Regular versus Premium Gas

[MoladoGuy]

Coming from the thread about "Performance Enhancer" I was interested to know if running Preimum help many of us who have lifted the KJ and added some weight such as skids, bumper, etc..TJKJ uses Premium but that's due to the extra 1400 pounds and JeT 2 device he has setup on his rig.

Would Premium improve pick up and/or MPG on just a standard lifted KJ?

 

[Atrus]

I am by no means an expert, but this was heavily debated in the community that I just left about my previous car. A lot of tests were done, and power/MPG was achieved using the lowest grade of gas that did not cause knock.

I don't know all of the Jeep terminology yet, but in GM's systems, when the PCM (powertrain control module) sees knock it retards the timing, causing loss of power and I guess inadvertantly loss of MPG?

There's no easy answer in my opinion. Some modded GM 3.8's had no knock whatsover. I had 5* when essentially stock. Every engine will be different, and modifications further differentiate them.

I have a great scan tool for GM applications. I am not sure if the Chrysler parameters work as well on it, but check out www.autotap.com. It can be run on a PDA or laptop. Unfortunately, no tweaking of the computer can be done with this, only scanning.

I chased that knock around, trying to put in colder plugs, etc etc. It's a losing game IMO, I drove myself nuts with it. I finally said fawk it and ran 87 in it. I still have the car (for sale!) with 130k on it and it still runs great. Just drive the Jeep and enjoy it IMO.

 

[Marlon_JB2]

I ran a few tanks of premium in my '04 back when gas was... uh... cheap...er than it is now, lol.

Absolutely nothing changed. Same 15-17MPG.

 

[tjkj2002]

Coming from the thread about "Performance Enhancer" I was interested to know if running Preimum help many of us who have lifted the KJ and added some weight such as skids, bumper, etc..TJKJ uses Premium but that's due to the extra 1400 pounds and JeT 2 device he has setup on his rig.

Would Premium improve pick up and/or MPG on just a standard lifted KJ?

The main reason I use premuim is because of the chip and added extra weight,if yor KJ doesn't knock or ping there is no need for higher grade gas.If you are towing,carring and heavy load(inside),and/or driving hard through the mountains(or any of these in combination) premium could help.You are not going to see a mpg increase just your mpg's will not decrease as bad as using regular(and could prevent engine damage).The cost difference between the grades is so small that it's not any big deal for me(I would only save about $2 if I used regular per fillup).I can't recommend using premuim because every vehicle is different and will react different to different grades of gas,it just so happens that premium works for my KJ better than regular.

 

[JeepJeepster]

A heavy load creates more heat which in turn causes knock(where tjkj comes in). Thats the reason the OM says its a good idea to use Mid-Grade when towing since the engine is usually under more load and produces more heat.

I think I used Mid-Grade once when towing on a 3hour trip..

 

[myjeepwalks]

I normaly run regular. I filled it up once with premium when i was heading up the PA turnpike to my grandparents house in the poconos. Seemed to run better and pull better up the hills. Dont remember if the gas mileage was any better. Next time i fill up i'll put premium in it and do a comparison.

 

[indieaz]

Going to be a lot of factors. For example weather...you aren't going to see any knock on a 65 degree day with low humidity even fi you've added 400 pounds of accesories. The air charge is going to be plenty cool.

On a 105 degree humid day...probably some knock and timing will get pulled. In this case premium might help. But the reality is are you willing to sacrifice another .20 cents per gallon to save 5-10hp?

Our jeeps have a relativeyl low compression ratio (9.1:1 for the 3.7l). Under most driving conditions (likely 98% of the time you drivE) you aren't going to experience any knock. Except during adverse weather, when towing/going uphill maybe a little. During those times maybe filling up with 89 or 91 will be helpful, but the rest of the time why bother?

As for gas mileage...so long as you aren't experiencing knock whiel running 87 octane the difference in mileage is going to be zero between it and 91. If you are experiencing knock...the higher grade gasoline will result in better mileage...but only *slightly* better. Maybe 5% at best. So maybe 20.5mpg instead of 20.

 

[Lizzys07Liberty]

While I certainly agree that the Jeep probably doesn't need premium, that's all I use. In fact, I use it on every vehicle I own.
I first started using it on an older (older as in 2002) 4-cylinder truck that is underpowered. Make a long story short, I bought my little Ranger, new, as a means to put tons of mileage on while working as a photographer for various news agencies. With the A/C on and attempting to keep the truck at freeway speeds, you really needed to have somewhat of a lead foot. So I noticed that it knocked just a little. Put a tank of premium in it, was a little more zippy (still underpowered but better) no knocking, and better gas mileage.
That's all it took.
And as tjkj2002 stated, it's really not that much more for premium. If the Liberty tank was totally empty, it would be less than a four-dollar difference between filling it with regular and premium. Let's face it, $61 to fill up or $64...not really a big difference in the grand scheme of things.

And typically, as engines get a little more broken in; the better the MPG. My Liberty is doing well at only 3300 miles- In city I'm averaging just short of 21 MPG. If premium gas helps achieving that fact, great. If not, then $4 is a cheap way of thinking I'm giving the Lib a little extra attention. I mean we can spend $50 on some trendy air filter, $12 on a super fancy oil filter, and $40 on synthetic oil....but can fill up on the cheapest, most watered-down gas in town, caused it saved them a .04 a gallon? And until I started driving an older carbureted vehicle (by choice) you start realizing that there is actually better brands of gas out there.

That said, every vehicle I own, runs better on it. More pep, less strain, less knock and better mileage. No brainer.

 

[Ry' N Jen]

Gooday,
I have run 98 octane gas in our KJ for the last 4 tanks and to be honest, I first thought I noticed a differance when nailing it off the line and n Freeway trafic, but at the end of the day it didn't seem to make any differance miles per gallon wise. In fact, one tank of preium I actually noticed lower mph. (Must of been that lead foot syndom.)

ry'

 

[AZKJ]

While I certainly agree that the Jeep probably doesn't need premium, that's all I use. In fact, I use it on every vehicle I own.

Let's face it, $61 to fill up or $64...not really a big difference in the grand scheme of things.

That said, every vehicle I own, runs better on it. More pep, less strain, less knock and better mileage. No brainer.

I'm not using Liz's post as an attack, it's just a good post that sums up what a lot of folks think, that I want to address....

I'll toss my opinion in the ring, which is gathered from my experiences driving all makes of cars in this country and overseas, reading articles on the subject, talking with mechanics and a lot of car guys over the years.

You're wasteing your money and doing nothing for your engine or MPG averages using a higher grade of gasoline than the manufacture calls for. It is on a rare occasion that using a higher grade helps.

"Let's face it, $61 to fill up or $64...not really a big difference in the grand scheme of things.". I'd say that would depend on how much gas you use and your financial status. Paying on average $.20 a gallon more for high test and figure at least a tank a week, that adds up to $208.00 a year. So no, I guess it's not a big deal, unless you're a starving student or some young couple trying to make ends meet before they get established in life, someone down on their luck OR some retired guy on a fixed income (hey, that's me ). I know there's a lot better things I'd rather spend money on than gasoline.

Here is a quote from the Federal Trade Commision's website on the subject:

The Low-Down on High Octane Gasoline

Are you tempted to buy a high octane gasoline for your car because you want to improve its performance? If so, take note: the recommended gasoline for most cars is regular octane. In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual.

The only time you might need to switch to a higher octane level is if your car engine knocks when you use the recommended fuel. This happens to a small percentage of cars.

Unless your engine is knocking, buying higher octane gasoline is a waste of money, too. Premium gas costs 15 to 20 cents per gallon more than regular. That can add up to $100 or more a year in extra costs. Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need.

The entire article can be found here. You can do some Googles on the topic and you'll find tons of articles saying the same thing... use what your owner's manual recommends.

Again, this is just my opinion, so go with whatever works best for you....

 

[thecause17]

I've tried premium in every vehicle I've ever owned to see if it made a difference in performance and milage. The fact is, the only vehicle I've ever seen run any better on it for me was also the only vehicle I've ever had that premium was recommended by the manufacturer...moral of the story, read and listen to the manual.

Of course, that's applying to stock vehicles. If you have a chip or forced induction, then premium is probably the way to go. Other than that, stick to the book's recommendations...I've actually seen manuals that state premium gas can damage the engine (my dad's '98 Ram says this in it's manual, and so did my '97 Ford Explorer)

To be honest, I notice much more of a difference in milage depending on where I buy it, rather than what octane it is.

 

[daniyel]

I've actually seen manuals that state premium gas can damage the engine (my dad's '98 Ram says this in it's manual, and so did my '97 Ford Explorer)

This is also one reason to never use Octane Booster from the bottle to the tank. Reason being is the methanol used in a lot of those (some FI cleaners use methanol as well), which is bad for these engines. Sorry for adding to an old post btw.

As a funny, my XJ (Cherokee) with the tiny AMC 2.5 needs 89 Octane ever since the prior owner over-bored the cylinders (30 thousands of an inch).

 

[Dave]

The manual says 87 octaine and mine runs fine on that. Mine is stock. If you're running a chip or other mods, then a higher octaine could be necessary. (like tjkj). Otherwise the stuff AZKJ posted is probably true. You are probably just wasting your money running a higher octaine than you need.

That said, the manual also says you can run 89 octaine for towing. I tried a higher octaine for towing a couple of times and didn't notice a difference between that and the 87 octaine. So I just use 87 octaine all the time now.

Dave