Odd question about tires

Ok were on the right track here and let me explain why I'm concerned about the tires in the first place. I had MT Sportsman Pros on it in the same size and at 18lbs of air, they spun ever so slightly when I ran it a few years back. I switched to the ET Streets and they dont spin at all. With that being said the car 60 foots just as good spinning the old tires as it does hooking with the new ones, the 1/8 Mile ET and MPH have dropped with the new tires...about 1 tenth and almost 3 MPH the overal ET and MPH are relatively the same. I guess I'm getting at did I over tire my car? and will pumping them up till they spin a bit defeat the purpose anyways. Should I look at working this out in the carb or should I loosen up the converter?