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My wife was complaining about me putting in 91 octane (we don't get 93 here). It is generally 20 to 23 cents a gallon more.

So I decided to try 87 in my 97 Mark VIII (which is premium RECOMMENDED) with NO tune, 115k, JMod, 255/45/17 tires. This is as close to similar drive cycle and environment as possible. It is strictly city driving, no AC, lots of stop and go, rarely over 50 mph and not for more than a couple of miles at a time. Another test would to have my wife drive both, I feel that her results would be closer to cheaper with 87 octane as she is a non aggressive driver. I am fairly high on the aggressive list, WOT from stops and late braking.

  • economy
    1. 87 octane - 13.6 mpg at 3.45 a gallon is .25 a mile (.253)
    2. 91 octane - 16.8 mpg at 3.63 a gallon is .22 a mile (.216)
  • drivability
    1. 87 octane - transmission shifted smoother, traction control triggered rarely if ever
    2. 91 octane - harsher shifts, more of a sag when 1-2 at part throttle, traction control pretty much every take off, 50% of 2-3 shifts as well
  • fun
    1. 87 octane - no fun
    2. 91 octane - lots more fun
This is something I did in my 97 Cougar with a tune for each octane. I had similar results, nearly identical in it was about 4 cents a mile cheaper with 91 octane.

I realize that these results for a stock Thunderbird/Cougar are not obtainable, I got nearly identical mileage with 91 and 87 with the 87 tune and with no tune.
 

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Part of it is that with a higher octane, you can run more advance - which, as long as you're not going hog wild, can increase power output. Which, for the same driving, means less fuel consumed for the same power. Which means better fuel economy.

And on cars with knock sensors, it's really easy to get better fuel economy in the higher octane fuels - not because it's a better gas mileage fuel, but because the car's not pulling timing / enriching the air/fuel mixture to kill the knock! (Side effect is better fuel economy.)

RwP
 

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I didn't notice as much of a difference with my old 93, but I rarely put 87 into it. The manual said to use 91 or higher, but since there is a knock sensor on the 32v engines you don't need a tune to get the extra performance instantly.

I can say though that with 93 in the T-bird, I was getting the same fuel economy with my mods last year as I was getting stock (at least in the city).
 
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