Not true. Perf. shift on a stock PCM only raises the shift points while WOT. But increases line pressure in the tranny for firmer faster shifts (less wear on tranny clutches by the way) If you drive normal you will not notice any loss of MPG. If that was the case GM and there efforts to keep their CAFE numbers low would have left it driver optional in the later models not permanently program it in. Six months after I bought my car (50 someK on the ticker)I drove it across country Maine to California left the perf shift on all the time and averaged 32-34 mpg. With a high of 36.

So what's so special about it? On the earlier models it raised line pressure in the tranny, raised shift points slightly (under WOT)and various other sundry items. But mostly like Ord said it's fun to have an extra button. A buddy of mine drove my car when I had to pick up my motorcycle he called it the james bond car cause all the buttons and gadgets.

Some aftermarket PCMs need the perfshift to be engaged to take advantage of the tranny tuning upgrades. Hense the write up that started this thread