Yes the E63 is faster on the top end, but the X5 is faster to 60 and probably to 100, too. Where do you do most of your driving? You can put drag radials on an E63...you can put offroad tires on it, too. Just remember that drag radials have some downsides that will make it tougher to drive on the street...they are terrible in rain, and you can't drive them in cold weather. Of course you can afford a sports car...I didn't say you couldn't.

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2017 Nissan GTR Red/black - Tune/midpipe
2013 BMW X5 50i M-sport Alpine white/black - Dinan stage 1
2016 Jaguar F-Type R Gray/red