I've tried 87 and 93 on my stock 18. No change in mpg. My driving is to/from work in medium traffic, highest speed is 60mph, probably average 40mph, no heavy acceleration. I get 30mpg with both in summer, 27mpg in winter, which makes sense because they add 10% water to the gas in winter. FYI, 93 is 60 cents a gallon more than 87 here.yea I’m not worried about getting great mpg, I just want to get decent mpg compared to what Ford claims I should be getting g. I may trial using 87 octane. I was running 93on the last tank.