Old man at the Benz dealership says he’s been driving the same car for a long time and usually only puts in 87 octane in sprinkled with 91 every few times. What problems could this cause? Do I have to put in 91?
Disclaimer: I’ve owned (paid off) the car for 6 years and do put in 91, but trying to cut costs.
If the car is designed for/recommends 91 octane and is not modified, use 91 octane. Presuming this is a relatively modern car:
Using a higher octane than recommended is wasteful and you won’t see any benefit as the car can’t “take advantage” of the higher fuel grade.
Using a lower octane can cause poor/reduced performance, reduced fuel mileage, and increased deposits inside the engine due to an incomplete/in burnt fuel mixture as the vehicles computer will likely sense the lower grade fuel (through knock sensors) and adjust the ignition timing to prevent major engine issues.
I’ve also personally tested the savings on a few vehicles I’ve owned that require 91. Switching to a lower grade fuel only results in a few dollars savings per tank, but also got notably reduced MPG, negating any savings. In a 20 gallon tank, you’re usually only saving $6-$10, depending on the cost difference between 87-91.