So you are saying you run your tank dry on 87 octane gas M-F, then fill up with 91 octane for the weekend...run it dry, and then fill it again with 87 for the work week? Sounds like a PITA to me. Seriously, it costs like $6 more to put 93 in your tank. Plus your truck will run better, get better gas mileage, motor will run smoother, etc. Switching back and forth is too much trouble. Get a 93 octane custom tune and be done with it!