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There are alot of variables to consider. Is the engine the same temp every run? Are the tires inflated the same every run? Are you refueling after every run so the car weighs the same? Are you monitoring the track temp as well as the air temp and humidity. Are you staging the same? The car will pick up with cooler air, but I know that my car slows badly with high humidity. The main thing is to be consistent in everything you can control.