Town MPG in a CTD

[slowmover]

After joining Team Cummins a while back, and carefully reading what Diesel_Dave was doing on his truck I made some changes.

Several years ago had settled into a routine that was easy (the point of it) where all miles were covered between 1,700-1,900-rpm, town or country. The gearing of my truck (transmission and rear axle) meant that speeds of from 33-63 mph were easily covered by driving in 4th, and rarely in Direct in town, and OD for country (1,725-rpm at 58-mph). This might mean 33-mph in a 40-mph zone. A 7,500-lb truck is hard to stop, and keeping a bit of engine rpm to accentuate engine braking was found optimal by me. On a 45-mph road with little chance of cross-traffic I would upshift to Direct and run a bit faster.

This brought my town mpg from 18 to 19+ mpg, annual average over several years, and country mpg to 24 in all solo conditions (traffic, weather, payload, etc).

I gave myself room to be sloppy. And left room for when the weather was hottest (extended idling to maintain cab cool). The point of simplicity is that it is easy to follow with a minimal set of guidelines, and might be followed even if sick or injured or just plain tired.

I have always driven for mpg since acquiring a drivers license nearly 40-years ago, so a pretty fair amount of this -- EM suggestions -- was not new. More important to me -- much more important -- than mpg is vehicle longevity, followed by reliability. Mpg is third at best. Questionable practices -- EOffC -- were never considered.

So the second change I'd instituted just prior to joining here a couple of years ago was to reduce the total number of trips, even if each trip was longer by several measures available. The vehicle that doesn't move doesn't burn fuel. And when it was turned on, a route to all places was in hand with a considerably long warm-up drive favoring the local freeways to the farthest point first and then working back to the start point (home).

So far, so good. But I noted Dave's numbers, and -- even though I am not a commuter with set departure times and routes -- I felt I could improve my majority (at this time) town mpg. When he noted that Cummins had stated that the (later) 6.7L engine had it's best FE in the 1,300-1,500 rpm range and that he rarely saw above 1,600-rpm, and that it is possible to have higher town than country mpg the lightbulb went on.

So I moved things downward. All miles (as possible) at 1,300-1,500 rpm and shifts at about 1,500-rpm (and a hair upwards) to bring the engine back in at around 1,100-rpm. I was uncomfortable previously in bringing it back in at lower than 1,300-rpm (torque peak is 1,600-rpm) as that was part of my training in big trucks (respecting peak torque). City freeways are now covered at about 53-4 mph at 1,600-rpm.

I've recently averaged above 23-mpg "town" over more than a thousand miles with, again, these simple changes.

Thank you, Dave. First for me among the many around here as our vehicles are so similar; setting a larger context for me to read more carefully what others are doing.

For you other CTD drivers I've been able to up my average mph to 28+ in this period. Anything above 25-mph is optimal to the best of my experience.

Now I'm stoked to change my rear end gearing to 3.42 from the present 3.73 as that will bring me into the 1300-1500 sweet spot for nearly all town driving, and quite close on country driving. I'll give up a little perfomance, but as I am full-timing (living/working from a 32' travel trailer) this matched pair rarely exceeds 16,000-lbs -- low for this trucks capabilities; not considered "heavy" -- the change makes sense. A marginal change that will add up over the expected years and miles this vehicle will be in use.

I've also yet to explore slightly advanced engine timing and it may be that both changes will occur this calendar year (along with some small changes on intake and exhaust that mirror later factory practice).

I may also do some changes to reduce NVH (and hopefully extend vehicle life) that could have an effect on mpg, namely, changing to a lighter one-piece driveshaft and to a fluid-filled harmonic dampener (as the factory has, again, done in models later than mine). The effect may simply be indirect: a more alert driver on long trips.

A winter front, engine heater cord and a partial grille block have long been on the list. 2012 ought to be fun (more of the easy gains).

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[skyking]

Bravo! I think respecting peak torque when your engine has a boatload of it to spare is not necessary
If I light-foot my auto it stays around your goal RPM with the 3.55s. 55~60 MPH is around 1700~1800.

[Diesel_Dave]

Way to go, man! Keep up to good work.

I've been looking at doing a rear axle swap, too. I'm thinking it's easier to swap axles rather than gears (less to mess up). When I get around to it, I was going to just put a post up on Cummins Forum offering to swap somebody in my area a 3.73 axle for a 3:42. That way there's no cost involved, just a little elbow grease. The key would be finding somebody who has comparable miles and hasn't abused it.

[slowmover]

Originally Posted by skyking

Bravo! I think respecting peak torque when your engine has a boatload of it to spare is not necessary
If I light-foot my auto it stays around your goal RPM with the 3.55s. 55~60 MPH is around 1700~1800.

Thanks. I've been running country miles at 1,725-rpm or 58-mph since earilest 2009. I'll find a calculator and post what the numbers "should" be with a gear swap from 3.73 to 3.42 on these stock spec tires.

Loaded or empty, rain or shine, day or night, I put 16k on the truck commuting back and forth to another Texas city twice monthly that year and into 2010. Had to cross Houston each leg of the trip. Never saw lower than 24-mpg, with some highs around 27 (shown in fuel log where I went for "racetrack mpg": filled after warm-up and re-filled before shutdown to see what potential highest could be). Also, I kept the tires at the factory spec of 50/50 in psi FF/RR.

As to torque, it was pointed out that, again, the 6.7L (with a different manual transmission: the Mercedes-sourced G56) has an engagement torque of 400-ft/lbs. I'll assume my eariler New Venture 5600 (behind a motor with lower HP/TQ numbers) is similar. Indeed, I have to really come back in at a low rpm (sub-1,000) before I hear the auto-throttle take over and get a touch of rattle.

EDIT: Next step is in weighing this truck on a CAT Scale to get the current "solo" weight per axle and combined (driver, full fuel plus current bed load anticipated to change little) so as to dial in tire pressures ideally. I've experimented with 52/73 FF/RR recently and today dialed it back to 55/55 [hot] as the higher pressure in the rear screws the handling too much.

I have to move the trailer in another one or two weeks, and I'll get numbers for it as well at that time. As I have to install a new hitch receiver I'll be able to crank in the weight adjustment better than before.

Afterwards I'll have both solo and towing tire pressure numbers to work from.

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[user removed]

1725 X 3.42/3.73=1581.6

regards
Mech

[slowmover]

Originally Posted by Diesel_Dave

Way to go, man! Keep up to good work.

I've been looking at doing a rear axle swap, too. I'm thinking it's easier to swap axles rather than gears (less to mess up). When I get around to it, I was going to just put a post up on Cummins Forum offering to swap somebody my area a 3.73 axle for a 3:42. That way there's no cost involved, just a little elbow grease. The key would be finding somebody who has comparable miles and hasn't abused it.

Thanks, again. Your insights are a real help.

There is a fairly long thread on the 3rd Gen trucks on CF in re this gear swap. Several good reports. For the 6.7L guys, see posts by CourierDog a hotshot owner/op out of central Alberta seeing 20-mpg after this swap on his early (badly geared) G56.

As my truck just turned over 180,000-miles at 4,600+ hours I am working from the premise, that, as the average mph on the truck is above 35-mph during my ownership period, and was at 47-mph with the previous owner, I am hoping that I am now at the halfway point in time and miles (per 15-year standard life, and slipping past the B50 or MTBO of 350k due to miles/hours relation). Thus my plans are of running this truck another eight [8] years and another 180k miles. Any drivetrain related upgrades would then have the time/miles to pay out in a reasonable fashion.

I am, then, considering sending a new AAM 3.42 gearset and master overhaul kit to MICROBLUE RACING for their proprietary work (as well as axle end bearings) due to some highly favorable experience being shown on the Let's Truck forum (threads by Dice1). 4/10's of a mpg consistently on big trucks with wheel bearing changes; tractor and/or trailer. If I also source the TT wheel bearings through them I am hoping for a bit of elbow room, overall, when towing conditions are otherwise difficult. I would like to see a 1-mpg gain for the rig where both vehicles are as well set up as possible (drivetrain [and related] efficiency).

As of today were I to overhaul the NV-5600 I might also send it out for the same work. All of this is expensive with a payback that is more about friction reduction (temp control, thus fluid life) with a possibility of mpg help. Sort of a "create favorable mechanical conditions" scenario.

23-mpg around town wasn't new. But for over 1,100-miles it sure was. I'm pleased to accomplish that, and can now see where spending money will be able to make money as the payout will be of reasonable length. And to have some fun with gold-plating a few items.

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[slowmover]

Originally Posted by Old Mechanic

1725 X 3.42/3.73=1581.6

regards
Mech

Thanks, Old Mech. It does look pretty much like a 200-rpm drop.

As I tend to forget the details of truck spec and gear numbers I'm going to lay it out in this post I can reference in future; to show a range of what travel speeds can be made with the 3.42 across the ideal 1,300 - 1,500 rpm range per gear choice.

The calculators by Bowling & Grippo are an old favorite of mine from when I was researching how to get max FE from a big block Chrysler set up for TT towing a dozen and more years ago.

NEW VENTURE 5600: This six-speed is used in light duty Dodge pickup trucks from 1998-2005. All forward speeds are fully synchronized. This unit has an aluminum bell housing, cast-iron case & extension housing with a top mounted shifter. Available in both 2WD and 4WD versions, this unit weighs 360 lbs. The transmission is rated medium duty capable with a GCW of 26,000-lbs.

Transmission Ratios:

FIRST 5.63
SECOND 3.38
THIRD 2.04
FOURTH 1.39
DIRECT 1.00

OVERDRIVE 0.73

Stock tire size is 265/70R-17 31.4" tall at 657-rpms per mile
(MICHELIN LTX A/S Load Index 121 (E-Range)

With these book numbers entered it looks as though I'd be able to run:

* School zone at 20-mph in Fourth at 1,081 (or in Third at 1,494-rpm)

* Town at 30-mph in Fourth at 1,527-rpm

* Town at 35-mph in Direct at 1,281-rpm

* Town at 40-mph in Direct at 1,464-rpm

* Town at 53-mph (freeway) in OD at 1,416-rpm

* Country at 55-mph in OD at 1,470-rpm

* Target: 57/8-mph @ 1,523/1,550 rpm

(Versus current rpms of 1,725/50-rpm at this speed)

* Country at 60-mph in OD at 1,604-rpm

In other words, all 20-40 mph town driving, and all 55+ mph country driving is within or just barely atop the 1,300-1,500 desired rpm range. Can't ask for more than that! To my way of thinking, no need for a high-maintenance, questionably reliable Gear Vendors second overdrive unit (at $4000) or in changing tire size.

Winding out each gear a bit further than at present (time/distance, not rpm) ought to also make for some smoother transitions if I am correct.

As the range for towing is generally 1,400 - 2,200-rpm I ought (with this type of lightweight aero TT) to be able to maintain headway on the flats barring bad crosswinds at these same rpms, and still be able to "pull" the mountainous terrain at only a slightly slower speed than before (and nothing was slower than a few of the big trucks I've driven).

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[Diesel_Dave]

Originally Posted by slowmover

As to torque, it was pointed out that, again, the 6.7L (with a different manual transmission: the Mercedes-sourced G56) has an engagement torque of 400-ft/lbs. I'll assume my eariler New Venture 5600 (behind a motor with lower HP/TQ numbers) is similar. Indeed, I have to really come back in at a low rpm (sub-1,000) before I hear the auto-throttle take over and get a touch of rattle.

I think your 5.9L probably has pretty similar clutch engagement torque as the 6.7L. I'd say over 350 ft-lbs easily.

Regarding the "auto-throttle" you're correct. The low-speed governor will not engage until you dip below the idle speed. For me that's 800 rpm. I think it's the same for you. Basically the low-speed governor is there to prevent stalling--if you load up the engine so much that the speed dips under idle speed it adds more fueling to try to get the speed back up to at least idle speed. Of course, it can only do so much. If you try to start off from a dead stop in 4th gear and just dump the clutch it'll stall anyway, because it can't react that fast.

[Diesel_Dave]

By the way, do you know how to find out the "official" towing capacity of the truck (GCWR)? I assume that depends on the tranny and axle that you have, so do you have to have Dodge look it up for you based on the VIN number? All I can find on my door sticker is the GVWR.

[slowmover]

The Dodge Towing Guide is the way to finding specific of the question.

(The Dodge Bodybuilder Guide is the second source of this type of information).

On mine: (the closest choice set)

2004 Dodge Ram pickup 2500 SLT, QUAD CAB, 2WD, 8.0 FT Bed, 6-Speed HD Manual Transmission, 5.9L Cummins Turbo Diesel Engine - 610 lb-ft:

With 3.73 Axle Ratio Axle Ratio You Can Tow 13,100 lbs
Gross Vehicle Weight Rating (GVWR) = 9000 lbs
Payload = 2274 lbs
Curb Weight = 6726 lbs
Curb Weight Front/Rear = 4071 lbs/4071 lbs
GAWR Front/Rear = 4750 lbs/6000 lbs
Gross Combination Weight Rating (GCWR) = 20000 lbs

One takes the vehicle to a CAT Scale to find:

With driver, full fuel and items in truck that are always aboard, the "new" FF and RR axle weights plus combined numbers. This gives the proper "empty weight" to then calculate remaining payload capacity, etc. The first step in setting up a tow vehicle is to run the numbers (as is the last). While the truck is capable of higher towing numbers than those listed, one must respect tire loadings (and gamble a bit on RR axle rating; luckily commercial operators blazed this trail for us).

Proper tire pressure is about knowing the loading per wheel; generally, it is averaged across the axle. Note that combined GAWR's are higher than GVWR.

Hitch rigging is dependent on knowing proper "empty weight" as well as "hitched, but WDH inactivated" and "hitched, with WDH activated" numbers per axle [set] as it includes the trailer; the differences between axle values from solo to towing.

My truck weighs 7,360-lbs until recently, with topper, bed liner, driver, full fuel and a couple of tubs of stuff in the rear. It is now higher with full-timing load.

I'll acquire a new "empty weight", and then drag this trailer across the scales to find the other two number sets (plus trailer axle and tongue weights separately).

Start with numbers, end with numbers.

Much of what is constituted thus when we speak of longevity (component life, then vehicle life) is a matter of getting the numbers right. Reliability is on the same curve, so to speak.

FE is more easily achieved when weight is properly distributed (bed load, and any trailer loads) so that braking and handling is least affected. This includes tire pressure numbers: derived from actual loading.

The goal, as I see it, is to remain lane-centered going down the road with zero or very little driver input to maintain headway. Any driver input necessary to correct to this state of speed and position -- be it throttle, brakes, or steering -- is detrimental to longest life, highest reliability and thus to FE.

EDIT: The main point to working the numbers while towing is for the trailer, in a manner of speaking, to have no effect on the truck . . missed by many is that we also want the truck to have no effect on the trailer. It really is a two-way street, and the truck (when a truck is the tow vehicle) can be the weak link in the chain of road performance (which is also safety).

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