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jeepgcoman 03-13-2012 06:32 PM

Gas Prices and Fuel Economy....Again!!
Almost every time fuel prices rise to the outrageous, threads like this pop up. So after I express my "opinions", please feel free to add your handy hints on how to improve your fuel economy. Since our WKís are 4700 lb. barn doors (drag coefficient ~.041) going down the highway, itís always going to be an uphill (pun intended) battle. After I repeat some of the more obvious ones, I am going to suggest a couple ideas for the more adventurous and those mechanically inclined experimenters and tinkerers, like me. Bear in mind that, aside from the usual, the goal should be to build/mod. the intake/exhaust/engine to increase volumetric efficiency (VE) to maximize power (hp/torque) output in the RPM range where the engine spends most of itís time and, in the process, try to achieve a long, flat torque curve in that range. For most that should be ~1500-3000 RPM. There are others, but some important items to address would be vehicle weight (power to weight ratio, or lbs. per hp), lower center of mass, larger wheels/tires, if the engine has the torque to support them, and gearing, again if engine torque supports it. Hereís a good reference to look at for more information on improving fuel economy: Anyway, here are some of the more obvious, followed by the "tinkererís delight:
  1. Aerodynamics-any racks, rack crossbars, lights, or anything that increases drag coefficient should be removed, unless frequently used.
  2. Vehicle weight-Lose all excess items that arenít needed on a regular basis.
  3. Put an egg under your right foot, between it and the "go fast" pedal. I have a real problem with this.
  4. Make sure your tires are ALWAYS properly inflated. Be aware that the "recommended " pressure is probably not the optimum pressure for fuel economy.
  5. Front end alignment-Consider having it aligned in the configuration it is normally driven in. I always have my vehicles aligned with the wife and I sitting in it, as this is its normal configuration.
  6. Use the proper weight oil. Also consider using a good synthetic as, in general, they seem to work a little betterÖ least in mine.
  7. Engine tune is a biggie. If you donít know when it was last tuned, it probably needs it. Many of our WKís are sensitive to the type of plugs that are used, so use the recommended type. If it has spark plug wires (like the 05 Hemi), check the resistance of each one. If questionable, ask about it, but more likely just replace them. Also, do a search on EGR Valve, MAP and O2 Sensor issues. If youíre fuel economy sucks and you donít know the status of these items, either have them tested or replace them. On any of them, Iíd recommend just replacing them if the vehicle has over ~65-75K on it and you donít know the status.
  8. Check for excessive "rolling" resistanceÖtight/bad wheel bearings, dragging brakes, malfunctioning emergency brake, etc. 4WD vehicles have some parasitic drivetrain loss anyway.
  9. Speed-DONíT!! Just leave earlier. The "sweet spot" on most of our stock WKís is ~60-65 mph, or ~1700-2100 RPM. The faster you drive above ~60-65 mph, the worse (much) your economy.
The following is not for the faint of heart, and results may vary. No matter which engine you have in your stock WK the maximum hp is developed somewhere between ~5000-5600 RPM, and the maximum torque is developed between ~3500-4500 RPM. Since torque is what gets (and keeps) the vehicle moving, some folks that are willing to experiment may want to consider advancing the camshaft timing. Advancing lowers the power curve RPM range, making the engine work easier and deliver its power and better economy in the range that is closer to where it is driven most. There is a tradeoff though, as your top end (say above ~4500 RPM) will start to suffer. As an example, when I recently installed new aftermarket camshafts in both my Hemi vehicles, I advanced them both, even though the new camshafts had 4 degrees of advance already ground into them. On my WK I advanced it 2 degrees, for a total of 6, and on my LX, I advanced it 6 degrees, for a total of 10. The WK still runs very strong all the way up to 6200 RPM, but lacks a little on the low end. On the other hand the LX power curve really lowered to the point that anything above ~5800 RPM (stock Hemi shiftpoint) is pretty much pointless. From ~1500-4500 RPM it runs like a banchee. If I had it to do over again, Iíd have advanced the nearly 700 lb. heavier WK 6 degrees also. For info. sake, doing a cam advance should cost no more than ~$400, depending if youíre installing an adjustable timing set or advancing the gear(s).

I am now starting to tinker with transmission upshift/downshift schedules, graphing and loading different schedules the way I think will deliver the best all around results, in power transmission, driveability and fuel economy. Itís kind of fun and relatively easy with the Superchips Tuner Trans. App.

Knightrider03m 03-13-2012 11:20 PM

Re: Gas Prices and Fuel Economy....Again!!
I've tried not to speed...... and not to drive faster from a red light. However I just can't do it. That is why my Escape gets 19mpg now LOL

FlyinRyan 03-14-2012 09:17 AM

Re: Gas Prices and Fuel Economy....Again!!
Fuel trims tell a pretty compelling story. I've found that that the PCM tends to chase stoich whenever the fuel trims are other than zero. So instead of 14.7 (or really 14.1-14.4 is ideal stoich for E10 pump gas), + trims yield somewhere around 14.7-15.0 and - trims yield something like 13.8-14.3 . I usually tune for 0- to -5%, which may end up in slightly less mileage BUT better drivability, more torque (due to richer AFR), and tends to calm down cam surge. In some cases this may lead to no change in mileage because the engine is doing less "work" to propel the weight loaded upon it.

LE30vm 03-14-2012 09:45 AM

Re: Gas Prices and Fuel Economy....Again!!
What is considered normal MPG on a 3.7 4x4 WK? I am getting 13.7-14.2mpg....

jeepgcoman 03-14-2012 05:32 PM

Re: Gas Prices and Fuel Economy....Again!!
Knightrider, trying not to hot rod with a slightly modified 5.7 is probably much worse. I am like you.....can't resist, and don't know if I'll ever outgrow it. I hope not.

Escape 03-14-2012 06:35 PM

Re: Gas Prices and Fuel Economy....Again!!
I get 9.1 average MPG using 93+ octane

Your stock engines are very efficient as is.

Instead of trying to get better MPG from this heavy non-aerodynamic brick, that every year gets more and more safety features and creature comforts which equals more weight.

We should be looking to the root of the problem!

We should ask what happened to the value of the dollar?

We should ask why did we open up more federal land for drilling when we export 3X what we use in gasoline right now?

We should ask why if domestic demand is down why are gas prices up?

We should ask why gas prices are up 105% in the last 3 yrs.

We should ask why if someone thinks someone is going to fart in the middle east (doesn't have to even happen) gas prices jump when we have reduced our foreign oil dependance from roughly 60% to 45%? Canada is #1 double that of Mexico and Saudi combined, which are #2 and 3 respectively. Then we have Venezuela, Russia and Nigera which combined still do not equal Canada. In 7th is Iraq with less than 5% of our oil imported from them. All other imports per country of origin are in the 1% or less range.

We should ask why there have been no new refineries built in the last 30 yrs?

jeepgcoman 03-14-2012 07:25 PM

Re: Gas Prices and Fuel Economy....Again!!

Knightrider03m 03-14-2012 08:34 PM

Re: Gas Prices and Fuel Economy....Again!!

Originally Posted by jeepgcoman (Post 593696)
Knightrider, trying not to hot rod with a slightly modified 5.7 is probably much worse. I am like you.....can't resist, and don't know if I'll ever outgrow it. I hope not.

I'm guessing that I will outgrow it when I get older. Although I have found myself driving a bit more slower as of late. But I am on vacation from my job so that could be the reason.

Escape 03-14-2012 08:52 PM

Re: Gas Prices and Fuel Economy....Again!!
I usually drive the speed limit during the day and 5 under at night. But if I feel the need or have to get out of the way fast I know the power is there.

Just because it will go fast doesn't mean you have to go fast.

I'd rather have it and not need it , then need it and not have it.

jeepgcoman 03-16-2012 12:11 PM

Re: Gas Prices and Fuel Economy....Again!!
Knight, don't hold your breath. I am already (much) older, and it hasn't slowed me down much from when I was a "real" kid. Still like to go to the races and really like the horsepower kick in the seat of the pants.

Don't get me wrong, I do drive the highways safely at 55-65 most of the time, but still like the occassional blast from ~60-90 or so, conditions permitting. Lots of wide open highway where I am. In town I also drive at or under the speed limit.....most of the time. Again, I like the blast from 0 or 5 up through the 1-2 shift, again conditions permitting. I echo Escape's feeling about having it and not needing it rather than needing it and not having it available.

SweetZombieJesus 05-25-2012 10:00 AM

Re: Gas Prices and Fuel Economy....Again!!
I have to say, for some perspective, I went from a 2002 KJ with the 3.7L V6 (which now belongs to the wife) getting 18 mpg highway -- into an 09 WK 5.7L hemi and I just got 21.2 mpg highway on a long ride. A heavier, bigger vehicle with a bigger more powerful engine (almost twice the hp/torque) in full time AWD (the KJ had Command Trac so part time only) and I'm getting better mileage. In 7 model years. Amazing. I appreciate this little nugget. Yes, around town is still abysmal ;)

Yeah, not a Prius, but I knew what I was getting into.

jeepgcoman 05-25-2012 03:33 PM

Re: Gas Prices and Fuel Economy....Again!!
Sweet, a lot of that goes back to the power to weight ratio. Your Hemi kind of loafs along to get and keep the mass moving, while the 3.7 has to work much harder to get and keep it moving. Of course, there is a limit to this also. If you go "too high performance" you can start going the other way on economy. I tried to mod. mine for low end (~1500-3000 RPM) performance, which is where I drive it most of the time. I think I could have gone a little farther though, and improved it a little more yet.

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