Gasoline

[Nan Burton]

would it be ok to use the mid-grade gas as opposed to the high test grade gas? 2003 Thunderbird.

[Quickdraw]

I use the 91 octane in my Tbird but I only drive it about 1,200 miles a year.

My daily driver 2002 Lincoln LS has the exact same engine and I use 87 octane sometimes and never higher than mid-grade (89 octane). The cars computer will adjust to even 87 octane to prevent spark knock. I believe the claims that using lower octane will reduce gas mileage and performance so it may be a wash cost-wise but it does feel better at fill-up time.

[drj001]

I have been using regular grade (87) in my '02 since I purchased it over 1 1/2 years ago. I too was told the computer adapts to or retards the timing so, there is no knocking of the engine on the lower octane. Bought auto with 15,000 km.on odometer and now has 41,000 km. with no issues. Have also heard it decrease performance and decreases mileage, but not enough that it's noticeable. I did try 91 octane for a few tankfulls earlier on, but did not notice any difference in performance or increase in mileage.
If there is, it probably is only in the higher rpm range (over 5500) where these cars are rarely driven anyways.
A lot of owners may disagree with the above, but auto mechanics and I have discussed this a number of times. The computer adjusts in a fraction of a second to any octane rating.
The savings are substantial on a weekly or biweekly fill up and you will not notice any performance loss, other than a perception on your part if at all. Again, the engine will not ping or knock on acceleration or load.