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Old 09-19-2020, 11:36 AM   #1
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Why is gear ratio so important

I see where gear ratio is discussed on several threads. What difference does gear ratio make? Is it for the start of the pull or for highway speeds. Is it for gas milage or for power? Please educate me.

Thanks
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Old 09-19-2020, 12:01 PM   #2
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I assume you are referring to the axle differential ratio.


This ratio is the number of revolutions of the drive shaft turns for each revolution of the wheel. Higher numbers mean more torque and higher towing capacity but lower fuel economy.



Here is a link to a pretty good description:
https://www.edmunds.com/car-buying/h...kup-truck.html
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Old 09-19-2020, 12:04 PM   #3
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A lower gear like a 4:33 you will have all kinds of starting power and more power pulling hills but your top speed will be limited. You will run higher RPM and not as good fuel milage.
A high gear like a 2:55 will give you longer legs and better fuel economy. You will run lower RPM but you wont be able to pull hills as well.
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Old 09-19-2020, 12:12 PM   #4
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Ask a kid with a bicycle the benefit of gears.

On a bile having more gears makes hills easier, flat stretches faster and long rides easier.

Same with a truck pulling a trailer. Multiple gears lets you have multiple ratios.
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Old 09-19-2020, 12:39 PM   #5
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Quote:
Originally Posted by KenZ71 View Post
Ask a kid with a bicycle the benefit of gears.

On a bile having more gears makes hills easier, flat stretches faster and long rides easier.

Same with a truck pulling a trailer. Multiple gears lets you have multiple ratios.
Now you are talking transmissions
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Old 09-19-2020, 12:52 PM   #6
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Thanks for the help
Yes, I am talking about the rear end.

Ok glad I came for your help. Someone told me that the number 3.23 or what ever the number is how many times the wheels turn for every turn of the drive shaft. Sounds like its just the opposite by my thinking and from what I am understanding

Now of course my understanding my be wrong. "I know Nothink"
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Old 09-19-2020, 01:02 PM   #7
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Quote:
Originally Posted by TOO LONG View Post
Thanks for the help
Yes, I am talking about the rear end.

Ok glad I came for your help. Someone told me that the number 3.23 or what ever the number is how many times the wheels turn for every turn of the drive shaft. Sounds like its just the opposite by my thinking and from what I am understanding

Now of course my understanding my be wrong. "I know Nothink"

Yes, a 3.55:1 means the drive shaft turns 3.55 times for every 1 turn of the wheels. Strictly for best towing consideration, you want a higher number, like a 4.10:1. The trade off is, obviously more turns of the drive shaft for each wheel turn means lower top speeds and/or higher RPMs for a set speed. For most real world applications, this translates to lower fuel economy for a given speed.
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Old 09-19-2020, 01:07 PM   #8
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Quote:
Originally Posted by happy2rv View Post
I assume you are referring to the axle differential ratio.


This ratio is the number of revolutions of the drive shaft turns for each revolution of the wheel. Higher numbers mean more torque and higher towing capacity but lower fuel economy.



Here is a link to a pretty good description:
https://www.edmunds.com/car-buying/h...kup-truck.html
This helped a lot. My understanding was just the opposite as to what it really is.
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Old 09-19-2020, 01:46 PM   #9
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And all this time I've been thinking those numbers meant dollars per mile!
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Old 09-19-2020, 04:20 PM   #10
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Newer trucks, especially diesels, have more torque, and the transmissions have somewhat lower (bigger number) first and second gears, so a low rear end ratio like was used in the past is not needed as much.

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Old 09-19-2020, 04:35 PM   #11
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To get meaningful answers, please tell us what vehicle/chassis you have.


Said another way, rear axle ratio is a LOT more significant if you have a 3 speed transmission than if you have an 8-10 speed transmission.


And, on DP's with the very popular/good Allison 6 speed transmission the main issue is "at what speed do you go into 6th gear".


If rear axle is geared too high (low numerical ratio) you may not go into high gear until 65-70 MPH. That means there really is no "best economic gear" in terms of MPG-- with 5th gear being significantly better MPG than 6th at the high speed it would require.
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Old 09-20-2020, 07:54 AM   #12
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If you're looking at buying a truck or SUV then look at it this way.
Looking at gas powered trucks the rear gears can be lower (Higher numerically) 3.73, 3.92, 4.10, 4.30.
Looking at diesel trucks you can get lower gears such as 3.31, 3.55, 3.73.

Both versions benefit from higher numbered gears in that the higher the number the higher the tow rating as they allow the motor to rev more easily which in turn puts less stress on drive train and cooling components.

Generally speaking lower numbered gear provide better mpg's but are less efficient for towing. There's a crossover point in the middle where mid range numbered gears give decent towing and mpg's.
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Old 09-20-2020, 08:32 AM   #13
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The issue with gearing is what speed do you wznt to run while towing. When i orderedmy 2017 F350 i went with the 4.10 gear beacause i would be towing at 65 to 70 mph. At that speed with the 6 speeds .672 overdrive i am running 1950 to 2050 rpm. Tnta is what i figured before i orderd my truck. With the max torque at 1800 rpms i can run cruise contral and it very rarey shifts gears. Now empty my milage is 16.5 mpg on the road were my sons 2019 f350 with 3.55 gears will get 20 mpg on the road. I only use my truck for towing my 13k lb fifth wheel. The lower gears keep the engine at a higher rpm to make power.
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Old 09-20-2020, 08:34 AM   #14
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I think you are kind of new to this! Not only does the differential gear ratio matter it will also depend on transmission gearing and your tire diameters. This all plays into the towing equation with your truck. Everything about a towing vehicle is a compromised between fuel economy and towing torque when pulling. As point out an engine design also matters in this equation.

I like to have a diesel engine for towing and there are several reason. But for towing at least with a Cummins the torque is provided at a lower RPM and is maintained thru out the power band. So this means with a 3:73 gear ratio (which is a comprised) and factory size tires. The truck can start and pull my trailer with no effort. Never exceeding 2,000 RPMs, my transmission is a double overdrive style. So, I can tow in 6th gear at 1,600 RPM's and maintain 65 MPH all day long achieving 11 MPG in fuel economy.

Just saying you need to review everything not just gear ratio of a differential.
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