In the city, my truck (97 2500 with 196k) gets about 20mpg (I am usually tooling around at 50mph on average) but when i was pulling about 3.5 tons, it dropped to 12.5mpg. I was a bit shocked but I was running 70mph the whole 600+ miles (used 3 qts of oil too - that was spooky)

Anyhow, I have a buddy who works at Catepillar (he is a diesel cooling engineer) and he suggested finding out the rpm the engines runs most efficiently at and pull at whatever speed that comes out to be.

Anyone know what rpm the cummins is most efficient at ( as specified by Cummins)

Thanks,
bausman

 

I'm gonna go with about 2000rpms as that is still in the powerband and the motor is quite fuel efficient there too.

 

I'd have to agree w/ that. 1900 to 2200 RPM

 

A Dodge rep, at a Cummins open house, told the audience I was in that the 12 valve is most efficient between 1700 and 2000 RPM. With my 3 1/2 ton load I get 15 MPG, interstate, 62 MPH, overdrive, cruise, 1800 RPM. I think it would drop to your 12.5 if I pulled at 70 MPH, just because of the extra power needed to overcome wind resistance. You should not use 3 qts oil in 600 miles. Better investigate.

 

1700- 2000 rpm is my feeling also

 

Thanks for the input - I'll have to see how slow I'll be going at 2000rpm

As far as loosing the oil, I didn't lose hardly any the first 5000 miles I had the truck driving around town. Seems the pulling may have had something to do with it. Does the turbo eat oil? If so, whats normal under heavy load?

Thanks,
bausman

 

It's not normal to lose any oil through your turbo. My 1997 tows a 2 horse trailer a lot during the summer. It's on a 6000 mile service schedule and doesn't use any oil between services. It has 110,000 on the clock.

 

I agree with everyone on here

I love the midrange grunt, but getting that engine to the sweetspot at the 2000 rpm level just flat out saves fuel and your ride runs smooth..

Tx

 

Based on the graph that infidel posted, looks like 1500 rpm is the most efficient speed to pull at. My buddy at CAT agrees. Since I have a 4.10 in the rear and only 5 gears, that equates to about 45mph for me Don't think I'll be saving as much fuel on the highway as I'd like but I'm gonna pull it back from 70 to 60 or so on long trips. More than 1500 rpm but better fuel mileage at least....

bausman

 

Hey you gots a CUMMINS forget the Cat cat on this one.

 

Me too my 4.10s auto tranny with 285's and I'm turning about 2300 @ 75 mph

 

I've found that rpms make little difference in fuel economy, it's speed that kills it.
Lots of folks think they're going to jack up their mpgs with an overdrive gearbox, it doesn't pan out.

 

ok so if speed kills then what is optimal speed - I guess a couple guys have said low 60's seems best. Any other takers on that bet?

bausman

 

Well around 30mph rolling resistance is equal to aerodyanmic resistance, but I don't think it'd go over to well driving on the interstate going 30mph. No matter how much faster you're going there will be more aerodyanmic drag, it increases as a squared function of velocity. (5X speed -> 25X drag)

You'd kill the tranny running anything slower than 1600rpms with a 5 speed.
http://dodgeram.org/tech/specs/axle/5sp.htm

 

Well, like many of you I'm a tech nut. I watch those gauges and compare loads/mods and speed with what I'm getting.
MY General 2cents worth is;

At around 400hp is GREAT on these trucks! Easy to do, and increased my mpg about 2mpg, from 12-13 to 14-15. I can tow 14,000 and MY truck is 9,370empty, so go figure 25k at 70mph getting 12.5mpg. Plenty of useable power.

{a side note..HOW you carry your weight will also alter your mileage! I had a gasser, got 12mpg flat empty. I could CARRY 3,000 and get 10, or trailer 8,000 and get 11}

I have done MORE improvements and am unsure of HP? 475?550??(have not dynoed yet) My mileage as gone up to a peak of 18.8mpg on one all highway trip.(empty and @65-70)

The PEAK MPG is going to be where your injector pump is timmed at. With a careful ear you might be able to hear it. Currently mine in the 1900 range is a "normal" noise, 2000 is a deep tone, 2100 is quiet! ..or there abouts.I can't hear the valves. My timing is at 16.5..

So in my humble opion, upgrades that help make useable power(without smoke) will help your mileage. Your static timing will select your "cruse" rpm, unless your running propane, which will lower it (I think,due to the propane fireing first and burning the diesel prior to the compression flash point of the diesel. It improves complete combustion of the diesel).

Best cruse simply is the highest gear you have with the least amount of boost!

Drag on the trucks! I KNOW speeds above 65 starts to really hurt them. I heard once 58 is where it starts to show any signs. Stock autos seem to do well there.
Remember the stock autos timing is set for 1600rpm.

I know for me;
70mph is a solid 10psi boost
75mph is a solid 15psi boost
80mph is a solid 20psi boost
90mph is 25lbs boost and 1000+egts any slight hill and I'm at 1200+
I have enough fuel, but will glady wait for the twins before any higher, "tests".

Any sustaned high speed driving,EMPTY, will heat up the tranny. It gets tight to shift/etc.. SPEED COSTS!

3500 4dr/ext cab 4wd 5spOFE GREEN 4.10
US Gear/O-RINGed/4"exhaust/pod pyro-boost 387hp+?/945ft-lbs+?
370 inj/#100plate/4k kit/AFE(med) Upgradded brakes
HX-35-14 adj wastegate(40psi)
STaY Away from DrP!I'm out $3k
1of30 built in Cleburne TX