the terminal 3 . the characteristic converter 2 has the characteristic as shown in fig5 that is , the input θ - output ni characteristic . explaining the operation of this case , a reference output on the lead 2b is a high constant output voltage corresponding to the characteristic c 2 , as shown in fig5 which shows the scope representing a small amount of travelling of the accelerator pedal . when a detected signal at one terminal of the comparator , representing the engine speed , is larger than the signal at the other input of the comparator 4 , representing the amount of travelling the red lamp 6 turns on and this indicates that the engine speed n in fig1 is larger than no . in the scope in which the travelling amount corresponds to the characteristic b 2 , the output signal on the lead 2b represents a reduced voltage value as the travelling amount θ increases and , when the output voltage is higher than a voltage value on the lead 3a , the red lamp 6 illuminates . when an amount of travelling the accelerator pedal , θ , exceeds the characteristic a 2 , the output voltage on the lead 2b , corresponding to a reference engine speed ni , always becomes a zero value and , thus , the voltage value on the lead 3a is larger than the value on the lead 2b to thereby turn on the red lamp . consequently , it is apparent that the comparator 4 may compare an engine speed and an amount of travelling the accelerator pedal , one of which has been characteristic - converted . as understood from the explanation above , the fuel economy running indicator of the present invention functions to illuminate a lamp for warning an operator when an engine operating state intends to go out of the border lines a , b and c in fig1 . in obedience to such warning indication , the driver can continuously control the engine such that the engine is operated within the area defined by the lines and , thus , the vehicle is permitted to repeat the accelerating operation , approaching the minimum fuel consumption rate as closely as possible . accordingly , it is possible to reduce the fuel consumption within the limits of possibility , effecting the high - speed running required of an automobile and the repetition of the accelerating and decelerating operations . further , since it is easy to change an amount of travelling of the accelerator pedal by merely judging the illumination of one of the green lamp 5 and the red lamp 6 , the driver is not required to have any new operating techniques in addition to a conventional techniques . still further , since the present invention is constructed by a very simple circuit such that the output signal of the characteristic converter 2 is compared with a detected signal other than the detected signal input to the converter 2 , the unit cost of production is reduced . additionally , according to the fuel economy running indicator of the present invention , it is possible to detect the engine speed at the neutral point of the a . c . generator without using an expensive device , such as an electromagnetic pickup . therefore , the mounting cost of the overall system is economical .