A change in octane rating???

[Compaq888]

I have for the longest time refueled my car with 91. When the prices were $3+ I switched to 89. So far I'm refueling with 89. My car is designed to work on 87 octane but i'm scared my engine won't work as good in performance and the fuel economy will go bad. What do you think I should do?

87 is regular right??

Here are my car specs... http://www.nissannews.com/site_library/nissan/1999vehicles/altima/specs.shtml

Also I heard altitude has something to do with the octane too. I'm at 0 altitude.

And the cylender compression ratio is suppose to be 178 for all cylenders while mine are all 190. Having higher cylender compression builds up artificial compression requiring higher octane fuel.