I have a 98 disco and ive been using 87 and probably should be using premium what effects has using 87 done to my car and what differences will occur when I go to 91?

 

91 burns slower at a high flash rateand is for high compression engines. Using lower octane gas, depending on the compression of the engine(newbie to Discos so I don't know the spec's yet) could be minimal or you could be scorching everything from your block, pistons, valves, exhaust ports on your heads, exhaust manifold and O2 sensor not to mention filling your cat with soot faster. Again, I am moreaccustom to Corvettes and BMWs as they were my past cars but that was the case with them and most likely the long term result for Discos.

My 2c

 

Same here regarding BMWs, but you are spot on. Underperformance is the least of your worries if you consistently use an octane rating less than recommended. Once or twice if you can't find 91+ is fine, but not all the time. It could cost you considerable sums later on.