There is no need to be technical in gauging fuel mileage... head wind/tail
wind? Are you kidding?
I once put in 89 octane on my BMW with high compression engine... my average
with 93 octane is around 330 miles with reserve left over. I put in 89
octane and my tank is bone dry at the gas station with only 260 miles. I
only had splash of fuel when I thought I would have 2 gallons of reserve.
My friend who owns 2000 E320... when he first got the car, he would put in
only regular... money was tight for him... his car computer average said
17.3 MPG... he ran it for over a year. Then he switched to 89 and he said it
bumped to 20 MPG...
My folk's 2001 E320 always had 93 and the computer said 23.7 MPG... This is
60,000 miles worth of fuel. On long trip of highway only, we get 29 MPG.
This we have done it for 500 miles stretch at a time.
Air filter is quite important too. Our SUV with new air filter and high
octane fuel gets 19MPG local driving... when the filter is 9 months old, the
fuel mileage drops down to 17.5 MPG and will go down to 16 by the year end.
This is with 93 octane.
I once filled the SUV with 91 octane from Sunoco... with the fuel mileage
reading at 19 MPG with clean filter... the fuel mileage with that tank of 91
octane is 17 MPG.
I don't know how many more types of car do I need to say that will have
similar mileage reduction. Generally speaking, look at your compression
ratio, if it is high and the manufacturer said high octane is required, then
it is required and fuel mileage will drop if you use lower octane.
If you have Honda, Toyota, GM or Ford and all other with 9:1 compression
ratio and lower and the manufacturer said 87 is all you need, then 87 is all
you need. However, if you put in higher octane, you will see higher fuel
One lady in newspaper said if she used regular as recommended by Honda, she
gets what the car is rated for... 33 MPG, but she put in 93 octane and she
got 40 MPG... so calculating cost per mile with fuel only... premium fuel is
cheaper to operate than regular because of the extra fuel mileage.
Now, if you are stricty super uber city like NYC only, then I am not sure...
but you need to do some experiment yourself for your average driving
condition. Not 25 miles highway driving test... heck if you shut off AC and
windows up, you will get extra 3 MPG on that same highway test.