Increasing MPG

[SSWEBARRICK]

Own a 03 duramax.Crewcab/shortbed. all stock. 50,000 miles. looking to improve from 13 mpg I have gotten since purchase in 7-03. Don't want to sacrifice power as I plan to start towing travel trailer soon. Have heard many things from 4" now 5" exhaust, to engine chips. What do all the experts (Diesel Page Members) recommend! Thanks for the input. Rory

[More Power]

Two big factors that determine fuel economy are weight and drag.

The heavier the vehicle, the more energy it takes to get it moving and to maintain speed. A 3500 will generally get lower fuel economy than a 2500HD. Hauling tools, an auxiliary fuel tank, additional accessories, etc all add weight. Make the truck as light as possible.

A suspension lift, larger tires, wider tires, soft tires, and external accessories all add drag, which increases as vehicle speeds increase - exponentially.... Limit speeds to no more than 65-mph if you're concerned about fuel economy. Driving 55 will do better than 65, but no where near as dramatic as slowing from 75 to 65 (the exponential factor).

Not recalibrating the speedo/odo after installing larger tires is the single biggest reason for low fuel economy reports. Some of it is due to a simple math error, some of it drivetrain related.

So, drive slower, install stock size highway tread tires, run them at the max recommended pressure listed on the side wall, and make your truck as slippery and light as possible. Correct the speedo/odo... smile.gif

Jim

[SSWEBARRICK]

I am due for new tires. Presently have 265 75r 16 on/off. Would a quality all season highway tread, stock size be what you are suggesting? What is the stock size for my truck? 245 75r 16? 235 75r 16? I do drive between 75-80 on freeway, which is 50% of my driving. I am sure the speed reduction will help as well. What will a 4" exhaust gain me? HP and Torq?

[rjschoolcraft]

For the LB7: http://www.kennedydiesel.com/photogal/pages/4-5exhaust.htm

For the LLY: http://www.kennedydiesel.com/photogal/pages/4-5exhaust_lly.htm

Dyno data is a very valuable "lie detector!"

[jbplock]

Originally posted by SSWEBARRICK:
What is the stock size for my truck?Rory,
Stock size is 245/75-16 ... If uncorrected 265/75-16's will yield a 3.9% error ... (see Tire Size Calculator (http://www.miata.net/garage/tirecalc.html)

smile.gif

[More Power]

Although straight-rib highway radial tires generate the least amount of rolling resistance (and longest tread life), I run all-season tires as a compromise here in Montana on my 2001 2500HD (currently Mich LTX). Aggressive M&S tires with a large block pattern generate more rolling resistance.

A tonneau cover helps reduce vehicle drag, as does a rear spoiler (wing). Running with the tailgate down creates more wind drag than when running with it up. Removing the tailgate makes it worse.

Bug deflectors create turbulence, as do mirrors and anything else sticking out on the vehicle.

As a general rule of thumb, light-duty diesels generate the best fuel economy (bmsfc) when driven at their torque peak. For the LB7/LLY, this is 1800 rpm. For the LBZ, this is 1600.

Jim

PS - Lil Red (http://www.thedieselpage.com/features/project2-16.htm) has 3.42 gears, which produce a highway engine rpm nearly identical to the new LBZ (80 mph = ~2000 rpm). This 5000-lb LB7 powered truck is lower than a 2500HD, and runs with a hard tonneau & wing and front spoiler. It has produced nearly 25 mpg at 75-80 mph on the Interstate. Weight & drag!

[ 12-27-2005, 03:23 PM: Message edited by: More Power ]

[Tough Guy]

Wow! 13 sounds a little low, even for your speeds and winter fuel...

I just got back from a 340 mile road trip with my F-250 Crew Cab Power Stroke...it weights in at 7500lbs all by itself, with 5 passengers and cargo, prolly 8500lb+ Its got All-Terrain 285's for shoes and I run them at a mushy 55lbs for winter traction. I drove exactly 78 mph on the freeway for the most part, and with some city driving the trip average was 15.6 mpg with winter fuel.

You either have some problems, such as maybe you need new fuel/air filters, or you drive faster then 80 mph....


Chris

[silverback]

Rory,
My early 04 LB7 is getting ~18 MPG commuting (24 miles on flat state highway) at 66 MPH on the cruise control. I'm running the same size tires (265 16s) as you but I have corrected the speedometer/odometer with the Hypertech which I run at level 1. I also have a 4" exhaust and the foam Uni filter. I think the shell on the back does help with aerodynamics. I'm also getting ~14 MPG towing (in tow/haul mode) but I run a little less then 65 MPH. I have noticed that tire inflation can affect the MPG so I run 61 PSI front and 64 to 65 PSI rear unless I'm towing (then I bump the rear up to 70 PSI plus). I'd bet running 75 to 80 MPH 50% of the time knocks down your MPG.
-Ken

[Mark Heiken]

I gotta agree with Tough Guy in that something doesn't seem right. I get at least 16 mpg running 75-80. I have the old juice on level 2, and a 4" exhaust. Also have a tonneau. To be honest none of the mods have made a noticable difference in milage. 265 tires did make for a slight decrease but not much. I do have a fairly smooth driving style and don't use the cruise much in hills so your results may vary.

[MartyB]

I've been getting a constant 12-14 under normal driving with my Dually. But have to admit, my ave speed is about 30 for the whole 11000 miles I have done. I have gotten as high as 18 on the freeway ave 55-60 tho. So if you are like me, and do a lot of city driving and very little interstate, you ave may be about right! I hate to say it that way.

I would think with a SW rig you would be about 1-2 mpg more than me with a DW rig tho.

[markrinker]

Improper alignment can cause lots of rolling friction, and rapid tire wear. How do your fronts look?

Also, brake drag - but that shouldn't be a problem with 4wheel disc, unless you have a sticky parking brake cable, and you'd usually smell the problem first.

My bet is you have both a heavy truck, non-standard tires, and possibly a heavy foot.

[Kennedy]

Starting/stopping (especially starting from cold vehicle) and driving at speeds below TCC lockup can run MPG in the toilet in a hurry.

[Deansel]

A guy I know has the same truck as we do and his empty hiway best mpg is 14 and our averages around 17. We just took a 450+ mile trip up to Michigan with a very light load and averaged 19.5 mpg @60-75mph with winter blend fuel. He also has very poor throttle response at low rpms and averaged only 9mpg pulling a 24' camper on a long trip. We average 12.5-15mpg pulling our 22' fifth wheel, depending on speed.

[markrinker]

Deansel - is your buddy running stock rubber? What if any differences in modifications to the truck?

Idea: Trade rigs on a loaded trip and see if you can get better fuel mileage. It might just be you have better overall driving habits. (Lighter foot, coasting before braking, etc.)

[ViperSmitty]

I recently purchased a 2005 Chevy 2500HD and have the 265 tires on it. How can I tell if the speedo was recalibrated or not? I bought the truck with 6000 miles on it with the 265's already on it. The guy had to sell due to divorce. Who can I have do this for me or do I need to get a tuner to do it?

Thanks for any reply!

[More Power]

You can drive by 10 or more mile markers on the Interstate to check the accuracy of the odometer. Use the trip meter to get tenths.

You can also use a stopwatch to check speedo accuracy. It should take exactly 60 seconds to travel exactly one mile while running at exactly 60-mph.

Either method will likely produce a speedometer/odometer error. The percentage of error can be used to correct your fuel economy calculations.

Jim

[Kennedy]

A GPS can also be used to compare indicated speeds.