Fuel Milage/Wind Resistance

[JLinder]

Right now my truck is mostly bone stock (see sig) and I hate the milage I'm getting. I drive quite a bit of hwy miles, and I average about 17. 5mpg (calculated by hand). I make a lot of 250-350 mi one way trips for work at about 68-72mph. One thing I noticed is I regularly see about 8-10 lbs of boost on flat ground, and it goes up as soon as my truck sees a hill. I'm lazy and use the cruise a lot, and I know it tries its best to maintain the speed I set it at (lots of fuel when I slow down). What can I do besides slow down? When I added the Lund Lunar visor, I noticed a lot more wind noise in the cab above 65mph so I added a bug guard that seems to have helped some. The other thing I've done it put the flatbed on, and I'm wondering if that has something to do with my milage. I've read that the Dodge body was designed to have the tailgate up for min. wind resistance and I'm wondering if the flatbed is breaking up the wind when it leaves the cab. Could this be part of my problem? Thanks for any insight you might give me.



-Jeremy

 

[azcummins]

My suggestion to all new owners is to drive it hard. When I first got my 1 st RAM, my 1 st tank of gas was "fun"... lots of fast starts, driving like th 18 yr old I was. Got 17. 5 mpg out of a dually. My second tank was "be nice". Took truck up to 2000 rpm, shift. Got 14 mpg. I asked my trucker friends what was wrong, then just said, "gas on it. " Drive it hard: you won't hurt it. You don't have to race off every stop light but take it above 2600 rpm to shift. When do you shift, if you start out in the new gear below 1500, then run it up higher before you shift. Same goes for cruising around - Don't let it get below 1500.



I know it is a bit off subject, but that might be your issue.

 

[JLinder]

So if you take it to easy,

Then your motor might be burning more fuel at a lower rpm than if you kept the revs up? I guess that makes sense. One thing that really has me puzzled, my first tank of fuel I got about 18mpg and it went up from there. Towards the later part of fall/early winter before I had the flatbed and drove so much (commute to work at that point was 11 mi each way) I would usually get around 19-20mpg with city/hwy driving mixed. Now I can go an entire tank of fuel on the interstate, and be lucky to get 18. What's up with that?

 

[duaneparks]

Some of that may be aerodynamic drag, some of it may be the cruise control itself. Keeping a steady speed doesn't always translate to better fuel economy. Constant throttle settings will accomplish that (in most highway cruising instances) better than a cruise control.



Allowing the vehicle to slow down on the upside of a hill, then accelerate on the down side, while keeping the throttle position constant, will do better for fuel consumption, in my experience. Just pick a minimum and maximum allowable speed and roll on or roll off the throttle as you approach those. Not as comfortable as cruise control, but more efficient.



Hope this helps.



Duane

01 Ram 4x4 "Brandy"

 

[JCockrell]

cruise problem

I don't know if this will help. I hardly use the cruise control anymore. The mileage sucked on cruise. I was like you, Jlinder, I got about 19 with cruise and 22 without. Makes sense with the hills and the cruise trying to keep your set speed. For long trips, this sucks, but I'll take the mileage and a sore foot. My . 02, for what its worth. Oh yea, drive it like you stole it.

 

[The Sequel]

I too get a little better mileage with the cruise off. I drive about 50 miles round trip to work every day and can tell a difference in my mileage when I use the cruise vs. when I don't. Remember a year or so back there was a guy on the board (jnutter I think) who did some tests using a stick to hold the pedal in the same spot and was coming up with some pretty amazing mileage figures. If I remember correctly he had one tank at 25 or so mpg. I know he was driving just for mileage but it probably goes to prove the points mentioned above.

 

[roadranger]

Don't use the cruise control.



The big rig drivers are taught to use the boost gauge for fuel economy. Pick a boost setting, say 10 psi, and NEVER exceed it. Sure, you'll lose ground at times in the pack you're running with on the Interstate, but you're looking for fuel economy- NOT beating everyone to the exit ramp.



Oh yeah- don't forget winter blend fuel gets less mileage per gallon than straight #2 diesel.

 

[duaneparks]

Rember a year or so back there was a guy on the board (jnutter I think) who did some tests using a stick to hold the pedal in the same spot and was coming up with some pretty amazing mileage figures.







That's closer to what I was trying to describe. Thanks!



Duane

01 Ram 4x4 "Brandy"

 

[illflem]

I gained 1-3 mpg just by restricting the vacuum hose to the cruise servo with a 1/4" barbed hose coupler full of glue with a pin hole poked though it. The cruse is very slow to respond, much like driving with the pedal in one place but it saves fuel. Did this after noticing the pedal all over the place when driving in cruise into a gusty headwind and getting poor mileage. Not sure if the newer rigs even have a vacuum servo.

 

[Mundgyver]

I just put a bed cover that I built for right at $100 on the truck. It is flush with the top of the bed. Picked up a solid 2 mpg with this experiment. Mileage will get better as your engin breaks in.

 

[JLinder]

Anybody know more about the servo?

I gained 1-3 mpg just by restricting the vacuum hose to the cruise servo with a 1/4" barbed hose coupler full of glue with a pin hole poked though it.

Who knows about this? This might be the ticket. I really hate to drive without cruise because the 24v is so responsive to a small amount of throttle movement, makes maintaining a reasonable speed nearly impossible. Plus I'm lazy.



Illflem- What am I looking for (servo)?

 

[illflem]

Restricting the vacuum line was part of an experiment I was doing last year in an effort to convert the cruise control from constant mph to constant throttle. Never finished the experiment the way I had it planned because just the 25¢ restriction had the desired effect. I posted about it back then, don't if others have tried it. All the restriction does is slow down the cruise response so it isn't constantly moving the throttle. My restricted servo lets my speed vary around 5-7 mph instead of the zero the stock setup did. The vacuum servo is under the drivers side battery on 12 valves. I'm not sure how the cruise/throttle system works on a 24, if it's electronic the PCM would have to be reprogrammed for the same effect.

 

[JLinder]

I searched the archives and it seems in late 2001 the cruise function was moved to the PCM/ECM (not sure which). I'll try and confirm this with a second source. Anyone know if you can have Cummins reprogram the ECM to make the cruise not so hyper? I know they can so that is the bigger motors, so I'm sure they could with the ISB. It just a matter of could they do it to an ISB in our trucks.

 

[Boldt's Wagon]

Illflem's experiment on restricting the vacuum line to the cruise control is about half way down page 3 of:



http://216.235.147.117/forums/showt...rpage=15&highlight=vacuum cruise&pagenumber=3



I tried a similar restriction. I did not want to remove the battery tray to get at the cruise control actuator so installed a restrictor in the cruise vacuum line at the vacuum tee under the hood seal at the top of the firewall. I stripped 2 1/2 inches of insulation off a 20 ga stranded wire, pushed the wire insulation through a 1/8 barbed connector, and put 2 inches of 1/8 inch rubber hose on the connector. I then put the other end of the barbed connector into the soft connector on the firewall tee and put the original hard vacuum line to the cruise actuator in the other end of the 2 inch rubber line. Makes for easy removal and installation.



Installing a restriction at the cruise actuator should be somewhat more effective than at the firewall. The 3 ft or so of 1/8 inch line is a a small vacuum tank.



I experienced much more moderate throttle actuation (feather foot). It seemed like 4 to 5 seconds to get the same boost response as 1 to 1. 5 seconds without the restrictor. The throttle position did not seem to overshoot as intrepeted by the the boost pressure.



More or less length on the restriction orfice (length of stripped electrical wire insulation) would allow you to adjust the sensitivity. I will try towing the fifth wheel before fooling with it again.



Illflem's vacuum restriction may be way of returning the cruise control function to the highly bombed Rams that have made the cruise too sensitive for use.

 

[Dennis Garrett]

Make sure you factor in odometer error.

 

[BillGotthelf]

I had a bed cover and got 21mpg (99) at 75 and I changed out to a fiberglass cap that sticks up over the cab by 6" (sloped upward from the cab) and my mileage dropped to 19. The cap only weights 400 lbs. When I tow with my 96 at 65 mph I get 12 mpg. Trouble is now I want to get there so most of the time I'm at 70 (11mpg).

 

[JLinder]

Maybe the cruise is my problem...

I had about 60 mi. of city driving on my tank when I took off today to take a buddy about 150mi to look at a truck. The drive was all highway and interstate. I never used the cruise control (my foot was asking me to) the entire trip. I filled up when I got back into town and my milage for the tank was 18. 7.



That seems more inline with what I should be getting, I think. I've got a few trips to make after the long weekend, and I won't use the cruise and see if I can replicate the results of this last tank. Hopefully we've found the problem.



Thanks for all the replies.