We have made countless trips from Oklahoma City to Denver for a number of years in more vehicles than I care to admit I’ve owned. We leave OKC with 87 octane and Denver with 85 octane. And we almost always get better mileage returning to Oklahoma City. I’ve used 89 octane in the Taco a couple of times while towing with no noticeable difference. Pure gas might increase the mpg less than one mpg. A year ago several tanker trucks were mistakenly loaded at the OKC terminal with E85 ethanol and distributed to a number of local stations. The wife pulled the Honda CRV into the station with the low fuel light on and filled the tank with the E85 before the stations were notified. I noticed the mileage had decreased for the first half of the tank. I kept it full of 91 octane pure gas for a couple of weeks.