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Old 03-12-2011, 03:45 PM   #155
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I am really looking forward to the Diesel experience. I have started pricing the truck. One dealer in Jacksonville would only go 4 percent off of MSRP. Another dealer in the Panhandle told me that I could choose from what they had in stock (no 3500's at all) or wait until they got more stock in. The salesman at this dealership told me that they had a Silverado 1500 in Victory Red in stock and that I should jump on that since it was the right color. I can't believe that people that stupid are allowed out by themselves.

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Just a normal car salesman. Sal.
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Old 03-12-2011, 05:08 PM   #156
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I've really enjoyed this thread. I share the common happiness with the Duramax & Allison combo. But, I've also had a very different experience with my truck. She's a 2009 2500HD LTZ 4x4 (LMM Duramax) bought brand new at the end of Oct 2009. I couldn't resist naming her "Allison". Since new, I've put over 53,000 miles on her. Obviously low miles for a diesel, but not too shabby for only 17 months. I absolutely love everything about it...until I customized it!

Customizations:
- Ranch Hand bumper replacements both front & back.
- NFAB 6-step access steps (three per side)
- DeeZee bed rail caps on all four rails
- all the above parts were black from the factory. I had a shop paint match and powder coat to match the Chevy factory white of my truck
- BD Suspension Lift (7" front & 5" rear with add-a-spring not a lift block in rear)
- XD Addict 18" wheels & ~35" BFG All-Terrains
- HyperTech tuner to adjust tire size and use Stage 1 performance tune full time.

The truck looks great. I'm very pleased with the ride and performance of the lift kit. The tires are great and road noise is nothing to complain about. I don't notice the additional performance of the tuner unless I stand on the throttle (which very rarely occurs).

Here's where things start suckin' for me! The larger diameter tires have totally screwed my gearing. I tow a 2011 34' Classic (her name is Abigail). I'm a full timer, so Abigail is always heavily loaded. I weighed the rig on a CAT Cert Scale last week on a trip...17,400lbs with no driver or passengers but full diesel and fresh water tank.

Always use tow/haul with AS connected. Tranny will never go into 6th gear. It will stay in 5th and often downshift to 4th with only a slight hill. Worse yet, it often will go back to 5th before cresting the hill only to downshift to 4th again and dump fuel chasing the cruise control speed. On this last trip, I found the best "experience" was achieved by using manual mode, locking the tranny in 4th gear and settling for 67mph at just shy of 2500 RPMs. On a longer downhill, I'd click manual up to 5th to coast more but that really didn't seem to increase fuel efficiency or reduce tranny temp. The tranny consistently ran 100-120 degrees hotter than whatever the ambient temp (ie. 10 degrees outside & tranny would be ~110 degrees. 80 degrees outside & tranny would ~200 degrees. Engine temp never indicated a noticeable change on stock dash temp gauge. MPG averaged 9-11 depending on the terrain. Without towing, I get 14 around town and 16/17 on the highway if I stay close to 70.

This thread has referenced 1800 RPMs as peak efficiency for the Duramax. I can't keep it around 1800 for my life. It will downshift as soon as it tries to maintain the speed.

Thoughts? Ideas? Concerns? I'm very interested to hear what y'all have to say.

PS...sorry for such a long read!
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Old 03-12-2011, 05:31 PM   #157
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~~~
Thoughts? Ideas? Concerns? I'm very interested to hear what y'all have to say.

PS...sorry for such a long read!
Go back to the original tire size?

You don't offer a detailed description of how it performed under load originally, but it seems like you were happy with the performance, and then it became a point of concern when you changed the wheels and tires.
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Old 03-12-2011, 06:57 PM   #158
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Re-gear to match factory spec (or more likely, numerically higher than that to offset increased pressure drag and rolling resistance). Better, lose the lift and the offroad tires, IMO. The cost of looking cool isn't worth it: higher operating cost per mile and shortened life span. Offroad tires are poor (to be generous) for highway towing; in a number of important ways (a separate subject to be searched).

Take the specific question (cruise rpm) to one of the D-Max boards and see what others have done plus what their advice may be. You want the specific dyno graph and BSFC numbers of your engine spec to use a gear calculator. To tailor shift points the engine must run each gear within a particular rpm range. Once it is outside of this it can be some real work to re-create.

There may already be a table of this -- transmission rpm range and shift points under several scenarios (light, heavy, WOT) -- which others can link you to. A diesel, even a wannabe like the D-Max, has a smaller rpm range than a gasoline engine. We can't just rev them once we put in a hotter cam (as with gas) and push the shift points higher to compensate (as one example).

Find the tire manufacturer tire height (published) and measure the tires as well. Take your time. Small measurements being off can accumulate and create less than desirable results. The difference, say, between 4.11 gears and 4.33 may be huge in perceived performance, yet small in mpg.

Here's a set of calculators to play with. Also, scale the truck (as you already should have) set up for extended travel (heaviest likely weight) with per axle numbers.

It is one thing to alter a truck, it is quite another to re-create the overall quality of the stock truck (as you've learned).

On mine the highest BSFC numbers come just under 2000-rpm (most work for least fuel), but the road speed is too high to use that when solo once the effects of pressure drag (aerodynamic resistance) is calculated. The small change in rpm reduction to travel at 60 mph (1,750 rpm) is accompanied by a huge reduction in fuel burn, so even a less than "perfect" cruise rpm works to main advantage. 15% higher fuel burn or 5% less than perfect rpm? The dyno numbers must be "translated" into real life conditions.

To extend this, the range of 1,700 to 2,100 rpm is where the most efficient cruise rpm for all conditions will exist on a CTD of my vintage. The transmission shift points must work with this in order to "stair step" their way up as we accelerate. Peak torque and peak horsepower must be accommodated by trans gearing and torque convertor rpm design in an automatic. If OD is unusable then it seems obvious that the shift out of the powerband needs correction.

It's nice looking and a cool trailer, but either return to stock or continue with the $3k+ gearing changes, and possible custom TQ converter at above $1k. I would expect another $1k on top of that to round things off. Call it $5k at the outside.

In the diesel world it is always tempting to add a magic box to "fix" things. I disagree. It's fundamental abuse as no magic box comes without a cost to offset it's promised advantage. Do the mechanical relationships first, test, and then maybe a software tweak. Maybe.

Good luck

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Old 03-12-2011, 06:57 PM   #159
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The larger the tire size in the rear, the more torque required to push the weight forward.
The smaller the tire size, the less torque required to push the load forward.

I experienced this with the old Yukon I had with the 265 x 75 x 16 tires vs the 245's that other people had who were towing similar loads. The bigger tire size killed me....it looked cool, but it wasn't worth a darn for performance.
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Old 03-12-2011, 07:13 PM   #160
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Quote:
Originally Posted by DKB_SATX

Go back to the original tire size?

You don't offer a detailed description of how it performed under load originally, but it seems like you were happy with the performance, and then it became a point of concern when you changed the wheels and tires.
The performance was what you'd expect from the stock Duramax limited by the required emissions controls. The MPGs were better than a gasser, but not that of older, less restricted diesels. Plenty of torque and great tow capability!

It still has plenty of power to tow my AS and 27' boat...just costs more to do so since the "effective gearing" isn't "tuned" anymore to stay in the small sweet spot.

The ride is arguably the same or better than stock with the BD lift. Very happy with the lift product!

Be nice to have unlimited funds and have two trucks...one to tow with and one for ground clearance operations.
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Old 03-12-2011, 07:44 PM   #161
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@rednax - you've given me lots to research, starting with some acronyms. I appreciate it!

@dscluchfc - I knew about that relationship before I did the larger wheels and tires. I underestimated the impact it would have on the economy. The Duramax has plenty of torque to turn 'em, it's just costly at the pump.
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Old 03-12-2011, 07:50 PM   #162
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one of my fav sayings is:
"Speed costs money, how fast do you want to go?"

Modifications cost money.....
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Old 03-12-2011, 08:21 PM   #163
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Man, those are big-a$$ tires. Not sure they are compatible with rv'ing. But they must be great for whatever your high ground clearance activities are.

Happy trails - Pat
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Old 03-13-2011, 08:06 AM   #164
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There are some additional calibrations for fuel, trans shift, speedo, etc. available to the dealer for changed wheel/tire sizes. They may not have one for tires as tall as yours, but ask them for the largest available. They may have to call their factory Tech Assist to get the calibration downloaded to their tool.
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Old 09-12-2011, 10:33 PM   #165
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Still happy

It has been some time since I posted here, I want to chime in on my satisfaction of my 2011 GMC Denali 2500HD with the Duramax/Allison. As many will say, all of the new diesels are good trucks, they can pull a freight train. The truck has power to spare and it is a beast with luxury.
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Old 09-13-2011, 07:13 AM   #166
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Hey time...how goes it?

I don't hang here much any more either but happened to see your post...good to see your still travelin'!

We just returned from a 6k mile trip throughout the West...beautiful!

Bill
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Old 09-14-2011, 08:03 PM   #167
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Thumbs up Duramax loyal...

My current(2008)and last truck(2003)have both been the Duramax crewcab dually 4x4. Awesome trucks. But with any truck comes problems. You expect it, you deal with it, you laugh at it and move on. My 03's warranty had just expired at 70K. Around 90K the truck started to play havoc. The motor dropped a glow plug and was playing pingpong for who knows how long. The truck started to smoke and finally ended up on the side of the road. We towed it back to my shop. I found a used engine with about 100K on it. I wasn't going to pay 7K for a reman. We swapped it over the weekend and took it camping the next with our then 87 excella 32 footer. The truck ran like a champ until we got to town. We pulled into a Walmart to stock up when all of a sudden the engine stopped running. Huh? I tried to restart but no go. Finally I got a local tow company to bring out two flatbeds. Good thing for AAA roadside assistance. We were close enough to the campground to get free towing. So up went the dually onto one flatbed and hooked up the Airstream to the other tow truck. I went online to Alldata DIY and finally troubleshooted the problem. Bad fuel filter adapter. It was leaking and wasn't keeping the fuel pressure. Got home replaced the unit and all was good. Drove the truck for another 30K until I upgraded to the 08. I bought the 08 used with about 110K on the speedo. It needed a lot of major work being that I found out later it was a ranch truck out of Texas. I went through the entire drivetrain to freshen it up. Serviced all the fluids and filters, replaced all the ebrake cables, ebrake shoes, trans cooler lines, rear discs, rear calipers, pads, axle seals, wheels studs, front discs, calipers, pads, wheel studs, tie rod ends, ball joints, swaybar bushings, powersteering pump and box and hose. Wowie Zowie what a difference. The truck drives like new. The interior is LTZ minus the DVD player and sunroof. The best feature I love is the manual 6 speed shift. As in the 03 and 08 if I put the truck into the tow mode it never fails that the truck downshifts when I don't need it. Especially on a downhill run. I was constantly punching the pedal to upshift. Now I turn off the tow mode and control the speed with the manual shift. I love it. We are now pulling an 03 safari with a slideout. What a fantastic set up. Life is good.
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Old 09-14-2011, 10:31 PM   #168
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Quote:
Originally Posted by BillTex View Post
Hey time...how goes it?

I don't hang here much any more either but happened to see your post...good to see your still travelin'!

We just returned from a 6k mile trip throughout the West...beautiful!

Bill
Bill,

Thanks for the howdy. How was the trip? Maybe in the future we can make it to the same rally or just connect ourselves. Safe traveling!

Time (John)
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