02-05 What octain gas do you use?

I have a couple of old cars and I was told to use the lowest octain gas you can (without pinging) because the higher the octain the more eythinal there is in the gas. And that is what causes problems on older cars.

Would that be the same for our T birds? Does only one use 87 or 89 suscessfully?


2004 Merlot
If you search the forum you will find a (spirited) discussion on all things octane. . .

I've used 87 octane since purchase in May 2018 ('04 w/60k). No pings or performance issues. (I tend to drive "gently" and enjoy the scenery.) On the few rare occasions where I had to jump on it I only got a few short pings - no major protests.


Forum Moderator
Staff member
Please include a model year(s) on future posts in the subject/title. I added this for you.
In Central NY, I am fortunate to have access to many stations that carry ethanol free premium gas.. I always use the Octane rating that in the manual which is 91.. Never heard a ping/knock yet from our 02, and she also runs very smooth.. That being said, ours is not a daily driver. With the cost of the non ethanol premium 91, at about $.50 -60 cents more a gallon I'm only filling it maybe once every 6 or so weeks during the driving season here in NY.. If it was a daily driver then I would reconsider..
Last edited:
I wanted to chime in. I have an 04. I always used 93 octane until 6 months ago. My hubby and I bought another car, a Jaguar XJ and he researched and inquired with a lot of experts who all seem to agree that any high performance engine will adjust to any grade. I haven't noticed any change.
I have a couple of old cars and I was told to use the lowest octain gas you can (without pinging) because the higher the octain the more eythinal there is in the gas. And that is what causes problems on older cars.

Would that be the same for our T birds? Does only one use 87 or 89 suscessfully?
My dad is a car enthusiast as well and we both agree in higher performance engines or engines with upgrades either way we both use nothing but shell 93 octance v power or the top of the line Exxon 93 octane I think neither or is the best although I ha e gotten better fuel economy out of Exxon 93 Hope this helps


Forum Moderator
Staff member
In newer cars, you use what the manufacturer calls for, or you are just wasting your money unless you have the SCT Tuner with tunes optimized for different Octane fuel. For example, in my Mustang I have tunes for 87,89 and 93 octane fuels.
I found an excellent write up about using various octane levels in our 2002-05 Birds. Unfortunately I did not get the authors name to attach to this.
There fore I'm telling everyone who reads the these are not my words.
You will find this, I think very interesting & probably answers many of your questions.

Should or shouldn’t use hi octane premium fuel

I hesitate to chime-in here because I always end-up getting beat-up when I've either posted this or opened this dialogue with friends and/or automotive enthusiasts. There is so much myth about gasoline and how it impacts a MODERN engine. I assure ALL of you that you are flushing money out your tail pipes.

I'll explain...

The 3.9L engine in your Thunderbird is a high-compression (performance) engine, with a compression ratio of 10.75:1. Regardless of what gasoline you put into the tank, the compression is NOT going to change. ALL gasoline has the same energy content...PERIOD!

• ALL gasolines sold in the United States have fuel additives which clean the combustion chamber. There is no increase or decrease in the amount of (nitrogen-based) cleaning additive in any given gallon of U.S. gasoline. These additives are required by FEDERAL law. These additives will clean carbon deposits as the additive in the gasoline comes into contact with the internal engine surfaces. Though, different fuel retailers will label their fuel additives by differing marketing terms, it's ALL THE SAME cleaning performance. Adding a bottle of fuel injection cleaner, etc. to a tank of modern gasoline is wasted money (and depending upon the chemicals in that fuel-injection cleaner, potentially harmful to engine sensors). The same cannot necessarily be said if you fill-up in Mexico.

• Gasoline-air vapor will self-combust (that is, ignite/explode) when compressed. Modern gasoline has additives in the fuel which helps it resist this phenomena...dependent upon the level of additive in the fuel, the more resistance that fuel-air mixture has to combustion under compression. That level of resistance is known as 'octane'. There are several different ways to measure its resistance, but in the United States we use the (R+M/2) method. This is the number posted on the pump.

• All MODERN engines have acoustic "knock" sensors installed on them (your Thunderbird is no exception) which detect pre-ignition/pre-detonation (a.k.a. "knocking" or "pinging") constantly (hundreds of times per second). If the sensors detect engine/spark knock, the ECM automatically and instantaneously adjusts the spark timing to back-off of the ignition advance to eliminate knock.

• If you put 'regular' (87-octane) gasoline in your tank, and start your car, it will run and idle just fine. As you drive it gingerly down the road, it will run and perform just fine. As you accelerate hard (put a demand on the engine for more performance), the ECM will advance the ignition timing to account for the milliseconds of expanding ignited fuel energy so that the engine can get the most efficient use of that energy to push the piston down. At the same time, the ECM is listening for spark-knock. If the ECM detects spark-knock, it will back-off of the ignition advance (slightly) until the spark-knock is no longer detected. This happens hundreds of times per second, and is indiscernible to the human ear or other senses. When the ignition timing advance is 'retarded', the engine is not operating a 'peak' performance, and therefore, you are not able to eek the maximum engineered horsepower out of the engine at that
moment. This would be known as a performance 'degradation'. Since your Thunderbird engine is naturally-aspirated (not turbo-charged or super-charged), the instances of pre-ignition/detonation is minimal (comparatively-speaking).

Now, with all that said, what does this really mean to a lay-Retro bird-driver? Well, if you decide that you don't want to pay an additional ~50¢/gallon at the pump and opt to fill the tank with 'regular' (87-octane) fuel, you're going to save about $9.00 per fill-up. If you drive your Thunderbird as a cruiser, you're going to realize little to no difference in the performance of your car. You will NOT gain or lose any MPG (remember, there is no difference in energy content of premium fuel versus regular fuel (or anywhere in-between))! Now, if you drive your car like you stole it, accelerate hard from a stop, and put your foot into it when your "accelerating" on the highway, you may be able to notice, or I should more accurately say "measure", a minor horsepower reduction as opposed to the same driving-style with a tank of 91-octane. Why? - Because the ECM is detecting spark-knock and retarding the ignition advance to eliminate it, which takes away from the maximum-engineered engine performance potential. So, if you don't drive your Thunderbird hard, you can opt for 87-octane fuel and save yourself approximately $450 annually which you can put into your maintenance fund, or spend otherwise.

Ford has included this information (from its engineers) in your operator's manual:

"Your vehicle is designed to use 91 “Premium” unleaded gasoline with an (R+M)/2 octane rating of 91 or higher for optimum performance. The use of gasolines with lower octane ratings may degrade performance. We do not recommend the use
of gasolines labeled as “Premium” in high altitude areas that are sold with octane ratings of less than 91. Do not be concerned if your engine sometimes knocks lightly. However, if it knocks heavily under most driving conditions while you are using fuel with the recommended octane rating, see your dealer or a qualified service technician to prevent any engine damage." (Keyword: "optimum", "may").

If you understood all of that, consider yourself a wiser consumer. Don't let myth, marketing, or outdated information ruin your wallet
all above makes me wonder?? I think I heard in forums that our cars "learn" our driving habits and perform accordingly so if the car is changing its timing to compensate for the knock ,does it learn what its doing till it can't compensate any more? I have friends with a subaru that *requires* 91+ and if the run 87 it slowly starts running bad then it gets really bad they put 91+ back in and it runs good again.maybe because it does not have to compensate any more?.
on the other hand I awn a big bore bike that requires 87 and those that put 91 in them everything about it just goes to crud.like mpg,speed acceleration
I will stick with the "ethyl". (thinkin some of you have not heard that term in awhile) ÖÝÖ


Well-Known Member
For everyday driving Shell 93 octane from specific local Shell gas station that keeps fresher fuel and does not dilute (mix) with other grades.

For track days 100 octane race fuel from pump at secret station.

My car is modified and dyno'ed.

Great information here. Sadly I have always used regulat 87 octane, never
noticed any difference. Even in my daily driver which will take regular but
prefers premium, I can not see a difference.

Owners manual in my daily driver says 420 hp with regular, and 430 hp with
premium. With regular I got it up to the governed top speed of 157 one time
and it got there quick, with no knocking.

Have to remember here in Texas we have a road with a 85 speed limit, and
people pass you going 95 or 100 every day.