float s = 0;
    FILE *g;
    g = fopen("时空图.txt", "a");
    for (i = 0; i < car_num; i++)
    {
        temp_1 = p[i];
        speed = v[i][temp_1];
        gap = FindFront(i);
        if (speed + 1 <= vmax)
            speed++;
        if (speed > gap)
            speed = gap;
        if (speed > 0 && rand_double() <= pi)
            speed--;
        temp_2 = (temp_1 + speed) % lane;        //位置更新
        p[i] = temp_2;
        v[i][temp_2] = speed;
        a[i] = speed - v[i][temp_1];
        if (speed > 0)
            v[i][temp_1] = -1;
        if (a[i] > 0 || a[i] == 0)
        {
            tor[i] = (mg*(0.0076 + 0.000056*speed) + 0.5*0.3*1.2258*8.25*speed * speed + 1.1*mg / 10 * a[i])*r;  //扭矩
            pe[i] = tor[i] / (0.8*0.95*0.8)*(speed / r*0.8*0.95);
            pm[i] = pe[i] / (367.1 * 7);
        }
        //fprintf(g, "第1辆车的油耗：%2f\n", pm[0]);
        /*if (temp_2 == p[(i + 1) % lane])
        {
        printf("撞车了，位置为：%2d ,编号为;%2d", p[i], i);
        }*/
        s += pm[i];
    }
    fprintf(g, "耗油：%2f\n", s/time_count);


结果是文本文件g中什么也没有

先化简测试一下看看问题所在
#include <stdio.h>
main()
{
    FILE *g;
    g = fopen("时空图.txt", "a");
    fprintf(g, "耗油：%2f\n", 123.456);
    fclose(g);
}