on my 72 road runner, 572 hemi TTI big tube headers hooker areo chamber mufflers 3" X pipe exhaust, i had a exhaust drone. i changed the 3" tail pipes to 2 1/2" and now it sounds perfect. i put the car on a chassis dyno to measure hp loss. i lost 2hp with the change, but it was also 10 degrees warmer when i dynoed it the second time, so in my opinion there was no loss of hp with smaller tail pipes.