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I am soon going to purchase a new or slightly used GM full size truck, I would like to know if there is a Engine, transmission, rear end that will give me the best highway mileage. I see 20 and 21 being advertised yet everyone I ask says 18 is the best I can even dream about?

thanks

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There are a few reasons why nobody duplicates window sticker. Most having to do with the EPA's flawed testing, and the factory tires being LRR so most replacements will come with reduced fuel economy.

 

If you set the cruise on my truck at 50mph I might top 20mpg but who is going to travel any distance at 50mph? Higher you set the cruise the worse the mileage is, my cars aren't like that within reason, but the truck pushes so much air it just doesn't like doing 75mph.

 

FWIW mine is a 2005 GMC Crewcab Z71 with the 5.3/4l60E/3.42s, I would bet 3.73s would improve mixed mileage without hurting highway, I doubt 4.10s would even hurt on the highway, have to consider the size tires these things got. I am getting about 16mpg average on my rural commute with warmups and a heavy foot. If I reset the DIC I can see a solid 18.3 keeping the cruise set under 70mph on the highway, soon as I get into anywhere I have to stop and go that number tanks hard.

 

Acceleration KILLS mileage with these things as they are quite heavy.

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Ridiculously overgearing like 3.08s with a double overdrive is NOT the answer to fuel economy. If it was then using overdrive in town would be right too and we know that is not right. Lugging an engine is a bad thing.

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Ridiculously overgearing like 3.08s with a double overdrive is NOT the answer to fuel economy. If it was then using overdrive in town would be right too and we know that is not right. Lugging an engine is a bad thing.

 

 

Fully agree, but the OP said highway milage which is where the 3.08s are good at, the only place.

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The 2008 extended cab, 4x4, 5.3, with the 3.73 gearing seemed to be a great combination for me but 18 mpg would probably be on the high side for gas mileage although it would do it on straight highway driving. Mileage is an illusive figure as if I filled up then headed right down the highway the DIC would probably read 18 or 19 but after a few hills or stopping at a stop light & accelerating a couple of times the mileage would drop down to 15 point something or 16 point something pretty fast. The best I ever did was 20 mpg under ideal conditions, no wind, just fill up and drive 120 miles without stopping, then fill again. Normally got numbers like 16 and 17 mpg and was happy with that. Had a 4x4 regular cab with the 4.8 engine and 4.10 gearing which got worse mileage than the 5.3 with the 3.73 gearing. Just got the new 2011 with the 3.42 gearing and it is too early to tell what kind of mileage it will get.

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I am soon going to purchase a new or slightly used GM full size truck, I would like to know if there is a Engine, transmission, rear end that will give me the best highway mileage. I see 20 and 21 being advertised yet everyone I ask says 18 is the best I can even dream about?

thanks

 

 

5.3/6speed/308gears...18 inch wheels....bit over 18 around town...have only towed a one car trailer over the road...confident I would get 20 or better not trailering....1800 rpm or so @ 70 mph

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i have an 04 sierra ext cab 2wd. 4.8 4l60e 3.42, slightly bigger than stock tires. i average 16.7-17.4 thats about 70% city 30% hwy. no towing at all. i also put my foot in it quite a bit.

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You can get 20 in a truck. Most trucks won't at the speeds people like to drive though. I drive on a 55 mph highway to work. I get 20-21 on that highway. My overall hand calculated mileage because of a little city driving and some warmups puts my overall average about 19. Best tank I ever got was 21. But it gets 19 without to much straining. Mainly just keep a steady gentle foot and keep top speed under 55.

Mine is a 5.3 4 speed 3.73 GMT-800. You can get some newer configurations that would probably allow you to get that at a little faster speed. I would probably be looking at a 5.3 6 speed with 3.42. 3.08 does seem a little extreme.

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