Well, to be honest, higher octane gas is slower combusting, allowing you to advance the timing to get more compression and more power. Lower octane gas combusts faster, and most cars are designed to run off of it anyway, and with an engine that's designed to run off of regular grade gasoline, it won't make too much of a difference performance wise to run premium through it. Plenty of difference in your wallet though. Now, if you have upgrades to your engine, then you can run a higher octane gas and see a lot more modest to great gains, depending on the modifications. A stock engine won't be much better off.

At least this is true for fuel injection... carburetors, in my experience, have always done better with higher octane.

 

I think it probably matters what kind of car you have, I owned a brand new VW golf (2007) and I could get one HUNDRED extra kilometers per tank on the highway, but my friends new (2007) Pontiac g5 on high grade gas made little difference, if any. funny huh?

 

High grade gas helps for me, but more so than that, gas with no ethanol. Try to find a station that sells it (there is only one within 40 miles of me) but it does run better and more efficient. Also it smells nice unlike nasty gasahol.

 

I've heard that removing the roof rack will improve mileage. (I have not done any testing)

Replace the clutch fan with an electric.

Clean all the crap out of your jeep. Remove any excess weight.

If i had time i'd like to mess with the tune to improve highway mileage.

I disagree on the higher octane waste of money unless your current setup requires it but a stock set up will perform not better. Just use quality clean fuel.

And please don't burn corn eat it.

 

Octane is really just an anti-knock index and IMHO, the myth continues that higher octane will get you better gas mileage.

The stock 4.0 is designed to run very well on 87 octane.

Save your money.

Tuneup parts, fresh fluids, correct tire pressure go a long ways towards optimizing fuel mileage. A bottle of quality fuel system cleaner (BG44K/Techron) can help optimize fuel injector spray pattern.