Okay so I am also a conservative with money especially cause of the gas prices. Here in LA it's almost $4 a gallon for premium. I have tested both using 89 and 91. I do allot of in town driving so my MPG already sucks only averaging 11, but I do notice a huge difference when I use the 89. Yes I spend less for a full tank but I suffer for how much I get out of my tank. If I fill with 89 I average around 170-200 miles per tank but when I use the 91 I get closer to 250-270 depending on traffic conditions. Plus I have also found that when I only use Shell gas I get the higher of the 2. I have even gone as far as trying the 89 octane and adding a fuel additive that states it will increase mileage "which doesn't work". So in the long run I decided to only fill on Shell 91 cause they add cleaners to their fuel which also helps the life of your car. I figure I'd pay that little bit extra to prevent a major problem down the road. Just incase you do highway driving when I do use the 91 I get average 320 miles to the tank and on 89 around 279. Hope this helps.