DCWinterXJ

Just wondering what octane gas you guys use/recommend considering the ethanol blend now in everything. I use supreme I guess 91 or 93 octane(will have to confirm) Seems to run fine on that however I never tried regular since I got the spare engine installed. Price is not really a big deal for me because I have a very short commute....I was just wondering as to whether anyone had any insight as to whether this is a benefit to the engine or not/ Thanks in advance!

tjwalker

The best rule of thumb here for your XJ or any vehicle is to consult your owners manual. If the manual recommends running regular, run regular. If it is designed for mid grade or premium, run that. The owners manual for the Jeep 4.0 calls for regular gasoline. Running a higher octane than necessary isn’t better for your engine, nor will it result in better gas mileage. Especially the low compression Jeep 4.0. Thinking that you are “treating” your engine by running premium is a long running myth that dies hard.

The AKI (anti-knock index -- also known as "octane rating") of the fuel is an average of two different methods of computing the fuel's ability to resist engine knock. Regular unleaded gas in the U.S. has an anti-knock index of 87, while mid-grade is usually 89, and premium typically is 91 - 93. The higher the number, the more anti-knock protection the fuel offers. The 4.0 liter engine runs just fine on 87 octane.

Premium fuel does not necessarily pack more energy content than does regular. Rather, it allows more aggressive engine designs and calibrations that can extract more power from each gallon of gasoline.

The higher AKI (anti knock index) of premium gasoline does not in itself make it any cleaner than regular gas. Oil companies, however, like to advertise that their premium fuels are ‘specially formulated” to clean fuel injectors, restore lost power, cure world hunger, etc. With the money you save by running the recommended octane of gasoline for your engine, you can afford to put a bottle of quality fuel injector in the tank once in a while to help optimize fuel injector spray pattern. The key word here is a “quality” fuel injector cleaner, like BG44K or Techron. Stay away from the el-cheapo fuel injector cleaners that you see lining the shelves of the parts stores. They really don’t work well.

How much will you save by running regular? That depends but for an average driver, you can probably save up to $200 a year. Gasoline retailers and refiners love to sell high-test premium fuels because it's more profitable than regular grade gas is.

To add one more thing to choosing fuel, your best strategy for choosing where you fill is to use a TOP TIER gas station. Regular use of Top Tier fuels over time does have its advantages to be sure with fewer carbon deposits on injectors, valves and combustion chambers. I always choose a TOP TIER station when possible. Click on this link to learn more about Top Tier fuels and to use their "Locator" feature to find out which Top Tier stations are near you. In my area, Top Tier fuel is the same cost as non Top Tier.

https://www.toptiergas.com/

Cummins93

Even with the crank sensor advanced and summer temps I run 87 without issues. I run 93 or higher with forced induction.

DCWinterXJ

Ok thanks you guys! I am gonna put regular in my next tank....I always get Exxon which is a local top tier. Very interested to see how she runs or if there is a change. But pretty cool.

EEVEE

i stick non ethanol in mine here and there and i notice a bump in mpg but imo it isnt worth it price/mileage wise

00t444e

In basic terms the higher the octane rating the more resistant to combustion the fuel is. Higher compression and forced induction engines require higher octane fuel so you don't get preignition. Running higher octane fuel in an engine that doesn't need it is nothing but a waste of money.

dmill89

^This, the 4.0L I6 has an 8.8:1 compression ratio, in general NA engines with 10:1 or higher compression may see benefits from higher octane, and 11:1 or higher usually require premium (some exceptions for direct-injection). Top Tier gas is always recommended as others have mentioned but the XJ or any other vehicle with the 4.0L I6 won't see any benefit from higher octane.