I'd think the fuel economy would only get hurt if there was spark retard from knock..therefore decreasing your timing..and when it decreases it, it decreases much more than is needed..thus robbing you of power and fuel economy. I'd think 87 could be ran if lots of timing were to be removed across the board...which in turn would effect your power, and if not operating at peak efficiency of the motor, then fuel economy would be hurt agin.... So the bottom line is use premium fuel and save lots of money that would be otherwise wasted for a rebuild from running 87 and getting knock all the time. ...and just because you can't hear it knock doesn't mean its not knocking...if you are hearing it ping, then its upwards around 10+ degrees being pulled and all it takes is driving around several thousand miles with only a few degrees to totally kill the rod/main bearings...

Lower the octane, the hotter it burns and thats one of the key factors in getting knock as the cylinder gets so much heat that the fuel/air mixture ignites upon the compression stroke before the spark plug actually ignites it..thus all that pressure from the pre-ignition is pushing the piston back down while its still being pushed up...imagine how thats hammering the bearings..