I also keep a log of all the gas used on my vehicles but have not run the
experiment you descibes.  How many tanks of gas was the experiement
conducted on, and was it using the same brand of gasoline?

Based on your figures, it would take a price differential of more than $0.50
per gallon to justify the lower cost of the regular (I assume 87 octane)
gasoline.

For example, for 100 miles driven using regular (87 Octane), you get 18.2
MPG and therefore use 5.49 gallons of gas.  At $3.29 per gallon it costs
$15.37 to travel 100 miles.
For 100 miles driven on mid grade ( 89 Octane) it gets 21.4 MPG, and
therefore uses 4.67 gallons.  At 3.09 per gallon the cost is $16.98 to
travel 100 miles.

I used those examples because in our area the cost difference between the
two grades is often about 20 cents.

I'd sure like to know how many tanks were purchased to come up with those
figures because if that is the only difference then the mid grade may be a
better choice also because it may reduce the possibility of pre-ignition and
that grade of gasoline may have better detergents. etc.

If the figures are right I would be interested to know what the mileage
would be if using a premium grade, like 91 Octane.

Mark

I've had similar results with my '06 Frontier 4.0L V-6. I don't have
the mileage log in front of me, but as I recall I averaged around 1.4
mpg better on Premium than on Regular (8% or so). For the first year I
owned the truck I used nothing but regular. However, Nissan clearly
states that although regular is OK, for maximum performance and fuel
economy, premium should be used. When gas passed $3 a gallon I decided
to try premium. I used premium for 2 months (6000 miles or so) and the
fuel economy was consistently better. However, it was not enough
better to justify the added cost of premium. I recently switched back
to regular and plan to verify that the mileage drops back to the
previous level. After that, I think I will try the plus grade. If, as
was the case for you, it provides a 12% increase, then I could justify
using the plus grade all the time.

I have tried switching to premium with other vehicles I have owned
including two Ford Expeditions. These vehicles had knock sensors and
it was claimed that the power and performance would improve if premium
gas was used. I never could detect any difference in gas mileage for
the Expeditions when running premium.

Ed