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Originally Posted by freakshow
So you're saying that there would really be no negative effects with regards to mileage or engine health for most modern stock cars to run 87 even if they require 91?
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The negative effects would be a loss in performance. The engine ECU would compensate for lower quality fuel so you're not going to blow up your engine or anything serious like that.
Keep in mind this is for more modern cars. It would be bad to run a low octane gas in an older car without some form of knock control system.
While companies always list the recommended fuel, some actually give ratings for their engine using the recommended and lower octane fuel (for example, the new Coyote engine in the Ford Mustang quotes power/torque outputs on 87 and 91 octane fuel).
You have to remember manufacturers have to allow for all sorts of scenarios from dumb drivers putting in the wrong fuel to people on a road trip at a station that doesn't sell fuel they need. Vehicles will run OK on lower octane fuel - they just perform better when running on the recommended fuel.