问：汽车失去了汽油行驶里程，没有太多动力

我的汽车一直在不断失去汽油里程。它开始获得20 mpg，现在只能获得18 mpg。每周都会稳步恶化。引擎没有错过。我尝试使用两周前使用一罐喷油器清洁剂，但这无济于事。我的房子距离镇上15英里，我开始沿着道路驶回，以便我可以慢慢开车使汽油持续更长的时间。但是该计划也没有工作。另一个问题是我的汽车没有太大的力量。自从给我以来，今年夏天一直呆滞。也许我应该切断催化转换器，并在没有转换器的情况下将其直接运行到排气管上。 Yet, another problem is that the brakes are making noises and they also bind. The transmission shifts in a strange way and 1st gear feels like it is slipping a little. The 2nd gear is quick, hard, and jerks (sometimes it jerks me forward). I recently had a problem with a dead battery and yesterday I found that the cables were loose, so I tightened them. I am hoping to do the repairs myself and would you please tell me on which area I should start?