A friend of mine just got a 2001 Stratus R/T. The manual says to put in Premium. But he doesn't want to splurge the extra 10 cents a gallon and he puts in mid, or even worse, regular octane (87/89).

I tried to explain to him why he should put in premium, but I'm not positive. What I could think of, is that his car is (probably) somewhat high compression, I'm guessing around 10:1. Maybe more, it's a Mitsubishi engine.. and that higher compression engines are more prone to detonate and therefor he should put in a higher octane because it is less prone to detonating...

Couldn't really think of any other reasons though.

 

Simple. Because the owners manual said so. If a car is design to run on 87, then we can debate on the need/benefit of running 91or 89. But if a car is design to run on 91, then there is no debate, and only use to lower grade when you have no choice like you are stuck in no-where-land.

I didn't know the Mitsubishi Stratus R/T run on 91. Does the Eclipse V6 run on 91 too?

 

Use the lowest octane that doesnt produce detonation.
The motor will run better and make more power. If he doesnt have any
ping or knock using lower octane, thats fine. But its important to be sure
that its not detonating.