I also put an AT muffler in my Grand Cherokee (w/V8), and experienced an incredible average mileage increase of 5 mpg (from 13 to 18). A friend was so impressed that after making a trip together to Wyoming he put one in his van (not a mini, also w/V8) as soon has he got back.
Anyway, please be advised that this type of muffler is NOT recommended for heavy off-road use, since dents could interfere with the functioning of the turbine.
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