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Old 09-10-2020, 10:32 AM   #1
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'Rear Ends' in Your Tow Vehicle

No one seems to consider their Tow Vehicle's... Rear End gear ratio. You choose a color, an interior and options inside the cab you like and make the purchase. Rarely does anyone ask about the Rear End. Often the heavier trucks like a 3/4 ton or heavier have HIGHER REAR ENDS for a reason. They haul or tow larger weights.

Mechanics call them Differentials. Layman call them Rear Ends. Street mechanics understand both terms.

My F350 Diesel has a 3.55 Differential. It works very well towing a 27 foot Airstream. If you are towing a 30 to 34 foot Airstream... a 3.73 differential would probably be better for your needs.

The ‘caveat’ is that the HIGHER the Ratio number, the easier it is for the Tow Vehicle to pull a heavier Trailer... but your Gas Mileage will suffer. Lower Rear End... higher gas mileage when not towing. Higher Rear End... lower gas mileage when not towing.

Fords have an identification plate for their... Rear Ends. So would other pickup trucks. the 4x4 have matched Front and Rear Differentials. I did not check, but when in 4x4 if they are mismatched, trouble when engaged.

The Lower the Rear End... the better your gas mileage without a trailer in tow. The Higher the Rear End... the worse gas mileage. This is the trade off when towing and if you spend more time Not Towing.

I remember in the 1960’s that the Hot Rodders wanted the larger Differential Casing for a 4:11. This gave the ‘hot rod’ a great boost offf the line... but topped out the engine RMP into Red Line quicker at higher speeds. A 3:08 Rear End was a bit sluggish at the start, but when the RPM began to increase... so could its Top Speed.

I wanted a Higher Rear End in my Hot Rod... a 4:11. I could get a 3:73. Why? Because the Casing was larger in the 4:11 and would not fit into the casing of a 3:73. Learned that at 20 years old. My rear end had to stay as it was.

Take some time and check your specifications on YOUR Tow Vehicle. If you notice it is a bit of a struggle to get moving from a stop light... it could be YOUR REAR END.

Google: Ford Differential Options as an example.

Those bragging about GREAT gas mileage probably have a LOW Rear End Ratio. Those with a lower gas mileage having a HIGH Rear End Ratio.

A drive shaft turns 3.55 times to turn a wheel. A 4:11 Rear End turns the drive shaft 4.11 times to turn a wheel. This is how I see it. Your engine will have a Higher RPM with the 4:11 but be moving slower than the 3:08 at the same RPM. We called it "Top End". Red Lining... your engine's maximum expected RPM before things become 'iffy'. Two bolt mains. Four bolt mains. My Diesel may have Four Bolt Mains... I would not be surprised. I need to check that out.

What is your Rear End... experience? My 3:55 pulls our 27 foot International easily and the additional cost of fuel is just the price I pay to tow in comfort.

Please correct me with any errors. My 'Hot Rod' days ended when I had married and a family in the last century. Sometimes... you have to consider that a Rear End does have better uses... and that is for towing.

I like mine, how about yours?
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Old 09-10-2020, 11:25 AM   #2
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I've got a 3.42 axle ratio in my TV. I also learned about this back in my hotrod days. My Pontiac Firebird had a 3.73 axle ratio, and it had cherry bomb mufflers. It was LOUD. I miss those days sometimes...
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Old 09-10-2020, 11:34 AM   #3
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Our F150 3.5EB tows out 27FB with 3.55. If I had a choice I would go higher just because I feel it would be less wear and tear on the drivetrain. Pulling a grade is never an issue with it. I don't think it matters as much with the 10 speed transmission as it would with a 4 speed.
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Old 09-10-2020, 11:39 AM   #4
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Ray, it is not a simple as it was in the hot rod days. 8 and 10 speeds have multiple overdrives, allowing for numerically lower gear sets. This allows for both high mileage, both solo and towing. Narrow ratios in 6,7,8,9, and 10th allow for optimum torque curve placement.
There's very good mechanical reasons for extraordinary mileage towing with modern drivetrains.
Most folks cant select a rear end by itself from the OEM. The ratios are forced by such things as max tow package or tire and wheel packages.
Also, many times, bigger is more efficient than smaller engines. I compared my company 5.3l maxtow to my 6.2l maxtow. With a 30 classic, the 6.2l gets more than 2 mpg better towing than the 5.3. Both with 8 speeds and 3.42 rear gears. Maxtow also gets you a 9.75" gear set, with GM, as opposed to a 9.5". The entire axle housing, shafts and pumpkin are physically larger than a non-maxtow.
Proper spec'ing is a science, more than ever.
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Old 09-10-2020, 12:39 PM   #5
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You forgot about the other half of the equation....

Ray,

In days of yore, the selection of a rear end ratio was much more important than it is today. Back when you only had "three on the tree", you had to choose a differential geared for "fuel efficiency / speed" or "towing prowess" or some mushy space in between. This is no longer the case.

Driving and towing and gas mileage and all the other factors we know are defined by the overall drive ratio for a given gear. This overall ratio is the product of gear ratio and differential ratio. To focus solely on rear end gearing is a red herring at best, and disingenuous at worst. When GM brought out the redesigned HD trucks for 2020, lots of un-informed fanboys (and quite a spattering of Ram and Ford aficionados, too) were quick to jump on the "why did they lower the ratio?" and "why is there no 3.73:1 option for the diesel?" bandwagons. I submit that most of these guys don't understand that overall drive ratio (the product of differential ratio and the respective gear ratio) is what matters. The engineers and designers have figured out how to create vehicle drive trains that can do it all. How is this possible?

For example, my 2020 Duramax has a 10 speed transmission (with the top 3 being overdrive gears) that is only available in a single 3.42:1 ratio. GM has selected the respective ratios of the 10 gears to cover an extremely large range of drive ratios. Then the smart computers take all the input and decide which gear is most suited for the conditions at hand. Lower gears are great when you need to pull heavy loads from a standstill, and the higher gears are great for keeping RPM minimized when the rig is up to speed. (With all the gear ratio possibilities, you can almost think of this as the modern equivalent a dual-speed differential!) Unless I'm in tow/haul mode, 1st gear is completely locked out and I start in 2nd. I really only reach 10th gear with I'm on straight freeways with no load/trailer. I'm using GM as my example because that's what I'm familiar with. The other brands have similar engineering philosophies as well.

So now that we're willing to shell out $60k or $80k or $100k for trucks, the manufactures can design and provide massively complex computer-controlled transmissions go a long way to negating the impact of a fixed rear end ratio. And we're all better off for it.

At least until the transmission fails. But that's probably a topic for a different thread on a different forum...

POI: When I was researching the upgrade from my 2015 Duramax to a 2020 version, I made this comparison. Look at the range of drive ratios the 10-speed Allison transmission covers:
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Old 09-10-2020, 01:51 PM   #6
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Hi Ray, Iíll play...

2019 Ram 3500 6.7 L high output Cummins with 3.73 rear end. Towing 30 ft International.

Fuel mileage around 12.3 mpg running 68 mph. Engine turns about 1,600 rpm at 68 mph.

Happy with gear ratio. Plenty of torque to start. Accelerates to 68 mph by the time I merge onto Interstate. Camping this week at Mama Gertieís in Asheville, steep climbing to the upper tier sites. I was impressed with how easy it pulled up mountain.
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Old 09-10-2020, 10:57 PM   #7
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I would have preferred 3.73 gears in my Ď18 Ram 3500 SRW but only 3.42ís were available. I wonder why they went to 3.73ís for 2019?
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Old 09-10-2020, 11:25 PM   #8
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Hi, I don't know of any Ford differential housing that has to be changed to go to 4:11 or 4:10 gears. Most Fords with the tow package came with 3:73's gears up until the game was changed by the 10 speed transmissions. One of the reasons that we call them differentials is because that is what they do. [while making a turn] Oh, and would you ever call the front axle on a 4X4 a "rear end"?
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Old 09-11-2020, 07:06 AM   #9
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The gear sets are not differentials. The ring gear bolts to the differential housing. The pinion attaches to the axle housing. The differential is made up of shafts and spider gears. The whole assembly is a "rear end" , or rear axle assembly.
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Old 09-11-2020, 08:04 AM   #10
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My rear end limitation is the size of my own rear end, and the ability to lift myself in the cab of my Silverado 2500. js..
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Old 09-11-2020, 08:08 AM   #11
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Originally Posted by Acheron2010 View Post
My rear end limitation is the size of my own rear end, and the ability to lift myself in the cab of my Silverado 2500. js..
I got a bony butt. Does that make it a tall or short rear end?
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Old 09-11-2020, 08:44 AM   #12
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Gearing. Some people get it - some people don't.

My current F150 has 4.11 gears since I'm running 35's. Yes it runs higher RPM's but my mileage went up and it tows like a dream.

With how I'm building my Airstream, I know I will have to upgrade to a 3/4 or 1 ton truck.

When I do that, it'll be getting the lowest gearing possible. 3.55 for a f250 or you can get the f350 dually with 4.10's.
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Old 09-11-2020, 08:51 AM   #13
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True. Whatever you have and whatever you get, just be sure that you calculate "effective" gear ratio, including the top gears in whatever transmission you get, and including tire diameter increase. Compare that to the engine torque curve, as built. That will tell you what gear set to get. With modern transmissions, I wouldnt assume the tallest available is necessarily the right one given your relatively light(er) load.
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Old 09-11-2020, 09:20 AM   #14
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True. Whatever you have and whatever you get, just be sure that you calculate "effective" gear ratio, including the top gears in whatever transmission you get, and including tire diameter increase. Compare that to the engine torque curve, as built. That will tell you what gear set to get. With modern transmissions, I wouldnt assume the tallest available is necessarily the right one given your relatively light(er) load.
Agreed. I'm one of those people that can't leave a truck well alone so for me and playing with bigger tires - gearing always helps.
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Old 09-11-2020, 10:27 AM   #15
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Yep, our F-150HD with max tow has 3.73 rearend - in 2013 it was a 6 speed trans. Hauls our 2004 International like it's not there, have to check the rearview mirror to make sure we are still hitched .
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Old 09-11-2020, 10:29 AM   #16
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Our 2019 NV 3500 has a 3.54 rear end.
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Old 09-11-2020, 12:03 PM   #17
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Did someone say rear ends?

Agreed with comments above that today, it's more about overall gearing, including transmission and tire size. Rear ends are no longer a good indicator to bias of overall gearing. As modern 8+ speed trannies have much wider gearing spans for wheel torque and cruise efficiency.

Low gears are great for pulling power but it should also be mentioned they are equally good for engine braking.

Don't forget the front end too if you're re-gearing a 4WD. Re-geared to 4.3s on my tow vehicle but it's also be cause I have 35" tires. Added a rear locker while I was in there.

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Old 09-11-2020, 01:35 PM   #18
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Originally Posted by ROBERTSUNRUS View Post
Hi, I don't know of any Ford differential housing that has to be changed to go to 4:11 or 4:10 gears. Most Fords with the tow package came with 3:73's gears up until the game was changed by the 10 speed transmissions. One of the reasons that we call them differentials is because that is what they do. [while making a turn] Oh, and would you ever call the front axle on a 4X4 a "rear end"?
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Old 09-11-2020, 03:49 PM   #19
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I have a Ford 460, C6 3 speed auto, and a 2.75 gear set vehicle. We have towed 3 AS trailers and a few square trailers. It always got the job done and still does.

I have a Ford 6.2, 6 speed, and 4.30 gears in a truck that weighs almost twice what the first one does. It makes way more HP, probably a little more TQ. It does a better job. It should. It's got 40 years of technology on option #1.
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Old 09-11-2020, 04:06 PM   #20
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Towing Differential Choices

The Airforums has had numerous Threads and Posts about Which Tow Vehicle is Best for My xx foot Airstream. Engine. Half Ton, 3/4 Ton, 1 Ton, Diesel, Gasoline, 4 wheel drive, comfort, seating options...

Jaybauman with his Post #5 opened a can of worms... This is Good! We need more of these conversations. I was using my experiences working on mid 1960's cars with four speed clutch transmissions. Now with 10 Speed Automatics... I am ignorant, but getting smarter as of, today.

Because of our Off the Grid Boondocking... the 4 wheel drive option is very important to... ME. When I needed to use 4x4, the option is worth the cost. It is also an excellent Sales Question when you are selling.

The length and Gross Weight of the Airstream and the ability to Tow with any vehicle is complicated.

Some owners have success towing a 30 foot Airstream with a 1/2 ton vehicle...

Read some of the 'numerous Rear Differential' Threads on searching Towing Differential Choices. When you have found something NEW and INTERESTING, please add your discoveries.

Rear Ends DO and CAN make a difference. I have a 3.55 Differential in my June 2016 F350 4x4 Diesel Ford King Ranch. It sat on the Dealer's Lot in 2017 as these new models had a larger fuel tank and aluminum beds. Maybe hood? I wanted the steel bed as I haul rock, fossils and agate in buckets. I acted as if 'I wanted the aluminum bed' for the weight reduction. I got a 'price reduction'... happy, happy me.

Below is an ad for the 2020 Chevy Silverado 1500... Attention is now coming to those buying Tow Vehicles for their Trailers and Fifth Wheels and Boats and... whatever.

(Search) 2020 Chevy Silverado 1500 Truck Engine Options & Towing Capacity (on Internet)

"The 2020 Chevy Silverado 1500 provides two different axle ratios for you to choose from. Whether youíre looking for a higher towing capacity with the 3.42 rear axle ratio or better fuel economy with the 3.23 rear axle ratio, the 2020 Silverado 1500 has your back."
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