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2015 8-speed Silverado/Sierra fuel economy set at 21mpg highway


Zane

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Well the 6.2 with the 8 speed it great lot of power very fast for a truck but the transmission on the highway and easy starts is good but for speed up slow down traffic ( 10 - 40 - 30 -10 - 40 )it feels worse then the 6 speed clunks and hesitats and the AFM really kicks in and out very hard it's very noticeable during this time also. But so far doing great on gas for 420 hp looks like I should be getting about 17 to 18 mpg a combined I have been going easy on it crushing at 63 mph and 40 in town I guess the real test will be when I tow something to see how the new transmission works out is there anyone else that has one yet with anymore input

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Well the 6.2 with the 8 speed it great lot of power very fast for a truck but the transmission on the highway and easy starts is good but for speed up slow down traffic ( 10 - 40 - 30 -10 - 40 )it feels worse then the 6 speed clunks and hesitats and the AFM really kicks in and out very hard it's very noticeable during this time also. But so far doing great on gas for 420 hp looks like I should be getting about 17 to 18 mpg a combined I have been going easy on it crushing at 63 mph and 40 in town I guess the real test will be when I tow something to see how the new transmission works out is there anyone else that has one yet with anymore input

 

:noway:

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  • 3 weeks later...

21 mpg is really terrible in the big scheme of things. That is less than 3 times the mpg of my semi truck and trailer, 5 axles, 70' long, and fully loaded to 80K lb, while the pickup weighs in (empty) at 1/10th the weight. Amazing that they still can't seem to get pickups into the 30 mpg range.... on a bad day.

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Every semi I've ever driven has got 6 mpg ... but I've been out of that line of work for almost 10 years now. Straight 3 or 4 axle trucks got the same.

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Full disclaimer I am not a truck driver, nor do I work on diesels, nor have I ever sat in the seat of a big rig.

 

With that out of the way, the big difference between a Big rig and a truck is obviously the Diesel part of this. With that said, a Diesel is designed to really operate at a particular RPM for best efficiency. I have read that Auto Big rigs have as many as 10 gears, and the manuals have 9, 13, or as many as 21 gears.

 

First off a manual shift car or truck is just more efficient (if you know how to drive) and with so many gears, and a little knowledge you can put the engine in that sweet spot of best economy/power for the haul you are doing. So to really compare an Auto 8 with a Manual 9 or 21 is really no comparison.

 

Secondly This is a Gas engine, which is not as efficient as your typical diesel. I have an acquantance that purchased a Ford Pickup with a turbo diesel and he was getting 36mpg in it... Of course the Dealership asked him to bring it back, claimed there was something wrong with it, then Ford called him asking for it back... Come to find out it had a european chip in it... He never took it to a Ford dealership for service...

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I average about 18-19 in the summer with my 2007 Tahoe LTZ, and right now it looks like 16-17 in the winter.

 

It is all about how you drive... For example, If your wheels are black from brake dust... You might have no clue how much energy you waste by heating up your brakes....

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As for a two stroke engine, Achates is working on designs of single cylinder, opposed piston engines that have shown in testing to finally break the 50% threshold in brake thermal efficiency. They can make engines from 45 hp to 5000 hp. A lot more compact, fewer components, and 30% better fuel efficiency. Surprisingly, this whole idea is not new. Junkers aircraft, in the 1930's Germany used engines of this design with great efficiency over typical piston engines.

 

http://www.truckinginfo.com/channel/fuel-smarts/article/story/2014/10/opposed-piston-diesels-about-five-years-away.aspx

 

As for current truck diesels, JSD... I typically average mid 7's mpg year round. Would be a little better if I ran some of it down south. I only operate in the upper tier of the U.S. year round. But the elusive 10 mpg ceiling was recently broken on a documented haul with a Freightliner Cascadia, Detroit 15L, and 12 speed trans, grossing 72,000 lb from California to N.C. Freight liner monitored the trip. I know the driver who did it. He has always been notorious for tweaking out mpg. Now if that kind of mpg can be delivered on a 70 foot truck, 13.5 feet high and 8.5 feet wide, on 5 axles can get that kind of mpg, it really speaks volumes about how the R&D at the pickup OEM's cannot get a diesel pickup much higher than 20 mpg

pulling a light trailer.

 

But engine designers are doing amazing things, just that the vehicle OEM's are keeping their heads where the sun doesn't shine. Like engine makers Ricardo and Cummins. Both have extensive test data results on inline 4 and V6 engines with HALF the displacement of diesels and gas engines they are putting to shame... and doing it on E85 ethanol. Imagine, a 3.2L V6 putting down as much HP and Torque as a 6.6L Duramax diesel and getting mpg numbers that would make even the diesel crowd sit up and take notice. Cummins 2.8L inline 4 running on E85 will shame a 5.7L V8 in power and economy. And since these are E85 engines, they do not require any more emissions junk that the typical gasser. Saving gobs of weight and cost. The diesels have to have a myriad of emission control components that can be a total nightmare when things go wrong.

 

Yet the OEM's keep feeding us engines that are so old school in just about every way and pump sunshine up their drawers about how much better their engines are. No, they are no all that much better, just more complicated. My 1998 454 V8 in a Chevy 2500 got almost identical mpg numbers as my 2013 5.3L in a 1500. Yep, that is darn right criminal. In that time frame, the average semi truck has improved mpg by 25-30%. But not pickups. No sir, they have gone backwards.

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Great info there Cowpie. I always hear from guys who have rented cars overseas, and the mileage they get out of the little diesels across the pond is AMAZING.

 

Somebody is making a killing off of us ....

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Why would GM drop the 454 if it was capable of meeting CAFE numbers and emissions as the current engines do? To put it simply, it can't. It is also amazing how every decade has its secret or "kept from public consumption" magic engine design, carburetor design, or fuel additive that is so evolutionary that it cannot be released for fear of it collapsing the entire oil industry.

 

Not saying there are not new designs coming, be foolish to do that. Only have to see the improvement that direct injection has made to see how something that already existed could be applied to another fuel type engine and make the improvements it did.

 

Comparing heavy duty diesel engine economy numbers to light to medium duty diesel engines is not even remotely relative. Basic things like rpm operating range alone will account for a fair chunk of the economy difference.

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Cowpie, on 12 Dec 2014 - 8:45 PM, said:snapback.png

My 1998 454 V8 in a Chevy 2500 got almost identical mpg numbers as my 2013 5.3L in a 1500.

 

 

That was one exceptional 454, or you have something wrong with the 5.3. I have a lifetime average of 16.2 MPG in my '13 Sierra 5.3. My daily commute is a short one and it's all city. In the winter, I use the remote start A LOT. I have it tuned, and use non-ethanol 91 octane fuel. Trips are always 20 plus MPG at 70 MPH plus speeds.

 

I have a 1972 Chevy truck, 402, Q-Jet TH400 4.10's. It is a 13.5 MPG truck empty if I stay out of the secondaries. While towing a load of almost 6000 lbs, I averaged 10 MPG over 300 miles.

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I get not much better than 10 towing that weight with the 5.3 & 3.73's .... so much for technology. Maybe 3 mpg better empty if I baby it.

 

We also have no choice in the Communistwealth of Massachusetts as far as ethanol goes - it's in EVERYTHING, no matter the octane rating. Just from traveling around the country, I know that's good for 2-3 MPG all day long.

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