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mikeyjd 04-25-2013 11:59 AM

My journey to 40mpg with a 1ton diesel.
Thanks to everyone who chimed in on my truck acquisition process here - After doing some research about this vehicle, and what can be done with it, it doesn't seem outside the realm of possibility to eventually achieve a 40 MPG fill up (I got 24.5 on my first 157 miles under poor conditions). I'll be logging my progress here as I go along. Any advice and feedback is always welcome and appreciated.

Truck Specs:
1991dodge ram
5.9L Cummins Turbo Diesel
Std. Cab
2wd manual transmission
V250 (sweptline with 1 ton suspension)

Planned mods:
Kill switch on shifter
Upper/lower grill block
Engine blanket
Timing advance
Raise tire psi (80? *max side wall)
Under belly pan
Electric fan
Custom aero cap
Any others if possible

Big Dave 04-25-2013 01:20 PM

This project has a lot of merit. My truck is fairly similar.

Quick question: Stick or automatic? Just on the face of it a stick is good for a 2 MPG gain. An automatic pretty much forecloses engine-off operation unless you can repair automatics economically.

Tires: Get E-rated tires a size smaller than what you have. I get by just fine on 225-75x16s. I air them up to 100 psi. Lacking any accurately tabulated RR values I default to OEM tires.

Gearing: Very important and huge opportunity for you. Your Cummins (like my IH) has vast low-end torque. You can actually operate at much lower RPMs than you think. I operate at 1325 RPM @ 70 MPH and 1050 RPM @55 RPM and can run as low as 700 RPM around town. I use a double overdrive (ZF6+ GV) and a no-longer-available 3.08 R&P set. By reducing engine RPM at any given road speed you reduce the engine frictional HP proportionately. It is a big deal. At 2000 HP my IH engines frictional power is about 10 HP but it drops by more than 40% at 1300 HP. When you are running at the ragged edge of a "lug" you've got it just right - in the most efficient place on the BSFC-RPM map.

Aerocap: Great idea. There are a number of threads on the ecomodder forums of various options for you. I had a crude aerocap and it was good for a 3 MPG improvement over an open bed and 1.5 MPG over a flat tonneau. Visibility and utility are issues - hence the diversity. Look at aerohead's truck. Its the slickest I've seen. He gets great MPG without any mechanical mods.

Bellypan: Should help but I've never tried it. Same with grille block.

Timing advance: That helps. When it won't start in the winter back it off and that's where you want to be. If you ever want to run propane or CNG back it down to the stock advance. Too much advance + gas fuel = thrown rods.

Engine blanket: Never tried that but I use a 203 degree stat and Evans coolant. If I could find a hotter stat, I'd use it. The Evans doesn't boil (at atmospheric pressure) til over 400 degrees. Hotter coolant makes the engine tighter and more mechanically efficient. Hotter coolant transfers more heat to the atmosphere and your electric fans wouldn't need to run as much. Caution: Hotter stat leads to hotter supply air from your heater and defroster. Mine roasts my fingers on high-dash heat and cracked my windshield in icing conditions where I had to run the defroster real hard.

Please do the ecomodder and pickup truck communities a favor: Don't make your claims based on driving 20 MPH below the posted limit. There is a substantial set of Cummins guys who claim high MPG while driving 40 MPH. That is phoney because nobody drives that way. Determine your MPG over a considerable distance. Any knucklehead can coax high instantaneous MPG on the ScanGuage going slow downhill with a tailwind in perfect weather.

I use a set routine 35% Interstate (70 MPH) 40% state roads (55 MPH) and the rest urban/suburban (running along with traffic) and use a minimum of four fill-ups to to average out the error in determining fill level.

While you are getting other things together, hone whatever hypermiling you feel comfortable with (I'm Sir Coastalot with my stick shift and heavy truck but other than trying to time lights, not much else) and do a rigorous baseline. Then do some mods and run the same routine again.

oil pan 4 04-25-2013 03:43 PM

Get a more modern turbocharger, maybe add a second one, straight pipe, intercooler if you don't have one.
Do the belt driven fan delete first.

UFO 04-25-2013 04:59 PM

I think that is a way cool project. 3.08 gears would be amazing.

Shortie771 04-25-2013 05:13 PM

I would suggest you start off with the simple things, to keep things cheap, easy and get you a little extra mileage while you are getting familiar with some hypermiling techniques.

Max sidewall in your tires: Free, 5 mins.
Antenna Delete/Relocate: Free, an hour or 2.
Mirror Delete/Interior Mirrors: Free-a few bucks, an hour tops.
Partial/Full Grill Block: Free-a few bucks, an hour - a few days depending on complexity.

Those small and quick upgrades should net you a few mpg and not take long at all to do. It would be a good start and since they are so cheap and easy, there are no excuses for procrastination (my biggest problem).

After you have done those, you can move on to the more complex and time consuming mods (belly pan, aero-cap, wheel skirts, air dam, etc.)

mikeyjd 04-25-2013 05:52 PM

Big Dave:

It's a manual trans (I've been doing a little PNG and allot of eoc with my manual 1.3l ford festiva already so I feel like that will transfer well over to the truck) I agree that its better to avoid minimum speeds on the highway but I do like to go 55-60 when I have time.

I have quite good access to tires so getting more ideal ones won't be an issue.

I've never done any kind of gearing swap so this will probably be a long term goal.

I have a plan in my head for the aerocap design and it will include a lexan window and a hatch style opening system.

Oil pan:
I love doing engine mods but am limited in my experience. I need to make friends with a good diesel mechanic as I'm much more of a hands on learner. Reading the threads is helping learn what I want to do, but I don't feel that much closer to being able to do it myself.

Thanks. And I agree that ideal gearing would be a worthwhile thing to do.

Thanks and I agree with you about the low hanging fruit approach. But I am definitely in this for the awesome numbers (let's face it its a game to me, albeit a practical money saving one). I'll probably end up pushing the limits with what I can do mechanically and aerodynamically as well physically.

Thanks for the suggestions and support. I think the first thing on the agenda is an oil change then I'll hit the basic free mods :-)

slowmover 04-25-2013 08:05 PM

There is a substantial set of Cummins guys who claim high MPG while driving 40 MPH

BigDave, I've seen you make this same statement on FORD forum threads. I've never come across it. Hard enough to find anyone who doesn't speed (runs 60-mph plus) or who keeps the average mpg updated. Now, if you meanAverage MPH your comment may be on track.

Trucks are a compromise. And part of that compromise is poor handling/braking. A propensity to roll-over where cars just spin out. Trucks are geared to run 55-65/mph most efficiently. And highest mpg claims are within this range. The foolishness of running a truck faster than this has no place in any discussion of performance, or from any angle. It is beyond the pale.

When I claim no less than 24-mpg at 58-mph @ 1,725-rpm that is over tens of thousands of miles. My truck spec, climate and terrain comparisons with others makes this consistent (with those others who keep records). I've also run over 1k miles in town at 23-mpg, but this was in practicing hypermiling techniques. And I've seen just above 30-mpg in a run across southernmost Louisiana on a road where the limit was 50 for many, many miles. A fluke, in any case.

If you've other sources to cite, start a thread on the topic. FORD is at the back of the pack for good reasons . . but all turbodiesel trucks have some magic to discover in re fuel efficiency. We're all friends around here, so let's not muddy the water.

The OP has his work cut out for him in establishing the best mechanical baseline. Sounds great here at the beginning (pics seem to show an unmolested truck), but at over 20-years it's time to go through it all in re systems.

I'd start by weighing it. A CTD is fairly impervious to weight changes, but getting the weights for each axle and each wheel where the truck has driver, full fuel and any permanent supplies aboard is the adjusted empty weight over vehicle manufacturer shipping weight numbers. IOW, a weight that will never go lower. Truck tire pressure is within manufacturer specs (especially FF/RR bias) and then according to load. Accurate steering and braking is not helped by overinflation, etc. Baseline, to know how it feels when it is to spec.

The underhood electrical harnesses and then dash harness would be my main concern in where to get reliability back to new. Get the service manuals, identify the harnesses and work hard to get those OEM parts. Dealers and parts stores start eliminating stuff for 15-yr old vehicles and they then become incredibly difficult to find (at a similar level of quality). It might cost $$$, but every penny will be earned back.


kafer65 04-25-2013 08:22 PM

I'd love to see someone do the template overlay on this body style. I've got an 87 longbed, but I can't get it to upload a pic.

oil pan 4 04-25-2013 10:54 PM

You can put a number of holset turbochargers on that cummins with nothing but off the shelf parts and adaptors.
First you have the turbos inlet, most holsets use 4''.
Most holsets use the T3 exhaust manifold flange, you exhaust manifold will work with most of holsets. Some HX40 turbos use the T4 flange.
Most holsets use a 3'' down pipe flange there are some 4'' and a 3.5''.

mikeyjd 04-26-2013 01:04 PM


Originally Posted by oil pan 4 (Post 368284)
You can put a number of holset turbochargers on that cummins with nothing but off the shelf parts and adaptors.
First you have the turbos inlet, most holsets use 4''.
Most holsets use the T3 exhaust manifold flange, you exhaust manifold will work with most of holsets. Some HX40 turbos use the T4 flange.
Most holsets use a 3'' down pipe flange there are some 4'' and a 3.5''.

Any good advice for someone wanting to learn this kind of thing hands on?

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