They don't even sell 93 around here so all I can get is 91 cost me $4.48 per gal today. I don't drive her very often so the $.50 a gal more than the 87 stuff isn't as much of an issue for me as I would rather be safe than sorry. If I drove her every day with one fill of the tank per week that still is only $8 per week. I think I would still go for the 91. A few years ago I figured I was spending a similar amount of money on tires as gas (the roads here have a lot of curves)
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Ken