Your first step is to learn the most basic lesson of fuel economy - don't pay any attention to "miles per tank" (or kilometers per tank).

The ONLY correct method is to divide the actual distance traveled by the actual fuel used.

Distance is measured by an accurate odometer, corrected for non-stock tires if needed.

Fuel used is measured only one way - how much fuel you pumped in as reported by the gas pump.

Your fuel gauge has NOTHING do with it. It's not accurate enough to do anything but lie to you.

Until you are recording the distance traveled and the fuel pumped, you have no clue what your actual mileage is.

 

I don't think there's a temp sensor on the AW4.


The torque converter locks up in 3, or at least it does in my '98. It'll lock up under light load around 40-42 mph, and it'll stay locked with more throttle input than it will in D.

 

That's what I was thinking.......and like cruiser said, only when "3" is selected.

 

Almado:

The loose vacuum hose allowed the motor to run lean which gave you better mileage under light load at highway cruise. You must have a good cooling system because lean mixture can cause motor to run hot. Also risk of damage/detonation under high loads. I doubt if you were getting very good mileage off the highway.

 

I should have clarified. I have an AX-15.

~65mph in 3rd= ~3000 rpm's.

It feels right in 4th though at ~2200 rpm's.

5th is way too low.

31" tires, stock 3.07's for now still.

Moving to 4.10 or 4.56 soon and I do anticipate a slight increase in economy for 2 reasons.

1. Less power required to move in each gear.

2. Better rpm's at any given speed.

 

See I was told the 4.0 liked the rpms to be closer to 1900-2200 cruising. Which makes sense, to me at least. With 3.73 and 35s 60mph is 2.1k in 4th, about 1.5 in 5th

 

How efficient an engine is does not translate directly into what is most fuel efficient overall.

For instance, the 4.0 may be thermodynamically most efficient at 2200 rpm but fuel consumption rate might be lower at the lower rpm even if the engine is not as efficient at that range.
Best thing to do is get a real time fuel economy gauge such as the scangauge or ultragauge. Then with a 5 speed...you can see the change in fuel consumption (injector flow rate) when you shift from 4th to 5th at a set speed.

 

This exactly. With something as aerodynamically challenged as an XJ, the higher the speed, the more drag is working against you. So even if you speed up to get into the proper rpm range, you could be killing you mileage with the increased drag.

I have an 85 with the 2.5l and I get my best mileage if I a running back roads at about 50mph, once I hit the interstate and get up to 70mph(3000rpm) I can pretty much watch the fuel gauge move. Of course, having 3.73s and no overdrive on the A904 doesn't help either with only 29" tires.

 

No one was discussing speeding-up to achieve a higher RPM. We were discussing achieving different RPM using different transmission gears, different sized tires, different gears.

 

True, however it is important to understand that the engines "most efficient" rpm range does not equal what is most fuel efficient for the vehicle, payload and terrain.
In other words, get a way you can accurately track fuel consumption and then you can see how different RPM ranges (on a 5 speed) will affect MPG at a set speed and load. Do this before you invest in gears (front and rear) and find unsatisfactory results.

 

Vacuum gauge works great for this^^.

 

When Mrs. Klutch drove her '97 Cherokee, I did a few things which helped just a little with MPG:

- Installed a K&N air filter. I know there's a lot of hype around these things, but it really did make a difference. It also gave the engine just a tiny bit more pep.

- Running full-synthetic, 5W-30 oil. I ran the "Supertech" full synthetic oil from Walmart with a Mopar oil filter also from Walmart. The Walmart full synthetic oil is actually very good oil. It's reasonably priced; especially when you buy the 5-quart jug plus a one quart bottle.

- Install good quality tires. Over the years, I have found cheap tires tend to have more rolling resistance.

All in all, it's not a big jump in MPG. But every little bit helps. I also read an article about this very issue recently, specifically applying to the Jeep 4.0. It said a Cat-Back exhaust actually increased MPG and performance pretty well. Other mods were miniscule or nothing at all.