Engine/Transmission (1998.5 - 2002) fuel mileage increase when going faster!?!?

[CRSmith]

For most of my trucks life (39000 miles) I've not really run it much over 2000 rpms when cruising on the interstate. I just didn't like the sound and though I would get better mileage. With 4. 10 gears and 265 stock Michelins this equates to about 65 mph. Well, I guess I've become less anal and was driving around 70-73 mph at about 2300-2400 rpms on this last trip. I know that's gettin a little crazy . Anyway, avg. mileage on the overhead console went from 17. 5-18 cruising at 65 to about 19. 2 cruising at 70-73. I thought it would surely go down. Was I running it closer to the peak hp or torque curve? Anyway, I guess I'll run it faster from now on. Makes me think twice about getting 315 75 r15's to turn lower rpms on the highway. Has anyone else noticed this correlation?

 

[DaveN]

I've noticed that on mine as well. I've got the 3. 54 gears so mine happens at about 80+. I think the fuel calibration changes somewhere around 2200 RPM that makes it more efficient. I typically gain about 1~1. 5 mpg if I just bump 80, at least according to the computer.

 

[rashwor]

Do hand calculations...

and see if it matches the overhead computer. Trip computers sometimes are inaccurate. :--)

 

[Got Smoke?]

I agree with the hand calculations comment.



I have 3. 55 gears, 5spd, & 33" tires and I know once I get above 70mph my mileage usually goes out the door. @ 80mph its usually just sucking it!:{

 

[dflack]

doubtful

I have check my mileage at 65, 70 and 75 mph on several roadtrips and find that as speed increases, mileage decreases. Having said that, I can say that at 75, I'm just barely turning 2200 rpm, so if there is some magic computer efficiency control over 2200, I'd like to know.

 

[rdanley]

MY RAM LIKES LIFE IN THE FAST LANE. SHE CANT STAND IT WHEN SHE HAS TO RUN UNDER 80. WHICH I HAVE TO HOLD HER AT OR WE BOTH WILL BE IN JAIL.



AS FOR FUEL MILEAGE BETWEEN 70 & 80 I ONLY SEE A DIFFERNCE OF ONLY 2MPG, PERSONALLY I THINK THE OLD CUMMINS LIKES IT BETTER WHEN IT IS TURNING ATLEAST 2200 RPM.

 

[CRSmith]

I think I like living in ignorance. I'm going to have faith in the mileage computer.

 

[mcoleman]

Anything over 2000 rpm causes my truck to loose fuel mileage but gain in fun mileage. With the Wife driving my truck to work and back she can't get it over 60 mph in traffic and regularly gets 22. + mpg.

 

[dflack]

discoveries by "ignorant" people

No one said you were ignorant. If there is something in the programming that changes efficiency at/near 2200 rpm, I'd like to know, and so would many others.



Please go ahead and make an accurate calculation by hand over a couple of tankfuls if you can.

 

[DaveN]

I agree it doesn't pass the common sense test. Something is changing past the 2200 rpm mark that tells my computer that it's more efficient.



I know that I get down the road much more efficiently at 2200 rpm than I do at 2000 rpm.



So if you value your time at $20 an hour, and convert those dollars to diesel fuel... .

 

[JR2]

Mileage an RPM (and speed) are directly related. The 24v's break specific fuel consumption is best at 2100-200 rpm (if I remember correctly). With that said here is what I have seen. If I drive my truck around 75 mph (2100-2200 rpm) with the fueling box on 1x3 (basically in EZ mode) I can get upwards of 18 mpg (hand calculated). If I drive the same strech of highway going the same speed with the fuel box on say 5x3 then I get about 15 mpg. If I drive the same strech again at 55-60 with the box on 1x3 at 75 I get 15-16 mpg. This strech of road is perfect for the test... there is a gas station at the start and finish (any one from SOuth Central Alaska its from Bird Creek to Soldonata) and has no stop lights for 140 miles.

 

[dflack]

JR2

JR2 - I don't understand your last scenario . . ?



This is exactly what may or may not be the question - at 65-75 MPH my truck, 2500 4x2 3. 55, is at 1800-2000 RPM and I'm holding back in the interest of fuel efficiency and safety when I'm not in a hurry. If I knew that 2100-2200 RPM was more efficient, count me in. I have a 3000-3500 mile roadtrip planned for May, so anyone else that knows if there is indeed a magic sweet spot 2100 - 2200 rpm, please let us know. (or if indeed where the pot of gold is at the end of the rainbow!)