[leftfield6]

According to various sources (google is your friend), it's less than 1/3 gallon in almost all cases. This is regulated state by state, so check with your state's Dept of Weights and Measures. They are the people who inspect and certify gas station equipment to make sure the consumer is getting what they pay for.

If you are filling up your tank, it has around a 17 gallons of capacity. 1/3 gallon of the wrong octane rating is going to be so diluted into the right stuff as to be a complete non-issue. It would make up less than 5% of your full tank of gas.

Even if you are only putting in 3 gallons (and why the hell does anyone do that?) remember its still going to be diluted into the same approximately 17 gallons as well. Assuming that what you had in the tank already was the right stuff.

If you are cruising around on "E" then buying 3 gallons, then back to E, then 3 gallons, and on and on, then yes, you are going to have a higher percentage of "bad gas". But if this is what you're doing, you don't need a Boxster, you need a Honda Civic.

really, the only people who should even care about this are motorcycle riders who have a motorcycle that needs 93. My bike does, and I have a 3.8 gallon tank, so that 1/3 gallon is a bigger deal to me. But honestly, I've never had a fuel related issue with the bike, so I'm not worried about it at all.