3 Inch or 2.5 Inch TTI tailpipes ??

I had a 71 Challenger mild 440 stock heads 284-484 hyd cam holley street dominator 750 vac carb auto 3500 stall 3.91 gears 9x28 slicks. It had crappy 3 inch to flowmasters then 2 1/2 tail pipes. It ran low 12s 12.20-12.30. Running tail pipes or removing made no difference I even tried 3 inch tail pipes no change. open header with 9 inch collector ext, ran 11.90. 2 1/2 is perfect also it just gets louder with 3 inch.