I don't want to beat a dead horse on the fuel mileage issues. I have been lucky, that driving like an old woman going to church on Sundays, is allowing me to get an average of 21 1/2 mpg according to my DIC and hand caculations. Thats 50/50 city and highway. I do everything I can to keep the RPM's under 2 grand. My RPM's were always 1,850 at 55mph.
Today, I made a road trip Chicago to St. Louis and back. Going down I didn't reset my DIC and cruised down there at 65mph and it held a constant 21 1/2 mpg also verified by hand calculation.
Coming back as soon as I got up to highway speed I reset my avg. fuel mileage. It jumped up to 25mpg and then dropped to 24.3 at its lowest. I did tailgate as many semi's as I could all the way home. I am afraid to go fill up because when I pulled up to my house it was still showing 24.7 and I still have a half tank left after driving 327 miles.
My rpms at 55mph dropped from 1,850 to 1,650 is there any reason why this happened?
I read in a post here that someone questioned if GM programs the computor not to have the engine run at its peak until a certain mileage or hour is reached. I cannot find that thread for the answer to that question.
My 05 CC, SB, 4x4, AT, RPMS dropped at about 2,300 miles on the odometer.
I am sorry for the long post, I am a police officer so I am used to writing long winded detailed reports.

great, im glad you got great mileage. if i could keep my foot from hitting the floor, i would get better. i get 17.6 mpgs highway but running around 90 on the highway most of the time. i do know if i do drive slower, it will go up.
i don't know about the programming on the trucks but diesel engines are normally real tight when new. As time goes by, the truck really breaks in and the mileage gets better.

If the RPM dropped that would seam to be in the tranny. Maybe MikeL can tell us if the clutches break in or something similar. How many miles are on the truck?

It turned about 2,300 miles when the rpms dropped

I experienced something similar with my old 04 lly. I was really low on fuel and accelerated really slowly, basically going off the throttle as soon as it shifted up and then went on it a bit more until it shifted again and went off it a bit so it would stay in that gear. It seemed like the computer noticed i was driving slow and the fifth gear would rev a bit lower.

Now my '05 revs higher again at highway speeds:(

I'll try what I did with the blue (04) and see if it will maybe do the same thing...the '05 has about 3400 miles on it now and the '04 had about 35000 on it when it did the above thing.

turns out, he accidentally turned off the "tow/haul" button right at 2300 miles.


:gr_grin:

:exactly:!

:lol: sounds like the jist of it.

Seriously though the thread that he was refferring to I wrote about 2 weeks ago and it was in regards to a 6.0L gasser. And on that one at around 435 hours the truck all of the sudden started getting 15.5MPG instead of 14MPG and would light the tires up after that point. I was just throwing in my $.02 on a subject about break-in.

No I didn't turn off the Tow/Haul mode.......... you goofballs:joke:
I have tried the tow/haul mode a few times just to see how the truck drove and the bulb on the dash lights up when activated. So it can't be the tow/haul mode was accidently on and I didn't know due, to the bulb being burned out.

Well I am glad you have good luck, becuase my truck 04 LLY has 20,000 mi. and if i keep it under 2000 rpm I get an astounding 12.5 MPG unloaded....

Edge level 1 x 1 , 4" Exhuast and no cat. on 285's Hand calculated!!!

I still have to call B.S. on you 22+ mileage guys becuase the best milegae of any D-max's in my area is 15-16 mpg UNLOADED driving like granny!!! Some have 50,000+ mi. and are plenty Broke in!!!!

Can't call B.S. on me because I've had numerous duramaxes and the best I could get on an lly strictly highway (for at least a hundred miles) going not over 70mph unloaded on stock slicers I can get 18.8mpg.

My 04.5 lly could never get that.

As soon as I tow though with mine at about 9000 pounds through the mountains it drops to like 9-10mpg.

Time to get some meat on it, and maybe crank it up a bit more. Not that that will help fuel economy or anything lol but I hate how it looks right now w/ those crappy 245's.

I just picked up my 06 Sierra and drove it 440 miles home from the dealership I bought it from. When I started the trip, I had 14 miles on it. I got 20 mpg driving at speeds from 55 to 65. The dic indicated 18.4 mpg, and by hand was 19.89. If the mileage doesn't improve at all with break-in, I will still be tickled. I was used to my 91 GMC gasser getting 14 going downhill with the wind at my back.

SO what's the best mileage tire size? 245's? How about 265's?

Well I am glad you have good luck, becuase my truck 04 LLY has 20,000 mi. and if i keep it under 2000 rpm I get an astounding 12.5 MPG unloaded....

Edge level 1 x 1 , 4" Exhuast and no cat. on 285's Hand calculated!!!

I still have to call B.S. on you 22+ mileage guys becuase the best milegae of any D-max's in my area is 15-16 mpg UNLOADED driving like granny!!! Some have 50,000+ mi. and are plenty Broke in!!!!

Being too lazy to look it up I need to know how tall 285's are and if you have had your PCM corrected for tire size. I am running 35's and I have a best reading on the DIC of 12.3 which is about 12% to 15% off if the math is right. According to the speed radar signs at local construction zones I am 73 to 74 MPH while the speedodometer reads 65.

Well At 50 To 55 Mph ( Low Rpm's ) 22- 23 Mpg Plus Is Common In My 2005 Lly. Don't Care How You Figure It. It All Works Out The Same. 18 To 20 Mpg On Mixed Driving..

Towing Is A Different Story, But You Have To Know That Mpg And Towing Don't Go To Gether..

Sure Beats The Ghell Out Of That 6.0 Gasser I Traded In!!!

Arm

Well I am glad you have good luck, becuase my truck 04 LLY has 20,000 mi. and if i keep it under 2000 rpm I get an astounding 12.5 MPG unloaded....

Edge level 1 x 1 , 4" Exhuast and no cat. on 285's Hand calculated!!!

I still have to call B.S. on you 22+ mileage guys becuase the best milegae of any D-max's in my area is 15-16 mpg UNLOADED driving like granny!!! Some have 50,000+ mi. and are plenty Broke in!!!!

Will,

If you get only 12.5 MPG there is something missing here. Either that figure is while towing, you have a real mechanical problem, or you continually have your right foot on the floor.

If you are truly getting 12.5 MPG EMPTY, then there is something seriously wrong! The 12.5 MPG towing is probably not out of line depending on what is being towed.

Britt

Hey, I just put Michlin 265/75-16" a couple of weeks ago. They fill up the wheel wells real nice and give alot softer ride ( I got the load range C tires). They are only 3.6% taller than the stockers, but they do help to drop the RPM's down some on the hiway. By the way, i got 28mpg on the hiway the other day running @55. I dont know why those other guys dont get much for mileage. In normal 70mph hiway speeds though, it does about 20 to 21mpg.

I seriously doubt that you get 21-22mpg going 70mph. I've had a few duramaxes and they were broke in and wouldn't get close to that at 70mph.

Is it hand calculated?

And the best tires if you stick with the 75% series and stock rim size is the 265, given the 245 tire in comparison is the exact same. You will do a lot better with a smooth highway tire than an offroad one.

I could get 19mpg at 70mph on level ground. In fact though, you're getting less because the truck isn't going as fast as the speedometer says.

on my stock 04 LLY I get a consistent 18.5-19.5 @ 70 hand calculated runing empty on long highway trips. Fill it to the top of the filler neck every fillup. Never have been able to get above 20, but pretty close. The trips are usually from Portland OR to Central montana over the divide. flatland or hills don't make much difference

Being too lazy to look it up I need to know how tall 285's are and if you have had your PCM corrected for tire size. I am running 35's and I have a best reading on the DIC of 12.3 which is about 12% to 15% off if the math is right. According to the speed radar signs at local construction zones I am 73 to 74 MPH while the speedodometer reads 65.
__________________________________________________ ________________

Tire Dia ={2x215x65} + 16 =27'' DIA
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2540
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2005 chevy 3500 LT CC LB 4x4

My 05 averages 14-15MPG empty and towing approx 6500lbs it drops to 12. I sure would like to know the secret to getting 20+ and yea I tried the under 2K rpms it improved about 1MPG (all calculated with the dic I'm really afraid to see the hand calc method) I have 13k miles on my truck now

I seriously doubt that you get 21-22mpg going 70mph. I've had a few duramaxes and they were broke in and wouldn't get close to that at 70mph.

Is it hand calculated?

And the best tires if you stick with the 75% series and stock rim size is the 265, given the 245 tire in comparison is the exact same. You will do a lot better with a smooth highway tire than an offroad one.

I could get 19mpg at 70mph on level ground. In fact though, you're getting less because the truck isn't going as fast as the speedometer says. I must have mis-read this. Going from 245's to 265's will make the speedo read slower than actual speed.

If I reset my DIC coasting down the mountain I can get close to 59 MPG!... It slowly fades back to my 17 to 18 MPG I get around home in the rolling hills of the countryside (empty). Towing 9k, it drops to around 14 MPG. I would love to see 20+ MPG! Maybe if I surreptitiously hook a tow strap to said tractor trailer(s) I could see that kind of mileage! I jump back and forth with my Edge/Juice from 1 (mileage) to 3 (drive), with the occasional 4 (race). I've tried the granny thing... With all the ups and downs of the countryside where I live, I think even granny has a hard time getting good MPGs.

My truck just turned 7,200 miles into work this morning and it has yet to give me the RPM decrease - MPG increase you describe in this thread... I'll hold my breath 'til this happens. :uhoh2:

I must have mis-read this. Going from 245's to 265's will make the speedo read slower than actual speed.

Yes, the dic will show a worse average fuel consumption than with 245s because its working a little bit harder, yet the computer thinks its going slower than it is because it thinks its got smaller tires on it. So the dic will show less than 245s. But when you hand calculate it, it will actually be better in many cases than the 245s because the added circumference of the tire usually outdoes the tiny bit more force it requires to turn the larger diameter tire. That's one way of lookign at it.

The other way is that you can drive the same speed as with 245s at a lower rpm (even though the speedo will say you're going slower as a result of an uncalibrated speedometer).

The 265's will also make it look better for sure IMO and if you stick with the E load rating you actually have more payload capacity on the tire than on a 245 rated the same. They're also a lot better offroad and on ice. The stock rims will also handle the 265s without any problems at all whatsoever. And lastly, a 265 tire will last longer (compared to the same model in 245) but of course is also a little bit more expensive.

Also if you have upgraded your tire size, you can't just hand calculate by the miles driven that it shows on your ododmeter because that is still based on stock tires. So you add a certain percentage to it (3.88% to be exact). The gallons used will always be the same as those numbers come straight from the flow rates. Usually you can just add about 5% (easier that way) and probably be closer to the actual mpg, than if you were all stock and calculated by hand compared to dic.

Hope it helps.

I Put 265's on my truck the day I bought it and had the speedo verified at a certified shop after, the spedo was bang on!!!! with the 265's. The teck told me that the manufactures are allowed 4% either way from factory! that means that any given vehicle can vary 8% from one another,,,, so now my computer is.......acurate!

well changing the fuel filter at 12,000 didn't help at all, still getting 12 -13 in the city. I drive only 10 miles from work to home in bumper to bumper traffic every weekday avg 5mph, and the a/c is on #3. I guess the heat index around 115 here in Baton Rouge ain't helping either -:t

Well where do I start? Since 1999 I have owned 5 Silverados, 3 gassers and 2 D Maxs. Now don't get me wrong, I love chevy trucks but one thing I have learned is GM CAN NOT make an accurate fuel guage to save their life. Any of you that think they have a 1/2 tank of fuel left when the guage is at 1/2 are in for a suprise. I can go 100 miles before my guage even hits the F for full. The other thing I have learned is that RPM's make way more of a difference on the diesels than it ever did on the gassers. I live in Colorado at the base of the mountains and have made countless trips up into the mountains and out onto the plains. And no matter what I can get better mileage keeping the RPM's low. I have made 4 trips into the mountains and back (same exact trip) and each time broke 20 mpg. This trip I could not go over 65 mph because of the windy roads. Traveling east to Scottsbluff, NE I get just over 20 mpg, but only if I never break 65 mph. When I go 75 mph it drops to 18 or less. It appears that climbing or headwind has way less of an effect than speed. This by the way is always hand calculated. I have been trying to meticulously find an accurate way to increase mpg and have found that 65 mph or less and the edge on 3/2 is what's working for me. Hope this helps.
:rolleyes:

My truck gets over 20mpg (just a little) going 65 in flat Michigan (highway miles)with no load. It drops to 16 to 17 with a combination of city & highway.

Sdeines

I have noticed in the mountains that going from Denver to Breckenridge an hours + drive foot to the floor at an average speed of 75 I can get 16.5 MPG and the same on the wat back through the mountians. Now going to Kansas City from Denver at 75 MPH lucky to get 13

On stock tires, unloaded, I get 18-19 mpg around town hand calculated. On the highway it goes up to near 20-20.5

Sdeines

I have noticed in the mountains that going from Denver to Breckenridge an hours + drive foot to the floor at an average speed of 75 I can get 16.5 MPG and the same on the wat back through the mountians. Now going to Kansas City from Denver at 75 MPH lucky to get 13

Funny you should mention that. Pulling my 4000 lb cargo trailer around the front range I can get 16mpg at around 70 mph. However on a recent trip to Wichita, KS pulling the same trailer at 70 mph I got 14 mpg. Maybe the Kansas air is heavier lol:eek: . In any event I love my truck. Who'd a ever thunk we'd be pulling trailers around at 70-75 mph on leather seats, in air conditioned comfort, listening to satellite radio, making phone calls with the push of a button, and so on. I think we're all spoiled. My first truck, a 1960 GMC V6, was a far cry.

:grd: