If it's the same engine they put in the 300M's, then my 2000 300M does
just fine on 87 octane. That's all I ever use.
The horsepower, torque, 1/4 mile times they publish are probably using
89 or 91 octane. That's why they say the recommended octane is 89 -
so that you don't complain to them when you don't get the 1/4 mile
time that Road and Track says they got.
Recommended is basically bullshit. It's when they say "required" that
you have to sit up and take notice.
With electronic timing and ping sensors, and the fact that the lowest
grade of gas you can find is basically 87, even engines designed to
use 89 (which I'm sure the 3.5l isin't) can get by with 87.
I have an '06 Ram with the Hemi and in the manual it says to use 89
octane. I've read stories where people have used 87 instead and have
had to have all fuel lines and fuel rails replaced. I just look at it
as spending a few more dollars for something that's required.
I've got about 65K miles on it after almost 7 years of ownership. 85%
of those are city miles.
When I'm on the highway, with the cruise control set for 120 km/h (75
mph) my tach says about 2250 rpm and the overhead milage display shows
anywhere from 26 to 30 mpg (highways around here are pretty flat) but
there might be a headwind sometimes. Seeing the display at 28 for
long stretches on perfectly flat sections is not uncommon. Turning
the AC on will usually knock about 1 mpg off the readout.
I'm very happy with the milage I get on the highway.
I also keep my tires at 34 psi. That probably helps.
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