6th Gen Maxima SL...
When I run '87' octane gas I get 28mpg on the highway.
When I run '91' octane gas I only get 24mpg on the same highway.

I thought '91' Premium is supposed to be better gas to use????

I would think that the '91' would give me better mpg.....

 

That is weird mainly because when I put 91 in my 02 infiniti i35 (5th gen maxima) my car runs lower rpm and gets 30 highway otherwise if I put in 87 get 26 highway

 

I'd say something else was going on if you saw that big a difference. If it's all highway my car does the same with 87 as it does with 93. In my normal mix of city/highway though the 93 gets me a little better mileage (1-2mpg). Also, there's no way a different octane rating can make your car run lower rpms (unless possibly it's a cvt transmission, which isn't the case in an 02).