My VW Passat requires 91 oct or higher. I can put in lesser gas, but the engine computer will adjust the timing to prevent engine knock. The overall performance and fuel economy would go down. So, it's false economy to put in lesser gas.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member