The JRC systems have the advantage of joint waveform signals for both radar sensing and vehicular communication functionality on the same device. These systems allow the management of the dedicated radio resources more efficiently and reduce the hardware cost. To improve the performance and to guarantee the permanent availability of the dual functionality offered by this system, we propose to use one of the promising approaches in wireless vehicular networks is the adaptive modulation (AM) techniques for OFDM transmission. This paper proposed an optimal approach based on the predicted instantaneous SNR of the overall subcarriers to choose the modes to be used by the transmitter for its next OFDM frame. AM-OFDM transmission parameter adaptation is an action of the transmitter in response to time and frequency-varying channel conditions. We evaluate the performance of the joint OFDM scheme in terms of outage probability (OP) for the communications functionality and the Ambiguity Function (AF) for radar sensing. The obtained results based on MatLab simulations show that significant improvements in bit error rate (BER) and sensing detection can be accomplished, illustrating the superiority of the adaptive modulation schemes compared to fixed transmission schemes within the automotive scenario