Improving the JK fuel efficiency?

[Mel.Specs]

True. However I got very similar numbers with my 07 (moded out, but had 3.21 gears) if I kept it to 100/60 (and actually got better numbers on several occasions). But then I was under geared then, while I'm over geared now. But that was more to point out the vast differences speed plays into the mileage.

I'm not disputing speed differences, I agree with you on that. Driving style is a huge factor. The advertised numbers between the two V6's was my point. That's why with the Hemi swap you get the same if not better MPG and be able to go faster. Maybe even go double the speed and get the same MPG as either V6's...........

That does it, I'm getting a Hemi. Thanks!

 

[Dan Grec]

That's why with the Hemi swap you get the same if not better MPG and be able to go faster. Maybe even go double the speed and get the same MPG as either V6's...........

If you read through the AEV forums, you'll see nobody is getting better mileage in any Hemi than a 2012 3.6

People seem to think a bigger engine somehow magically gets better mileage - it's just not true.
The limiting factor of all engines is the volumetric efficiency - that is - getting enough air into and out of the engine for a given rpm.
That's why we have things like Variable Valve Timing, variable inlet runners and exhaust runners - it's all about getting the air in and out when required. It's easy to build/tune and engine to do it well for a single rpm, but as we know, we drive out engines from idle to 5000 rpm, which is the difficult part.

Diesels are better at mileage because their higher compression ratio simply means you get more energy out of the same amount of combustible fuel. (more bang for your buck)
Also turbos have an impact on all this, because they are good at cramming air in no matter what rpm.

-Dan

 

[The Swiss]

... Apart from removing any of the above components...

Disclosure: I have read your comment , but ...

... I still have to state the obvious: You added a lot of cool looking stuff to your Jeep which added weight and significantly worsened the already less than ideal aerodynamics. My own experience; just removing the stock air dam below the bumper decreased my mileage significantly.

So, besides making sure your tire pressure is well adapted and that you are wearing some lite shoes, take a good look at your Jeep and think what accessories you could easily remove without jeopardizing the coolness factor of your Jeep. Most of all: Think really hard if you really need that rack for that trip. I know, not what you wanted to hear. Don't waste your money in K&N drop in filters, chips, programmers (they do make the engine more responsive, but in my experience don't help fuel mileage). Same for higher octane fuel; again in my experience, fuel mileage does not increases to offset the higher cost of the better gas.

That Pentastar V6 also has an advantage over the 3.8L.

I agree; my calculated fuel mileage is on average about 2 mpg better with the Pentastar than with the old 3.8. Both Jeeps are set up identically, close to stock JKU Rubicons with a 2"lift.

 

[Deleted member 48574]

Driving style has a huge impact over a Rack, in my experience.

If I keep my JK 2 dr with a rack and RTT to around 2k RPM, it sips the fuel quite comfortable at around 11l/100km. On the flat, that puts me about 105 kph.

If I push it up to 115 kph, I'm pulling 2400 to 2500 RPM. If you think about it, your engine is going through 25% more revolutions at 2500 RPM versus 2000 RPM. That uses a ton of gas and I found very quickly I was up around 14l/100 km.

The trip I tracked these numbers on was from Edmonton to Vancouver.


I used to think "Big deal, whats a few more liters per hundred?". Then I did some math after my latest trip:

Edmonton - Vancouver - Banff - Edmonton = 2418 km. Google says that should take 29 hours at the speed limit.

At 11L/100KM, I used 265 liters. Gas in Alberta is cheap, in BC it's expensive, so for this example (and ease of math) let's use $1.40 per litre. That means the trip costs about 371 dollars. I happen to know this is actually pretty accurate given my credit card bill.

Now, the above cost is sticking to the speed limit -- the trip would have cost me 371 bucks and taken 29 hours.



Quick math -- 2418 KM divided by 29 hours = 84 km/hr average. Lets say we up that by 10 km/hr and my consumption is 14 l/100 km*


at 14L/100 KM I used 338.52 liters. With the above gas price, that means I would have spent $473.34 in gas. Now for time, averaging that extra speed, the trip would have taken me 26 hours.

So, 3 hours difference in travel time, but I would spend nearly 100 dollars more in Gas. That's the equivalent of getting paid $33.00 per hour if it were my day job.

My next trip to anaheim and back is twice the distance. So an extra 6 hours of driving = an extra $200 dollars in my pocket -- that's halfway to a nice Warn winch!


* a 10 KM/HR increase in average speed maybe generous or conservative -- I suspect generous; I'm basing that off of some calculations I took on my trip with a 10km/hr over the limit = an extra 500 RPM on highway 2 and on the Trans Canada. I would suspect that really pushing the engine up around 2500 RPM would net you a real world average of less then 10 km/hr over the limit, thus further reducing the amount of time saved by driving that speed, and therefore making it even MORE of a marginal gain due to speeding.

Now, be kind -- I haven't done math long hand since High School, which much of this is, so I may have made an error or 4


End of the story: If you can afford the extra few dollars and your time is worth 30 bucks an hour, Great. Me, I'll just drive slower

Cheers
Craig

 

[JKJenn]

Disclosure: I have read your comment , but ...

... I still have to state the obvious: You added a lot of cool looking stuff to your Jeep which added weight and significantly worsened the already less than ideal aerodynamics. My own experience; just removing the stock air dam below the bumper decreased my mileage significantly.

So, besides making sure your tire pressure is well adapted and that you are wearing some lite shoes, take a good look at your Jeep and think what accessories you could easily remove without jeopardizing the coolness factor of your Jeep. Most of all: Think really hard if you really need that rack for that trip. I know, not what you wanted to hear. Don't waste your money in K&N drop in filters, chips, programmers (they do make the engine more responsive, but in my experience don't help fuel mileage). Same for higher octane fuel; again in my experience, fuel mileage does not increases to offset the higher cost of the better gas.

I agree; my calculated fuel mileage is on average about 2 mpg better with the Pentastar than with the old 3.8. Both Jeeps are set up identically, close to stock JKU Rubicons with a 2"lift.

Actually, I didn't add any of that stuff for the cool factor. Winch is to get myself out of trouble because I anticipate being out alone many times, the bumper if for the winch and protection. The roof rack is for storage and RTT. The RTT is so I can get out to photo locations that I have not been able to access. I don't camp on the ground and this was the cheapest route to go. So, again, I don't see a way around the extra weight at the moment. Maybe in the future I will have some spare cash and can get a trailer, lose the rack, and mount the tent on the trailer.

Driving style has a huge impact over a Rack, in my experience.

If I keep my JK 2 dr with a rack and RTT to around 2k RPM, it sips the fuel quite comfortable at around 11l/100km. On the flat, that puts me about 105 kph.

If I push it up to 115 kph, I'm pulling 2400 to 2500 RPM. If you think about it, your engine is going through 25% more revolutions at 2500 RPM versus 2000 RPM. That uses a ton of gas and I found very quickly I was up around 14l/100 km.

The trip I tracked these numbers on was from Edmonton to Vancouver.


I used to think "Big deal, whats a few more liters per hundred?". Then I did some math after my latest trip:

Edmonton - Vancouver - Banff - Edmonton = 2418 km. Google says that should take 29 hours at the speed limit.

At 11L/100KM, I used 265 liters. Gas in Alberta is cheap, in BC it's expensive, so for this example (and ease of math) let's use $1.40 per litre. That means the trip costs about 371 dollars. I happen to know this is actually pretty accurate given my credit card bill.

Now, the above cost is sticking to the speed limit -- the trip would have cost me 371 bucks and taken 29 hours.



Quick math -- 2418 KM divided by 29 hours = 84 km/hr average. Lets say we up that by 10 km/hr and my consumption is 14 l/100 km*


at 14L/100 KM I used 338.52 liters. With the above gas price, that means I would have spent $473.34 in gas. Now for time, averaging that extra speed, the trip would have taken me 26 hours.

So, 3 hours difference in travel time, but I would spend nearly 100 dollars more in Gas. That's the equivalent of getting paid $33.00 per hour if it were my day job.

My next trip to anaheim and back is twice the distance. So an extra 6 hours of driving = an extra $600 dollars in my pocket -- that's halfway to a nice Warn winch!


* a 10 KM/HR increase in average speed maybe generous or conservative -- I suspect generous; I'm basing that off of some calculations I took on my trip with a 10km/hr over the limit = an extra 500 RPM on highway 2 and on the Trans Canada. I would suspect that really pushing the engine up around 2500 RPM would net you a real world average of less then 10 km/hr over the limit, thus further reducing the amount of time saved by driving that speed, and therefore making it even MORE of a marginal gain due to speeding.

Now, be kind -- I haven't done math long hand since High School, which much of this is, so I may have made an error or 4


End of the story: If you can afford the extra few dollars and your time is worth 30 bucks an hour, Great. Me, I'll just drive slower

Cheers
Craig

Craig, thanks for the explanation - it actually made a lot of sense. I do try my best to stick to cruise control, but usually drive around 70 on the highway...I will try to pull it back some and do some more research into tire pressure.

 

[JIMBO]

:sombrero: You obviously have the 3.8L V6/42rle auto/JKU there is only one MAJOR way of increaseing gas mileage/w added tire size/weight and overhead rack--

That is regearing to a 4.88/5.13 or 5.38 diff gears and the cost of this job will overshadow the increase in mileage--however

By regearing you will increase gas mileage 10/20%, improve throttle response and give your JKU added (applicable) power that you never had as stock--

The cost wil be somewhere between $1000 and $3000 bucks, depending on who doesit and what else you may do-ie-lockers/LSD/Ball joints-

Good luck

:costumed-smiley-007:wings: JIMBO

 

[Deleted member 48574]

Craig, thanks for the explanation - it actually made a lot of sense. I do try my best to stick to cruise control, but usually drive around 70 on the highway...I will try to pull it back some and do some more research into tire pressure.

No problem! Another observation - I drive a manual, and I found that Cruise Control really hurts my mileage in hilly country. I figure the reason is the jeep is constantly trying to keep up the speed, which means higher RPM, and the cruise is always responding and reacting. If I take cruise off, and I see a hill in the distance, I can creep my speed up very gradually and get a bit of a run at it. This means lower RPM on the climb, and the momentum from the flat bit previously keeps my speed up for me. So it's more proactive rather then cruise control which is reacting to a speed drop. It may not have the same results in an auto but it really depends on shift points.

Cheers
Craig

 

[Dan Grec]

No problem! Another observation - I drive a manual, and I found that Cruise Control really hurts my mileage in hilly country. I figure the reason is the jeep is constantly trying to keep up the speed, which means higher RPM, and the cruise is always responding and reacting. If I take cruise off, and I see a hill in the distance, I can creep my speed up very gradually and get a bit of a run at it. This means lower RPM on the climb, and the momentum from the flat bit previously keeps my speed up for me. So it's more proactive rather then cruise control which is reacting to a speed drop. It may not have the same results in an auto but it really depends on shift points.

Cheers
Craig

Cruise control sucks for gas mileage because it's trying to keep you at a constant speed, no matter the conditions. When going up a hill, as you mentioned, it's best to get a run up, and be willing to go a little slower by the time you get to the top (don't just keep pressing down on the gas). Conversely, when going downhill, you can get a little extra speed then use a fraction less gas for a while until you come back to your previous speed.

Some logic applies to wind and other conditions that vary as you move along.

-Dan

 

[life_w_vagabonds]

Just filled up my '12 JKU Sport w/ 6 speed. I got 23.4 mpg. Much better than my 3.3 Nissan and the Jeep doesn't have 2000 miles on it yet. I'm wondering if further break in will help.

Then again I drive like an old lady. My Nissan I just sold had 130k on the original clutch! I used to drive big trucks, so I try to keep the flow going, its the sudden stops and starts that kill you mpg.

I would have to disagree with the cruise control though. At 60 I am doing about 2000 rpm and under 65 seems to be a good spot for mileage. I like to watch the MPG estimate readout on the dash. There is a reason that 55 used to be the national speed limit.

 

[dugedug]

Just filled up my '12 JKU Sport w/ 6 speed. I got 23.4 mpg. Much better than my 3.3 Nissan and the Jeep doesn't have 2000 miles on it yet. I'm wondering if further break in will help.

Then again I drive like an old lady. My Nissan I just sold had 130k on the original clutch! I used to drive big trucks, so I try to keep the flow going, its the sudden stops and starts that kill you mpg.

I would have to disagree with the cruise control though. At 60 I am doing about 2000 rpm and under 65 seems to be a good spot for mileage. I like to watch the MPG estimate readout on the dash. There is a reason that 55 used to be the national speed limit.

23.4 mpg calculated? <skeptical>

There was a reason for 55, a reason imagined up by the gov. But the results, like most gov sponsored ideas, didn't support it. ('..."independent studies" found a 0.5% total fuel savings')

 

[Deleted member 48574]

It's a lot more complicated then a simple speed limit.

For instance, in my 4.8 L Chevy V8, I cruise 110 km at about 1500 RPM.

In my 3.7 L V6 Jeep, the same speed requires 2400 RPM.

(Both numbers assume a still day -- no wind -- and flat ground).

Which is more efficient?

Well, without a degree in mechanical engineering I couldn't tell you the exact math. But given the Chevy has over a full litre more of displacement, that 1000 RPM by the Jeep means squat -- my Chev gets about 18 - 20 L/100 KM and my Jeep gets 11 - 13 (depending on City vs Highway)

Now back to the real world:

Driving through Montana in February, I was in 5th gear the whole time. The wind was so bad, that being in 6th gear meant that even with my foot to the floor I was decreasing my speed (and for the sake of traffic behind me I was trying to maintain 60 MPH). That can happen anytime -- the gearing wasn't appropriate for the power the engine was putting out. Between power, gearing, and wind resistance, my jeep was a horribly inefficient little vehicle.

The thing is, also, those are fairly new vehicles I cited above. Back before fuel consumption was a concern, they produced things like the 307 V8 for Fullsize Chevy passenger cars. That is about 5 litres displacement. But where my truck puts out a couple hundred horsepower and cruises at 70 MPH at 1500 RPM, the 307 would only put out around 120-150 HP on a good day. Even MORE important, was how that HP was getting to the wheels. My Impala (which is in one million pieces right now) had an old Powerglide Transmission -- 1 reverse gear and 2 forward ones.

So, in High Gear, that old V8 might have revved to 3 or 4 thousand RPM -- or more -- to go 55. Bigger motor by displacement, but it needed to work WAY harder to get the same speed as the modern Vortec engine. The Jeep could do similar speeds to that at about 1400 RPM, which is less then the Truck even. Point being, engine size is only one part of the fuel economy equation -- your whole powertrain has to be considered as improvements in one area can VASTLY override issues in other areas (hence , perhaps, the reason why the next gen jeeps are rumoured to use Fiat's (?) 9 speed transmission).

Don't get me started though on the leaded vs. unleaded debate. The mileage my dad's friends used to get in their Minis on Leaded Gas would make a Hybrid owner turn up their nose (but then, what hybrid owner doesn't turn up their nose to the rest of us ) (no offense just a playful jab!)

Cheers
Craig

 

[brianjwilson]

23.4 mpg calculated? <skeptical>

There was a reason for 55, a reason imagined up by the gov. But the results, like most gov sponsored ideas, didn't support it. ('..."independent studies" found a 0.5% total fuel savings')

I wouldn't be surprised. The first tank on my 12 rubicon six speed was 19 mpg calculated, with a good amount of in town driving.

 

[life_w_vagabonds]

23.4 mpg calculated? <skeptical>

There was a reason for 55, a reason imagined up by the gov. But the results, like most gov sponsored ideas, didn't support it. ('..."independent studies" found a 0.5% total fuel savings')

Don't forget to wear a hat when out in the sun.

Every vehicle I have ever owned got better mileage at a lower speed. My Nissan would drop down to 17 if kept over 70 for extended periods. The OP was asking for MPG tips, not conspiracy theories about the government or my math abilities.

 

[dugedug]

Don't forget to wear a hat when out in the sun.

Every vehicle I have ever owned got better mileage at a lower speed. My Nissan would drop down to 17 if kept over 70 for extended periods. The OP was asking for MPG tips, not conspiracy theories about the government or my math abilities.

No conspiracy theory at all. Facts supported by third party and even gov. funded research proved that lowering the speed limit to 55 produced results that were in the noise floor... Driving 55 in a 70+ zone is not practical today, you do that around here and youre likely to get shot.

How does a vehicles gas mileage decrease while driving extended periods of time at speed after the engine has been brought to operating temperature?

 

[Longtallsally]

I think Craig's explanation a bit back is the best one yet. It really comes down to driving style and where priorities are placed. If you are in a hurry, you'll be paying for it, regardless of how you are equipped. IMO, the rack at higher speeds hurts economy more as it will take more power to cut through the wind the faster you go.

But really, taking the ME (Mechanical Engineering) side of what the "sweet spot" of a motor is depending on its designed purpose, the simple physics are that higher speeds, rabbit starts, and generally speaking higher revs will garner worse economy. Without analyzing to the nth degree the aerodynamic impact of various components as they relate to when the variable cam timing and intake runners come into play with rpms, tire size, and gear ratio... Again, I think Craig said it most succinctly; just slow down a bit. :smiley_drive: