Collection of MPG Fuel Milage Tips

[Grahame]

There is a ton of great information buried in threads containing tips regarding maximizing fuel economy. I have scraped through the forum and collected everything I found into this document. Hopefully this helps any future user running a search.

If you have experienced a sudden or gradual decrease in your fuel economy go through the following list and hopefully you make some gains. At this point, 2017, it is fairly well accepted that the average mpg for the 3.7 KJ is 14-16 City and 21-25 Highway*.

The List:

1: Driveline
-Jack up the Jeep.
-Spin each tire by hand and check for resistance.
-Front tires should have similar resistance to each other, back tires should have similar resistance to each other.
If you experience excessive resistance on one tire over another check for:
-Seized caliper
-Seized Parking brake cable
-Other driveline issues such as rear differential bearings, front wheel bearings, etc.

2. Consumables:
Spark Plugs - Remove and inspect plug condition and gap, proper gap will be on the emissions sticker under the hood. If plugs need replacement grab a set of cheap copper plugs and gap them to the specifications on the emissions sticker under the hood.
Fuel filter - Replace if past recommended service interval, check your service manual for this timeline.

3. Air Intake System:
Throttle Body - Check for carbon deposits and clean with: 02-05 Throttle Body Cleaner, 06-07 Soapy Water
Idle Air Control (IAC) (It's mounted to the throttle body) - Check for carbon deposits and clean with Throttle Body Cleaner
Pollution Control Valve (PCV) - Remove, inspect and clean or replace if cleaning is impossible.
Small sponge filter in the airbox - Change or clean it with some gasoline.


4. Recommended* Driving Speeds in Miles Per Hour:
40-45: City Optimal
60-65: Highway Optimal
70: Highway Sub-Optimal
80+: You can watch the fuel needle drop about as fast as you are moving down the highway.


Additives:

A synthetic cleaner works best.
Set up the cleaning rig as though your engine is hooked up to an IV drip line like they give patients in hospitals.
Hook one end of the IV drip line into a vacuum line that is located as close to the throttle body as possible.
Poke a hole in the side of the drip line right before where you have attached it to the vacuum line, this is to atomize the cleaner.
The cleaner you have chosen goes at the other end of the IV line
Start the engine and keep RPM between 1200-1400
Begin inducing cleaner into the vacuum line at a rate of one drop per second, it should take about 30-45 minutes to consume 16oz of cleaner.

*Based on averaging out numbers reported by forum users posted over a 15 year period, individual KJs will perform differently.

 

[GunnerSchenck]

Shoot I drive everywhere I go between 60-80mph and it uses about the same gas as if I drive 50. It's just learning how to hold steady speeds while keeping your rpms as low as possible...
Only edit I would suggest is where you state to clean the throttle body with throttle body cleaner/carb cleaner.. this procedure shouldn't be used for 06-07 with drive-by-wire.
Only soapy water should be used to clean the throttle body for those years as they have a special coating on them and carb cleaner will ruin it.

 

[tjkj2002]

T
Air Filter - Inspect and replace if very dirty.False for any vehicle that has OBDII diagnostic software,a dirty air filter will actually increase mpg's but decrease performance


4. Recommended Driving Speeds:
40-45Mph: City Optimalfalse,obey posted speed limits,your going to get bad mpg's in city driving no matter what unless your driving a hybrid
60-65mph: Highway Optimal again false.most newer vehicles are optimized for 70+ mph
70 Mph: Highway Sub-Optimal Again obey posted speed limits and stop ******* other off
80+Mph: You can watch the fuel needle drop about as fast as you are moving down the highway.Posted speed limit in many states now,get out of my way


Additives:

-Sea Foam
-Marvel Mystery Oil (MMO).


Sea Foam:
The following is a procedure I was introduced to around 2005 and I have used it on many vehicle engines including my own 2002 Liberty 3.7L.


Pour 1/3 can into a safe container with an open top, like a glass tumbler or measuring cup.
Find a vacuum line that feeds into the throttle body or intake so that the liquid you will be inducing to this line will reach all cylinders.
Remove vacuum line from engine and attach a similar diameter hose to where you removed the original line.
Have a friend start the engine and rev it to 2000-2500 RPM
With the hose you just attached to the intake line, take the other end of it and "skim" it along the top of the Sea Foam that is in your open container. (Kind of like if you were to suck water out of a bathtub using a central vac hose.)
The engine will shudder and be a little angry with you, this is where your friend keeping it at 2000-2500 rpm comes in handy.
Once your engine has consumed all the Sea Foam in your container (1/3 of the can), turn off the ignition.
Wait 5 minutes
Turn engine back on and watch the smoke pour out of the tailpipe.
Blue/Black smoke = LOTS of carbon was removed
White smoke = Some carbon removed.
Pour 1/3 can of Sea Foam into Gas Tank at next FULL fill up.
If you plan on changing the oil within the next 250-300 miles pour the remaining 1/3 can in the crankcase, then change the oil & filter after 250-300 miles.
if you do this your chancing hydrolocking the engine.Seafoam is a terrible cleaner for intakes and damages plastic(which our intakes are made out of),a synthetic cleaner works better and the correct method is using a medical IV drip line with a hole poked in the very end before going into the intake.You want a drip every second and the hole in the line before entering the intake is to atomize the cleaner.Engine should be between 1200-1400rpm's and should take 30-45 mins to suck in 16oz and you must use a vacuum line at close to the throttle body as possible. Plus smoke ='s bad as the cleaner didn't clean anything at all and is buring off inside the cat's damaging them.

Blue/black smoke means your burning oil and that is it,white smoke means your burning the actual cleaner or coolant. Neither indicates you cleaned anything in the intake or engine,but you did clean out your wallet for nothing.


Marvel Mystery Oil (MMO).
Add it to a full tank of fuel, or
Remove air intake hose from Throttle Body and slowly pour into Throttle Body until engine stalls out.
Let engine sit, off, for 5 minutes.
Start engine and watch the smoke show pour out of the tailpipe.
If you do this your just a idiot,sorry but you are and not sugar coating it at all.

From a 21 year ASE Master Tech with L1/L2 also.

 

[dude1116]

65 is about optimal at my Jeep. I used to be able to average 22 @ 65 and 21 @ 70

 

[tjkj2002]

65 is about optimal at my Jeep. I used to be able to average 22 @ 65 and 21 @ 70

Before the SFA swap I got just shy of 22mpg's(21.9) at 78mph with a KJ that weighed 5500lbs with 4.10 gears and 32" tires.

At 65mph I would get 12mpg's since the trans would constantly "hunt" for the correct gear so I was constantly switching from 4000rpm's to 2000rpm's and then back to 4000rpm's.

 

[dude1116]

Before the SFA swap I got just shy of 22mpg's(21.9) at 78mph with a KJ that weighed 5500lbs with 4.10 gears and 32" tires.

At 65mph I would get 12mpg's since the trans would constantly "hunt" for the correct gear so I was constantly switching from 4000rpm's to 2000rpm's and then back to 4000rpm's.

See...sounds like yours did with 4.10s and 32s what mine did at 50-55 with 3.73s and 29s.

Every Jeep is different I suppose.

 

[uss2defiant]

buy a car

 

[Dave]

buy a car

Yeah, any car will beat the crap out of a 4,200lb+ lifted pig of a Jeep with oversize tires like mine.

Hey, as tjkj said...."I didn't build it for the gas mileage".

Dave

 

[tommudd]

04, 32s and 4.10s 76-77 ( around that ) best mpg
03, 31s and 4,10s 73-75 ( same around that speed ) best mpg

 

[GunnerSchenck]

See...sounds like yours did with 4.10s and 32s what mine did at 50-55 with 3.73s and 29s.

Every Jeep is different I suppose.

That's where mine hunts stock.. especially trying to do 55 up a hill.. I'm either very slowly losing speed at 1800rpm or quickly gaining speed at almost 3k rpms.. then it will shift and the gear hunt continues.. so I just do 65 up the hills and avoid the whole thing

 

[Dave]

That's where mine hunts stock.. especially trying to do 55 up a hill.. I'm either very slowly losing speed at 1800rpm or quickly gaining speed at almost 3k rpms.. then it will shift and the gear hunt continues.. so I just do 65 up the hills and avoid the whole thing

Just lock out O/D. It will keep your rpm's up and eliminate all that shifting around which just heats up your ******. Then when you are past that area, push the button again and ba-da-bing, you are back to O/D with a cooler and happier ******.

Dave

 

[Grahame]

Air Filter -False for any vehicle that has OBDII diagnostic software,a dirty air filter will actually increase mpg's but decrease performance

I removed this suggestion.

false,obey posted speed limits,your going to get bad mpg's in city driving no matter what unless your driving a hybrid

60-65mph: Highway Optimal again false.most newer vehicles are optimized for 70+ mph

These city/highway numbers are derived from me averaging out 100+ posts by multiple users across several Jeep KJ forums posted over 15 years. Real world reporting averaged out using a very large data set, as has been done here, is a very reliable source of information. This included peoples posted photos of their after-market devices showing averages and instant MPG readings.

Seafoam is a terrible cleaner for intakes and damages plastic

Edited section regarding additives based on your suggestions.

Only edit I would suggest is where you state to clean the throttle body with throttle body cleaner/carb cleaner.. this procedure shouldn't be used for 06-07 with drive-by-wire.
Only soapy water should be used to clean the throttle body for those years as they have a special coating on them and carb cleaner will ruin it.

Done.

 

[tjkj2002]

This included peoples posted photos of their after-market devices showing averages and instant MPG readings.

Which all are notoriously known to be very inaccurate.

Mine says I'm doing a average of 32mpg city and 50mpg highway which is impossible for my KJ,by actual hand calculation and GPS verified distance I average 6mpg city and 18mpg highway. Imagine a 6400lbs SFA'd KJ on 35" tires getting 50mpg's?

Some vehicles respond well to slower speeds and increased mpg's(big V8's mostly),KJ's with the 3.7 V6 does not,they like higher rpm's which in return results in higher speeds.

 

[Grahame]

Which all are notoriously known to be very inaccurate.

Mine says I'm doing a average of 32mpg city and 50mpg highway which is impossible for my KJ,by actual hand calculation and GPS verified distance I average 6mpg city and 18mpg highway. Imagine a 6400lbs SFA'd KJ on 35" tires getting 50mpg's?

Some vehicles respond well to slower speeds and increased mpg's(big V8's mostly),KJ's with the 3.7 V6 does not,they like higher rpm's which in return results in higher speeds.

That is awesome information! Of course you have a unique situation that doesn't fit the averages. However averages derived from a large data set is a more accurate picture than a single data point.

I am about to take my 02 stock KJ w 3.7 about 3700 miles on highway driving. Should I run several tests over several tanks at different speeds and then decide the average is a good picture of how the engine is performing, or should I lock it at 70MPH the entire time and then post that number as gospel?

 

[GunnerSchenck]

That is awesome information! Of course you have a unique situation that doesn't fit the averages. However averages derived from a large data set is a more accurate picture than a single data point.

I am about to take my 02 stock KJ w 3.7 about 3700 miles on highway driving. Should I run several tests over several tanks at different speeds and then decide the average is a good picture of how the engine is performing, or should I lock it at 70MPH the entire time and then post that number as gospel?

Those average numbers are only as accurate as the person providing them is trustworthy. It's the internet, people lie. Most of us on here use ScanGauges, which provide true mpg's based on the information provided from your computer via the obdII port. Same numbers a scan tool would read. Not just roughly provide a generally accurate number by filling up, driving a certain distance and hoping the equation provides an accurate output to judge our mpgs.
If you're talking abut a site like "fuelly".. they're liars. There's people on there with the same mustang I have saying they're pulling 30 city and 40 highway. Bs. Best I've ever averaged is 27 and I used every bit of road to my advantage.

Averages derived from a large data set are more accurate that a single data point... if the data being provided are all verifiable and accurate. All it takes with a large data set is for a few people to lie and say they got an unbelievable mpg scenario for the "average" to raise above what it truly is.

If they all use scan gauges, and not a "mpg calculator" then, and only then, is it irrefutable. As of now, it's not verifiable information.

 

[Dave]

That is awesome information! Of course you have a unique situation that doesn't fit the averages. However averages derived from a large data set is a more accurate picture than a single data point.

I am about to take my 02 stock KJ w 3.7 about 3700 miles on highway driving. Should I run several tests over several tanks at different speeds and then decide the average is a good picture of how the engine is performing, or should I lock it at 70MPH the entire time and then post that number as gospel?

If it were me I would drive it normally and check it. Everyone is going to have a little different gas mileage anyway. Some are stock. Some are lifted. Some with bigger tires, Some with gear changes, some without, etc.

For a 3,700 mile trip I would change the plugs, air filter and pcv and oil change and check tire pressures and just go. I would reset my EVIC and keep a manual record also. But that's me.

Dave

 

[Grahame]

Those average numbers are only as accurate as the person providing them is trustworthy. It's the internet, people lie. ... Averages derived from a large data set are more accurate that a single data point... if the data being provided are all verifiable and accurate.

All it takes with a large data set is for a few people to lie and say they got an unbelievable mpg scenario for the "average" to raise above what it truly is.

If they all use scan gauges, and not a "mpg calculator" then, and only then, is it irrefutable. As of now, it's not verifiable information.

You are correct, data outliers will skew averages up or down depending. I ignored any data points that were large outliers from the main groupings and given my inability to verify anything anyone on anonymous forums posts I chose the numbers I did based on what I thought seemed reasonable. I didn't run them by anyone else before I posted them and perhaps that was a mistake.

My intention was to post something that people searching about MPG would stumble across, thus saving them the hours of research I put into finding all the information in my first post. I didn't factor in that people might infer from my post that the Recommended Driving Speeds would be accurate for all KJs in existence.

What would you suggest would be better for these numbers? Should I remove that section entirely? I have added a disclaimer (*) to that section as a result of the good information gleaned from this thread, do you feel that is inadequate?

 

[GunnerSchenck]

No, that's good. I wasn't trying to say it was a bad gesture, just that everyone online wants to pretend they're getting better gas mileage than is true. If you found those same people who post high averages and managed to ask them face to face for a ride to see their mileage , they'd probably stumble through their next sentence lol

 

[tjkj2002]

No, that's good. I wasn't trying to say it was a bad gesture, just that everyone online wants to pretend they're getting better gas mileage than is true. If you found those same people who post high averages and managed to ask them face to face for a ride to see their mileage , they'd probably stumble through their next sentence lol

Or You could be like me and just don't care what kind of mpg's I get,more so on long trips since averaging 3mpg's on the highway still is cheaper then most plane tickets(to include baggage fees and such) and the hassle involved by flying.

Yes I track them since a sudden drop for no reason is a sign of something not right,very helpful diagnostic tool.

 

[GunnerSchenck]

All I can say is chances are nobody on here bought a liberty for its gas mileage.