The most common reason why a car runs poorly is because of bad tires. Some people make the mistake of thinking that if a tire shows no visable damage then it must still be good. That is not the case, tires only work their best for "x" amount of runs, then lose some performance.
Are you using traction compounds, are you using the right body style, Mazda 6 works great for outside rubber tire, have you compaired your setup with other racers?
The more info you give, the better help we can be.
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