(57) Acceleration modulatorfor a hydraulic driving system of an automotive vehicle comprising an accelerator pedal, an engine whose output varies with the amount of operation of the accelerator pedal as transmitted to the fuel control terminal of the engine, a hydraulic pump driven by the engine, a hydraulic motor for driving the wheels of the vehicle, and a hydraulic circuit for transmitting the hydraulic pressure output from the hydraulic pump to the hydraulic motor; the acceleration modulator comprising: a wire for transmitting the operation of the accelerator pedal to the fuel control terminal of the engine of the driving system, the wire comprising a first portion and a second portion; a differential gear mechanism having a first input terminal connected to the first wire portion, a second input terminal, and an output terminal connected to the second wire portion, the differential gear mechanism being so arranged that a movement of the first wire portion transmitted to the first input terminal is modulated differentially by a displacement of the second input terminal so that the modulated movement is taken out at the output terminal; and a fixing mechanism for variably fixing the position of the second input terminal of the differential gear mechanism, the fixing mechanism being so arranged that when the hydraulic pressure in the hydraulic circuit of the driving system rises to a preset level slightly lower than the relief pressure of the hydraulic circuit, the second input terminal of the differential gear mechanism is moved to a posi-- tion where the amount of output from the differential gear mechanism decreases.
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