If your car is designed to run on 87, then running it on 93 will do nothing except cost you money.
If your car is designed to run on 91 which I believe is what the label on my X5 tells me, then put 91 in it. 87 will knock and probably result in lower overall mpgs.
Here is a Car and Driver article:
http://www.caranddriver.com/features/regular-or-premium
Here's a snippet: The turbocharged Saab's sophisticated Trionic engine-control system dialed the power back 9.8 percent on regular gas, and performance dropped 10.1 percent at the track. Burning regular in our BMW M3 diminished track performance by 6.6 percent, but neither the BMW nor the Saab suffered any drivability problems while burning regular unleaded fuel.
So you will be down on power too.
Frankly, I am driving a $75000 luxury SAV. Saving 20 x $0.30 = $6 per fill upto go slower and cause my engine to knock is not something I plan to do. But you go right ahead.