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How do you maximize fuel economy?

J

Jackhill442

NAXJA Forum User
Location
Atlanta
I have tried most things I can think of, and I simply can't get to 20 mpg on the highway. (19.0 is the most I can muster pretending there is an egg under the gas pedal traveling at 75 mph on road trips) I have a 96 country with 205K, 1" lift front and rear, 235-75-15 Michelins, cone air filter (not cold air, it gets hot air), 180 degree thermostat, etc. I have also tried running up to 37 psi in the tires, with little help. Does cold intake air increase or decrease gas miledge? (I know it increases power). Will switching back to the 195 degree tstat help? I know I have a lot of miles, but the engine is kept in tip top tune and I run 10W30 Mobil 1, never uses a drop. I always read on here about guys getting 24,25 or more mpg with 4.0s, 4wd, and auto, which is what I have, so help me do it!
 
Jackhill442 said:
I have tried most things I can think of, and I simply can't get to 20 mpg on the highway. (19.0 is the most I can muster pretending there is an egg under the gas pedal traveling at 75 mph on road trips) I have a 96 country with 205K, 1" lift front and rear, 235-75-15 Michelins, cone air filter (not cold air, it gets hot air), 180 degree thermostat, etc. I have also tried running up to 37 psi in the tires, with little help. Does cold intake air increase or decrease gas miledge? (I know it increases power). Will switching back to the 195 degree tstat help? I know I have a lot of miles, but the engine is kept in tip top tune and I run 10W30 Mobil 1, never uses a drop. I always read on here about guys getting 24,25 or more mpg with 4.0s, 4wd, and auto, which is what I have, so help me do it!
Click to expand...

I can always get a little over 20mpg during most highway trips at 270k miles w/Renix. Rarely under that unless 02 or something is failing.

Even a 1" lift ain't gonna help and while I don't agree with the 180 thermostat, it probably isn't hurting you much on the highway. If your city is around 15-16, it sounds like about an average XJ to me. If worse than that, I'd suspect the 02 sensor.
 
This is what I do:

1.) Use cruise control anywhere it makes sense. Always try to stay at a constant speed.

2.) Do not accelerate too quickly - I find the cruise control "resume" accelerates too quickly. Smooth, gradual gas pedal changes.

3.) Only run long distances with the Jeep - corner store stuff either walk or use your daily driver 3 cylinder Metro type appliance if you have one. Stay out of the city and run highway miles only

4.) Lower your speed to 55-60 mi/h - even on 4 lane highway. (this is a really hard one to observe)

Doing all that, I can just touch ~22-23 mpg with my ~90 mile daily commute. I've seen 24 mpg for a little while over a couple week period when I first got the Jeep.

HTH
 
bustednutz said:
drop your speed down to 70 from 75. You'll see a change.
Click to expand...

I agree. I drive x-country a lot, going from full to empty on the freeway, and I notice I get the best mileage by just setting my cruise to 70 mph or slightly above. Best I got was above 25 mpg.

Also, I think cold air intakes technically are suppose to increase mpg by giving you more air, cold air is more dense, for full combustion. It's probably negligible though.
 
19 mpg isnt that bad at all but it could be a little better. Changing back to the OEM 195 is supposed to be best for fuel economy. Also, I see your profile says you live in Atlanta do you deal with much traffic? That kills my gas mileage, air conditioning does too. Without the a/c on the open road with no traffic as long as I keep it under 80 I get 20-21 mpg. In my daily commute filled with traffic and running the a/c I'm getting a lousy 17 mpg.
 
Yeah, the traffic is definitely ridiculous, I hate Atlanta. But I am really talking about long road trips, like a 6 hour drive to the beach, and so on. I usually set the cruise at about 75, which I know creates more wind resistance than 55, but who wants to cruise at 55-65 mph with the damn trucks and other traffic flying by you hard enough to rock the vehicle?
 
Jackhill, there's several things you can do to improve mpg. A free-flowing muffler would be a good first step (to go along with your cone air filter). A fabricated cold-air intake also helps.

Because our XJs have the aerodynamics of shoeboxes, anything to improve the airflow will also help mpg. Maybe a front airdam, combined with an air deflector for the hatch area? The airdam will decrease air resistance in front, while the air deflector will act as a 'spoiler' and break up the rear end vacuum.

Which brings up another question; does anyone have a source for an XJ front airdam? :dunno:
 
Jackhill442 said:
I have tried most things I can think of, and I simply can't get to 20 mpg on the highway. (19.0 is the most I can muster pretending there is an egg under the gas pedal traveling at 75 mph on road trips) I have a 96 country with 205K, 1" lift front and rear, 235-75-15 Michelins, cone air filter (not cold air, it gets hot air), 180 degree thermostat, etc. I have also tried running up to 37 psi in the tires, with little help. Does cold intake air increase or decrease gas miledge? (I know it increases power). Will switching back to the 195 degree tstat help? I know I have a lot of miles, but the engine is kept in tip top tune and I run 10W30 Mobil 1, never uses a drop. I always read on here about guys getting 24,25 or more mpg with 4.0s, 4wd, and auto, which is what I have, so help me do it!
Click to expand...
Based on what you say above, the following should help:

1. SLOW DOWN. You will get NOTICEABLY better mileage at 65MPH rather than 75MPH. (Remember, the power required to push the car thru the air goes up as the SQUARE of the speed, and at 75MPH, most of the power you are using is in pushing the air out of the way.)
2. Use a 195 'stat. The engine will run more efficiently at the design operating temp.
3. LOWER the XJ. Take off the lift. The higher the XJ is, the more the air interacts with the BOTTOM SIDE of the vehicle. The chassis is MUCH less "slippery" than the hood and roof.
 
Jackhill442 said:
...who wants to cruise at 55-65 mph with the damn trucks and other traffic flying by you hard enough to rock the vehicle?
Click to expand...

Somebody who wants better gas mileage than they get at 75 mph.
 
Yeah, but I'd rather sacrifice the miledge to not go so slow. I thought about lowering it back to stock height, but I figured the 1 inch wouldn't make a noticable difference, am I wrong? I have offset wheels, and the tires rub the flares if it is at stock height, so I would have to go with smaller tires. The smaller tires and lower height would sure look worse.
 
I lost ~1 mpg adding a ARB bumper (stock height) so yes, it makes a difference.
 
Jackhill442 said:
Yeah, but I'd rather sacrifice the miledge to not go so slow. I thought about lowering it back to stock height, but I figured the 1 inch wouldn't make a noticable difference, am I wrong? I have offset wheels, and the tires rub the flares if it is at stock height, so I would have to go with smaller tires. The smaller tires and lower height would sure look worse.
Click to expand...

It takes more energy to accelerate the larger tires... stock height and stock size tires will help your mileage. Additionally, having them stick out wider increases your frontal area and adds drag.

Its all a trade-off... fuel efficient doesn't look cool.
 
I do a lot of highway driving, and I find if I keep my 93' Sport below or on 2000 rpm, my milage is really good. There's less wind and road noise, and at those low revs, very little stress is on the engine. Granted, in top gear, it's only about 60 mph, but most of the speed zones on I-95 down here in S.E Florida are construction zones, so 55 is the max "legal" limit.

On a longer drive to work (say, 60 or 70 miles one way), it might take an extra 5 or 10 minutes to get there, as opposed to doing 70 + mph, so I just take that into account and leave a couple of minutes earlier.
 
Tires with greater offset increase the frontal profile, and that makes the drag worse.

So does the lift. EVERY BIT of lift makes it worse, even 1 inch. (Hence why you see race cars with air dams and skirts on them--less drag.)
 
I once got 22-23 mpg on my old 88 XJ with 30x9.5x15 tires earlier this year. However, it does have a 5 speed manual tranny which probably makes a lot of difference.

My newer 00 XJ Classic with auto trans and 235/70/16 tires got 21 mpg once but normally gets 18-19 mpg.

Personally, only a fool would drive 55 on an interstate with a legal limit of 75. That much slower than the rest of traffic could cost you a hell of a lot more than a loss in gas mileage.
 
AZ Jeff said:
Based on what you say above, the following should help:

1. SLOW DOWN. You will get NOTICEABLY better mileage at 65MPH rather than 75MPH. (Remember, the power required to push the car thru the air goes up as the SQUARE of the speed, and at 75MPH, most of the power you are using is in pushing the air out of the way.)
QUOTE]
Let me clarify my point a bit about speed versus fuel economy:

If you slow down to 65MPH from 75MPH, you need only 75% of the power to push the car thru the air that you did at the higher speed. YES, you read that right, only 3/4 as much.

So.....by slowing down to 65MPH, your 19-20MPG should go up to somewhere between 23-24MPG, assuming no other changes.

Now if 65MPH is "too slow" as others have bemoaned, well then be prepared to pay the price in high fuel costs.

There is no such thing as a free lunch when it comes to fuel economy. I learned that 30 years ago in my Internal Combustion engine design class in college.
Click to expand...
 
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