With all this hype about muffler bearings I would like to point out that by far the best way to improve vehicle performance is a proper 4 corner Air-flush. Hardcore racers are well aware that stale air in your tires can decrease horsepower being transfered to the wheel by 20-25%. To check to see if stale air is robbing HP you need to remove the valve cap and press down to release some air, make sure that while releasing the air you need to be close enough to smell what is coming out. If the escaping air has an odor and/or does not smell like the air in your garage or where ever your vehicle is stored then you most certainly have stale air. To remedy this problem you must remove the valve core from each tire, after the old stale air has been purged form each tire immedietly re-install the valve core's and re-inflate with fresh air. To keep this problem from re-occuring proceed to you local auto parts store and purchase two items... A can of air-B-fresh additive for each tire and a new set pressure sensative self purging valve cap's. These cap's are a special order item and you will need to ask someone at the counter so they can order the specific model for your car/truck.