Point out to your daughter (if the wife won't listen) the difference in price between premium and regular gasoline. Also show her the difference in maintenance costs. Give her an allowance and tell her that she needs to pay for her own gas.
I've owned several Mercedes over the years, and I enjoyed them, but there is a reason people trade them in at around 88,000 miles. Maintenance starts to get VERY expensive. Even with a new car, I never took my car in for service where the bill was less than $1,000. It was a running joke -- what does x part on a Mercedes cost? The answer was always $1,000 (at least). A well-maintained Mercedes will last forever, but if you look at the Mercedes maintenance schedule, you'll see that's because you've basically replaced every moving part on the car.
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