Gas mileage


Whats the best way to optimze gas mileage in a 4.0 liter Jeep? Also, is it okay to use 10w30 oil in the winter and 10w40 in the summer?


If it gets really cold where you are, you want to use a 5W30 oil. The 5W or 10W that you see in the weight of the oil is its winter weight. This reflects the oils ability to flow at 32 degrees F. So if the temperature gets blow this, the oils flowability decreases. And this can cause engine wear at a cold start-up.

So, 5W30 in the winter, and 10W30 in the summer. But why stock two different weight oils? Why not use 5W30 oil all year round? Aint goin’ to hurt your engine none!



Maximum safe tire pressure and a front end alignment is about all you can do and it won’t help much.


In addition to the proper viscosity motor oil, higher tire pressure and properly aligned wheels, the most effective ways of increasing your gas mileage are:

*Modify your driving habits, i.e.–don’t use drive-up windows at banks and fast food joints. Park the car and go inside.

*Don’t tailgate! When you tailgate someone, you inevitably wind up hitting the brake more often than if you followed at an intelligent distance. Every time that you hit the brake, you then have to compensate by using more gas.

*Combine many short trips into one expedition with a few stops, thus allowing your engine to get to full operating temperature, which is when it is most efficient.

*When you are driving, make believe that there is an egg between your foot and the gas pedal. In other words–use a slow and gentle application of the gas and the brake whenever possible, rather than a heavy application of those pedals.

*Make sure that you replace spark plugs on a timely basis. Sure, they may be good (in theory) for 60,000 miles, but this can be false economy. If you replace them at 30,000 miles, you will usually wind up with more efficient combustion (no misfiring) and you will have no problem removing the spark plugs at the next interval.

*Get rid of unnecessary weight in the car. If you are carrying around extra tires, rarely used tools, junk that should be in the basement, clear those things out of the Jeep.

*If you are carrying anything unnecessary on the roof rack, remove it!

And, although I’m sure that you don’t want to hear this one, it bears saying anyway. If you really are interested in getting better gas mileage, a truck-based SUV is not your best choice of vehicle.


All very good advice! As was pointed out in other posts, oil viscosity choice determines both fuel economy and engine life. If you want both long engine life and good fuel economy, synthetic oil, such as Mobil 1 will give you both. Using regular 10W30 in the winter will not give you good gas mileage and in cold areas and can cause severe engine wear, especially on the valve gear. For opitimal gas mileage and good engine wear, I do the following:

  1. Buy a block heater if you live anywhere north of the Mason-Dixon Line. Plug it in the winter, with a timer to come on 1.5 hours before starting the car.

  2. Depending on the age of your Jeep, and condition of the rings, use 10W30 synthetic in the summer, and 0W30 or 5W30 in the winter. If you live in Florida or Southern California, 5W30 or 10W30 for year round use is OK. Believe me, 10W30 synthetic gives much better protection that 10W40 regular oil. If the engine is well worn, don’t use 0W30, but 5W30 instead. 5W30 synthetic is much more “slippery” than regular 5W30, and will improve gas mileage. Synthetic oil has more film strength, but with a thinner film.

  3. As mentioned above, driving style affects gas mileage the most. Take it easy!

  4. After starting the car, it takes only about 10 seconds for the oil to reach the valves and camshaft if the engine is warm. It can take up to a full minute to get there with 10W30 with a very cold engine. After plugging in the car, the start next morning will be easy, and no warmup is needed. Drive off gently until the car is up to full operating temperature.

  5. If you can’t get a hold of a block heater, still use the lighter grade synthetics; but resist the new 5W20 or 0W20s, they will cause oil consumption and smoking in older design vehicles; I assume you have the venerable straight 6 in your Jeep.

I have been practicing these activities for the last 30 yeras or so and have not rebuilt an engine in the last 42 years, while getting better than average fuel economy.


And to top it off, you should drive the speed limit on highways. The Jeep is geared to run at low RPMs when you are around 55 MPH. Faster equals higher revs and greater wind resistance. I go 55 MPH when that is the speed limit, though I also meet, but don’t exceed, the 65 MPH limit on some highways near me.