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Old 03-24-2008, 07:46 AM   #1
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Gear Ratio Change?

I'm currently towing a 23' tandem Safari with an '07 GMC pickup equipped with the towing package, 5.3 liter engine, and 3.73:1 rear end gears. I am wondering if anyone has first-hand knowledge of the improvement in a simular truck after switching to a 4:10:1 rear end gear?

The truck does a great job towing on the flat at up to 75 or so, but who wouldn't want more power on the hills?
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Old 03-24-2008, 08:30 AM   #2
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You may notice an improvement on the hills but you will also notice fuel mileage decrease as you will be turning more rpms with the higher gear ratio. I would have to toss around how often I went to the hills versus flat land driving. Especially since fuel costs here are 3.36 per gal for reg unleaded
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Old 03-24-2008, 09:17 AM   #3
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It looks like you have a good set up for the application and trading to a lower ratio will impact your mileage when not towing, not so much around town but if you drive interstate I would guess a drop of 3 to as much as 4mpg depending on the speed you travel. Also, if you have 4 wheel drive, I think you would have to change the front ratio as well. Big bucks. On such a new truck the 3.73 is a nice mix and work well on our half ton for nationwide towing a 28.
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Old 03-24-2008, 10:14 AM   #4
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Your truck has a 4 speed auto with tow haul, correct? I have the same setup
and I STILL tow in 3rd gear. Save your transmission! The difference is about 500 rpms and overdrive isn't involved. Your power band is 2000 to 3500 rpm which translates to 60 to 75 mph. plenty of power and no constant up and down shifting ( from overdrive (4th) and back to third). You still have the economy of 373 gearing when not towing.
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Old 03-24-2008, 10:41 AM   #5
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Steve, I studied this and my experience is that a gearing change can make a significant difference in a small block 1/2 ton. Factory gearing is a compromise.

I went to 4:30 gears and wish I had done it a long time ago. It is a completely different towing experience! The biggest problem it solved was "gear hunting" at interstate speeds. The rig just always seemed to be wanting me to upshift or downshift. Acceleration improved noticeably when towing.

The big surprise was that towing mpg went up 2 mpg! No significant mileage change around town so long as I do not get heavy footed. Just a broader more usable power spread now.

I used Ford Racing gears which were only a small bit more expensive and designed to take abuse.

..................Scott

btw: I built a spread sheet which put me at about 4:10's but I went with 4:30's, allowing me to to install larger diameter load-range D and E tires. The spreadsheet, combined with my engine's hp/torque curve let me find an optimal engine/road speeds in various gears for towing.
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Old 03-24-2008, 02:19 PM   #6
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Steve,

I did the gear change to my 2001 GMC Yukon. I went from the 3:73 to the 4:10. The difference is extremely noticeable towing. It is very easy to do with a 2wd truck because there is on one diff. I have over 30,000 miles towing with the rig. There is a little drop in mileage not towing, and towing I had no change in gas mileage. Before the change the rig was always seeking a different gear on the slightest incline. with the change, I can tow all day long in OD. The sweet spot is about 62 mph. Uphill it will tow at 2800 rpm and the engine will never overheat. I search for the gear that will gives me 2800 rpm.

I ramble, because I'm trying to do too many things at once.

PM if you want more info

Mark Wegman
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Old 03-25-2008, 11:52 AM   #7
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My truck is two wheel drive. I understand there will be sacrifices in mileage when not towing, and that sort of information is what I'm after. I checked today, and at 70 MPH, the truck is turning 2000 RPM in OD, and if I drop down to 3rd, the RPM goes up to 2800, and I do tow in 3rd if there are a significant number of grades, and/or winds to deal with. I'm dissapointed with my current towing mileage (10 on my last trip to Big Bend Park), and that was at an average speed of 65. We were, however, unlucky to have head winds both ways. Scott, I am surprized to hear your mileage went up with the gear change. I wonder if my GMC would do the same as I see you are driving a Ford. Mark, your information is sort of what I was expecting to hear. I did notice you are towing a thirty footer, and so now I need to figure out if the change would be best for me towing the smaller 23'. Thanks for everyone's input.
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Old 03-25-2008, 02:57 PM   #8
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Quote:
Originally Posted by SteveH
My truck is two wheel drive. I understand there will be sacrifices in mileage when not towing, and that sort of information is what I'm after. I checked today, and at 70 MPH, the truck is turning 2000 RPM in OD,
Steve, With a 4:10 rear end, your speado will read 65 at 2000. The additional torque to the road is substantial. I couldn't believe the difference.

I bought the gear and pinion online.

Mark
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Old 03-25-2008, 06:29 PM   #9
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I got the 2007 Silverado 3/4 gas with the 4.10 rear end.
I never run out of power, and I live and travel in the mountains.

As you get above 6,000 feet, power always goes down, but
though it gears down, I never am slowed down.

Though you could put a 4.10 in, I was advised that is very expensive,
since it is not just the rear end that needs mod...

Gas mileage is 11 over 1000 miles for summer and 10 over 2000 miles
in the winter. 16 w/o trailer on the road.
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Old 03-26-2008, 06:39 AM   #10
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Quote:
Originally Posted by SteveH
I am surprized to hear your mileage went up with the gear change. I wonder if my GMC would do the same as I see you are driving a Ford.
I suspect the towing mpg increased because my truck was just overgeared for towing the Safari. So the engine was never in the optimal power band at highway speeds (i.e. needed too much throttle or being over reved). The mpg jump would be not a Ford/Chevy difference- just more significant because of my smaller motor.

Does your tranny have a "towing switch" to disengage the torque converter lockup? If so, that would give you a very usable 4th gear for towing and really let you take advantage of the 4:10 gears. If not, you could call someone like Troyer Performance for a chip with 2-4 different shift patterns. That would give you an optimal towing pattern and the abilty to switch to an optimal non-towing pattern for mileage or one for high altitude towing etc.

I think that a chip and 4:10 gears would really make the Chevy's power available, increase mpg, and make you a very happy tow'er

.................Scott
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Old 03-26-2008, 07:01 AM   #11
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Scott,

Yes, my transmission has a "tow/haul" mode, but it just adjusts the shift points and the down shift points....it does not eliminate operation in overdrive. I too believe the towing would be much better with 4.10's, but I'm just trying to figure if it's the best move for me as the truck is also my "daily driver", and the unloaded mileage is part of the equasion for me. At minimum, I think I'll wait and drive the rig as is on our planned trip this summer to Alaska and see how it goes. I did learn the "feel" of the down shift points in the throttle pedal on our trip last week to the Big Bend Nat. Park, a trip of about 900 miles towing. We came back on IH10, and there are lots of grades out West of San Antonio that are pretty steep, but fortunately, not too long.
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Old 03-26-2008, 07:05 AM   #12
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FWIW; we had 2 indentical Burbs, one had 3:73 the other 4:10. No difference in mpg at all. Towing was always done in TH mode and sometimes in 3rd.

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Old 03-26-2008, 08:04 AM   #13
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Quote:
Originally Posted by SteveH
Yes, my transmission has a "tow/haul" mode, but it just adjusts the shift points and the down shift points....it does not eliminate operation in overdrive. .
Overdrive and torque converter lockup are two different aspects of your tranny. Overdrive is simply your top gear (usually a bit higher than 1:1, hence the name "overdrive"). Whether or not you can get practical use of OD for towing depends upon your overall gearing and engine power. The torque converter lockup feature "locks" the torque converter from slipping at highway speeds for mileage but is not recommended to use when towing. However, in some vehicles the only way to disengage the torque converter lockup is to drop down one gear, effectively eliminating one gear and forcing you into a less usable gear spread.

I do not know about your tranny's mode switch, but if it has a configuration to disengage the lock up and still run in top (OD) gear, you can make the top gear usable for towing with 4:10 or 4:30 rearend gears. Having 4 usable gears instead of 3 would greatly increase your gear spread, usable power and terrain options. You may want to call Troyer Performance they really know this stuff!

.................Scott
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Old 03-26-2008, 09:04 AM   #14
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Scott,

Yes, I'm aware of the different gears, and the torque converter lockup. My transmission goes into OD, and the torque converter locks up in Tow/Haul mode. I've watched the tachometer and it's the same in T/H, or out. It just goes higher in the gears, and down shifts quicker. While towing on grades, you can watch the tach, and see it first shift out of lockup, and then down shift to third gear. And, by the way, the TC will lockup in third gear also. As far as what's "recommended" by the GMC, I have searched the book and all they say is to tow in "Tow/Haul" mode.
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