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Old 07-09-2014, 07:06 PM   #1
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3.42 or upgrade to 3.73 gears

We picked up a Rockwood Windjammer 3001W in April to pull with my 2013 GMC Sierra 1500 Ext. Cab. It has the HD towing package, 5.3 V8, 6-speed tranny, 3.42 gears, and the GVW is rated 15,000 lbs. We weighed the whole rig on the way home from the dealer and tipped the scales at 13,060 lbs with two people and a full tank in the truck. I figure we have added between 500 and 750 in gear, but have kept a very close eye on weight. Currently I get from 8.5 to 9.5 MPG when towing around Texas and have had minimal issues pulling so far; I use the Tow/Haul setting most of the time. We are planning to pull to Oregon next year via Yellowstone (4,000 miles) and wondered if it would be wise to move to 3.73 gears for the mountains. I am adding the K&N cold air system and mandrel bent dual exhaust to boost power...and hopefully fuel economy also.

Has anyone out there made this swap, and did it help/hurt? I know my around town regular driving MPG would probably take a hit, but would it actually be better MPG when pulling with the lower gears?
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Old 07-09-2014, 07:15 PM   #2
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If my thinking is right, 3.73 would give you slightly lower mpg all the time. Changing the gears would be very expensive if its 4wheel drive.
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Old 07-09-2014, 07:50 PM   #3
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I have a f250 with the 5.4 and 3:73 gears in a 4X4, I tow a 3065w Windjammer . I get 10.5 mpg at best, that's keeping the speed under 60mph. I am also thinking of some mods to get a little more power. I agree changing the gear could be an expensive venture. Sounds like a good reason to get that diesel .
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Old 07-09-2014, 08:06 PM   #4
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2-Wheel drive

I haven't called the GMC dealer yet to get pricing, but mine is a 2WD so just the rear gears and some adjustments to the computer should be all that is required. I think pulling up the Rockies may tax the 5.3 with 3.42 gears; although I have pulled up some pretty good grades recently with no real issues.
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Old 07-09-2014, 08:15 PM   #5
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I haven't called the GMC dealer yet to get pricing, but mine is a 2WD so just the rear gears and some adjustments to the computer should be all that is required. I think pulling up the Rockies may tax the 5.3 with 3.42 gears; although I have pulled up some pretty good grades recently with no real issues.
It's much cheaper to just down shift and slow down a little. Unless you plan on spending a lot of time in the mountains over the next few years, it is not worth it to take the hit in mileage when not towing.
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Old 07-09-2014, 08:47 PM   #6
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I am a shop foreman at a GM dealer and own a 13 Chevy 1500 and a 08 Chevy Dmax. Going to a lower gear will hurt more then help. You are at your best setup with the engine, trans and gear you have. Also do your research on the cold air setup because some of them will cause issues with trans and may cause check engine lights. With that much weight I would upgrade to a diesel if you can. I pull 15k+ with my dmax at 70mph and get 15 to 16mpg. Whatever you do don't program you ecm with a power up programmer. It will void your warranty and even if you take it off we can still tell and is required to report to GM for warranty repairs on engine.
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Old 07-10-2014, 09:27 AM   #7
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changing gears

Excellent advice and just what I was looking for by posting. As I said, I use the Tow/Haul selector for most driving except on really flat stretches of road. I am not going to look further about the gear change, as I suspected the loss would be more than the gain and now it is confirmed by those with more experience and knowledge than I have. I have heard that the cold air sometimes doesn't help, but thought I would give it a shot to see. I am keeping all of the OEM stuff to put back on if needed. I am going to an upgraded exhaust with mandrel bent tubing and cat-back duals to pick up a little better breathing. The truck is only one year old (see photo), and upgrading to a new one is not in the cards (or budget) at this time. We picked up the trailer based on our ability to pull with the 1/2 ton, so we are still very pleased; I was just checking to see if something I could do would make it even better.
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Old 07-10-2014, 09:47 AM   #8
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I think pulling up the Rockies may tax the 5.3 with 3.42 gears; although I have pulled up some pretty good grades recently with no real issues.
Yes, you will have to go slow; sometimes it is hard to be patient on a 4,000 mile trip; the towing days can get long. You may be screaming up the grades at 4,000+ RPM; perhaps in 2nd gear. Just be prepared for it. Watch your transmission temperature. Some of the grades are steep and they go for miles.

It's wonderful territory though. Take your time and enjoy it. Have fun!
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Old 07-10-2014, 10:02 AM   #9
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I was pulling a Lacrosse 301rls about the same size with a 2008 GMC 1500 with a 373 / 4 speed. I upgraded to 2013 Chev 1500 with a 342 / 6 speed and it felt better than the GMC. It was shifting a little more when needed but there were no large jumps in RPMs like the 4 speed plus I got about one MPG.
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Old 07-10-2014, 10:06 AM   #10
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Why do you want a cat back system? The exhaust gasses have cooled significantly at that point and take up much less volume so bigger pipes aren't really needed. Just sayin'
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