People I keep talking to are whining about running premium, it is "too expensive" they cry. What? Since when?

Ok we have a 18 gallon tank. On average we don't run it to the E line, so likely you are putting in 20 gallons at a fillup.

Assuming you are paying $4.09 a gallon for regular that is $81.80
Assuming you are paying $4.29 a gallon for premium, that is $85.80

That is A total of $3. $3! How FAR can you go on $4? Lets say you get 18mpg (I know, you all probably do much better unless you keep your foot in it), that is 0.97 gallons, a total of 17.6 miles you can go on that $4. IF you get better mpg with premium (most cars do now that have the high/low octane tables properly setup), it is EASY to get better mpg with premium than low octane.

The % difference is minimal. The distance you can go can really end up being much improved.

So why NOT get a tuned ECM/PCM if you don't already have one, and enjoy SAVINGS by running premium? Mainly the LT1 cars, the L05's, L03, and 4.9L cars I can do now.

Now for the carbie guys, you can do it the old fashioned way, get a timing light and boost up the timing and tune it for 93 octane. Do the math, spend some time on it, premium might be a good thing for mpg...

You really think premium and a tune pays for itself? I don't mean the tune, I can do that myself.

Let's say premium is $4.35 and regular is $4.00.

Let's say mileage goes from 21mpg to 23mpg (no idea if this is realistic).

Getting 21mpg and with regular at $4.00/gal, for 500 miles of driving, you will spend $95.

Getting 23mpg on premium, at $4.35/gal, for 500 miles of driving, you will spend $94.50.

So to actually make this work, your cost of premium would have to be closer to regular than my example, or your mileage would have to increase by a larger amount. But wait - when people compare gas mileage on premium to regular, they ALWAYS compare the stock tune on 87 to a "performance" tune on premium (whether it be DIY or mail order). That's not fair. The stock tune can get some efficiency even running 87. That is what should be compared.

My first tank has netted 17mpg and a lot of it was highway, so I have a ways to go before I am even getting the correct mileage for a stock tune on 87. Then I can worry about tuning.

I'm the ideal candidate to test this. Currently, I am running a "premium" tune in my PCM (95 SS, T56, 4.10 gears). I am commuting on rural roads at 60mph, with approximately ten stops. About 30 miles round trip a day. I fill my tank each time, and calculate MPG. I have been using 87 exclusively (and I am with the current tank). I will fill the next two with premium, and post my results.

In this region (chicagoland and around 100 miles radius) it is consistent $0.20 more for premium. In St. Louis it is $0.30, my mom claims $0.40 in Florida.

A friend of mine has a 04 5.3L GM truck and just going to 89 octane noticed a mpg improvment. Those vehicles fully populate the high octane tables. So it will use them as it can.

Some cars ARE tune factory for both (NorthStars for example), some are not (4.9L cars and need timing adjust to run 87 octane, but factory are set for 93, now on my 91 Deville, it DID run better on 93, but on 87 to 93, mpg was identical). My 94 Fleetwood LT1 tuned for 87 (stock and modified cal) and 93 only, got better and more consistent mpg on 93. Between 1-1.5 mpg improvement and much less loss in the winter, where I often saw around 2-2.5 cold temp loss. I only saw around 0.5 to 1 mpg loss.

Each car is unique, and as ALWAYS (read my posts, I ALWAYS preach this, I have a "few" posts under my belt), record CAREFULLY your data and prove out the theory.

The whole point, it takes much less improvement in mpg now days than it does when gas was $1.00 a gallon. The $ diff was huge then, the $ diff now is pretty small.

A stock LT1 tune likely will have very little if ANY improvement in mpg on 93 than 87, the timing is just too conservative.

I'm the ideal candidate to test this. Currently, I am running a "premium" tune in my PCM (95 SS, T56, 4.10 gears). I am commuting on rural roads at 60mph, with approximately ten stops. About 30 miles round trip a day. I fill my tank each time, and calculate MPG. I have been using 87 exclusively (and I am with the current tank). I will fill the next two with premium, and post my results.

If you are getting knock counts and, by extension, knock retard, then, yes, your mileage would surely improve. But I suppose it's possible for higher octane to improve efficiency even lacking spark knock, based on the crank angle where the peak combustion pressure occurs...

In this region (chicagoland and around 100 miles radius) it is consistent $0.20 more for premium. In St. Louis it is $0.30, my mom claims $0.40 in Florida.

I'm seeing closer to 40 cents in the North East. At a mere 20 cent premium (haha), premium and a tune would surely be my choice. That's only 5%. Surely you can get 5% better mpg with a premium tune. That's only 1mpg at 20mpg.

A friend of mine has a 04 5.3L GM truck and just going to 89 octane noticed a mpg improvment. Those vehicles fully populate the high octane tables. So it will use them as it can.

That is interesting. I haven't seen much on octane sensing and handling. All I've seen is the low octane learn stuff in the LT1 8051s. I tuned my 95 Z28 to run on 87 and turned off all the low octane learn (and burst knock) stuff because it kept getting in the way. I imagine today's PCMs are really amazing.


My 94 Fleetwood LT1 tuned for 87 (stock and modified cal) and 93 only, got better and more consistent mpg on 93. Between 1-1.5 mpg improvement and much less loss in the winter, where I often saw around 2-2.5 cold temp loss. I only saw around 0.5 to 1 mpg loss.

That's interesting. I know they still change the fuel for the winter, at least in climates that actually have 4 seasons. I think it's a hold-over from carb days to prevent vapor lock in the summer and probably hard starting in the winter. I am sure there are also emissions reasons for the seasonal fuel mixes. I wonder what is different about the premium batches that make it take less of a hit for winter mpg.


A stock LT1 tune likely will have very little if ANY improvement in mpg on 93 than 87, the timing is just too conservative.

Right, which is why I wonder what kind of mpg improvement is possible staying with 87. With nice heads (i.e. homogeneous mixture promoting without the flow restricting swirl ports of the TBI heads) like iron LT1 heads, I never really saw 93 as necessary but that doesn't mean it can't help if you tune for it. I also feel like LT1s could withstand some lean highway mode tuning but I have never really heard of anyone try it. Only on the older gen stuff like TBI (factory in some apps) or TPI (not factory but easy to turn on).

My understanding of octane requirements and tuning is the following:
1. As you advance your spark, and the engine continues to pick up power into spark knock, then you need a higher octane.
2. However, if the engine can be spark advanced and get PAST the point of optimum power (or torque @ rpm) before actual knock, then your octane is fine.

I suspect #1 happens a lot and even a "good head" like the iron LT1s can benefit from a higher octane.

When I did my Z28 for 87, the only knock I had was at 90-100kPa from above the torque peak to redline. Everything in the part throttle or even high load low rpm region was fine. No knock retard at all. The stock 46° or whatever at cruise proved to be no problem.

I should also mention I am shifting 1-3 at 2500, and 4-6 at 2000. I'm actively trying to squeeze max mileage out of this thing these days. I don't think I have been WOT in a couple of months. (Too bad, because it's a beast). I do have a cable and a laptop, so I can check and compare the knock counts between the octane levels. One last piece of the puzzle: I use the LT4 knock module.

One last piece of the puzzle: I use the LT4 knock module.

Right, which is overall desensitized in certain frequency regions due to the roller rockers the LT4 ran. Works great for old clanky high miler engines, too, though I've never ran it.

i wonder if i should run premium on my '85 4.1? maybe something good might happen?

i wonder if i should run premium on my '85 4.1? maybe something good might happen?

I believe the recommended fuel IS premium for your car. Look in your owner's manual...

Unless you run your cars in livery business , you should try premium

In case you were referring to me, I wanted my spare PCM tuned for 87 octane. Why? Because it is a spare and why would I need two 93 octane programs?

I currently have my 93 octane tune in the car, runs well but needs some tweaking. I am getting it taken care of at the St. Louis Nationals next weekend.

I do not drive it daily, so the 93 octane program is perfect for that car. On my wagon, I choose to keep the 87 octane program because it is a daily driver. 90% of my commute is stuck in 10-30 mph traffic, so why waste premium while I am practically idling to work.

It is nice to be able to swap PCM's to get premium or 87 octane, pretty easy to do. It only affects your OLM, so you have to track that yourself.

We can do that, let me know when you wanna hookup