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2016 Ford F-150 XLT 2.7 Ecoboost 3.55 Axle
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Discussion Starter #1
I’m not dumb when it comes to mechanics, but could someone kinda give me some tips on what’s what with gear ratios? Like I hear guys saying, “yeah my f150 is a 3:55 rear end, or a 3:73” what does that mean? Is the 73 like a larger gear therefore the truck is faster?
 

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Numerically lower is for top speed and mpg, numerically higher will give you lower speed but take off quicker and offers lower mpg.
3.15 is 3.15 turns of the driveshaft equals 1 rotation of the wheel.
Numerically higher offers a better mechanical advantage for towing. Think of your bicycle in 1st gear vs 3rd gear.
3.73 and 3.55 are the two most common ratios for trucks, offering a mix of mpg and towing.
 

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Example: I have 3.55 gearing but wish I had 4.10 for burnouts.
 

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Shorter gears(higher number) does not automatically mean faster. Back in the day when 4 speed transmissions were the norm, they definitely made a difference. But as the number of gears have increased, the ratio spread for the transmissions has grown making the axle ratios less and less important. Now, with the 10 speed trucks, I doubt there is much difference between them at all. The first gear on the 10 speed is so short that you still get a really short gear for getting off the line even with 3.31's.

Also, our engines are so powerful and torquey that it usually overcomes the need to have a high axle ratio. If you had a 5.0 where the torque peaks at much higher RPM, you might see an advantage to higher axle gear ratios.

That said, it really depends on your specific scenario. My truck is a 6 speed trans with 3.73's. When I am climbing a hill with my trailer at 65mph, my truck physically cannot shift to 2nd gear because the engine would go past redline. If I had 3.31's or 3.55's, I might actually be able to drop to second and run closer to 5000 rpm where peak HP is.

So for me, a slightly taller ratio might actually make more sense even though the 3.73's technically provide more mechanical advantage. The key to going fast, pulling a trailer up a mountain, etc is being able to sit at peak HP. HP tells you how much twist an engine can make at the rear wheels compared to another engine. If my Honda makes 200 hp, and my truck makes 400 hp, I can twist my rear tires twice as hard in the truck. But my transmission and gear ratios need to let me reach that 400hp since it only happens in one narrow RPM window.
 
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2016 Ford F-150 XLT 2.7 Ecoboost 3.55 Axle
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Discussion Starter #5
Shorter gears(higher number) does not automatically mean faster. Back in the day when 4 speed transmissions were the norm, they definitely made a difference. But as the number of gears have increased, the ratio spread for the transmissions has grown making the axle ratios less and less important. Now, with the 10 speed trucks, I doubt there is much difference between them at all. The first gear on the 10 speed is so short that you still get a really short gear for getting off the line even with 3.31's.

Also, our engines are so powerful and torquey that it usually overcomes the need to have a high axle ratio. If you had a 5.0 where the torque peaks at much higher RPM, you might see an advantage to higher axle gear ratios.

That said, it really depends on your specific scenario. My truck is a 6 speed trans with 3.73's. When I am climbing a hill with my trailer at 65mph, my truck physically cannot shift to 2nd gear because the engine would go past redline. If I had 3.31's or 3.55's, I might actually be able to drop to second and run closer to 5000 rpm where peak HP is.

So for me, a slightly taller ratio might actually make more sense even though the 3.73's technically provide more mechanical advantage. The key to going fast, pulling a trailer up a mountain, etc is being able to sit at peak HP.
Interesting. I think I’m beginning to understand it.
 

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Numerically lower is for top speed and mpg, numerically higher will give you lower speed but take off quicker and offers lower mpg.
Numerically lower ratio is no assurance of superior MPG. If your statement was always true then the taller the tire you mount the better the MPG.

As a rule of thumb starting point, the most work for least amount of fuel occurs at or just under peak torque RPM. Poorer MPG at lower RPM. Poorer MPG at higher RPM. But if there is not enough HP there then you have to run higher RPM.
 

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I have usually seen numerically higher do better in town, but of course worse on the highway.

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Numerically lower ratio is no assurance of superior MPG. If your statement was always true then the taller the tire you mount the better the MPG.

As a rule of thumb starting point, the most work for least amount of fuel occurs at or just under peak torque RPM. Poorer MPG at lower RPM. Poorer MPG at higher RPM. But if there is not enough HP there then you have to run higher RPM.
Numerically lower ratio is no assurance of superior MPG. If your statement was always true then the taller the tire you mount the better the MPG.

As a rule of thumb starting point, the most work for least amount of fuel occurs at or just under peak torque RPM. Poorer MPG at lower RPM. Poorer MPG at higher RPM. But if there is not enough HP there then you have to run higher RPM.
It's been true for majority of situations.
I don't have the energy to argue this minutia.
 

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equating tire Size with rear Axle ratios is apples and oranges. Using a taller tire means a heavier tire with the weight further from the center of rotation which means it requires more energy just to accelerate the rotational speed and the linear speed. Plus they are most likely a wider tire with more frictional loss in contact with the road surface.
the best rear axle ratio is the one that lets the motor run at the lowest rpm that it can to maintain a steady speed over the terrain and load conditions. Since load and road are infinitely variable picking the most economical ratio for all applications is impossible. enter The transmission.
 

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2016 Ford F-150 XLT 2.7 Ecoboost 3.55 Axle
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Discussion Starter #11
I have usually seen numerically higher do better in town, but of course worse on the highway.

Sent from my moto z4 using Tapatalk
Where do I find what ratio my truck is?
 

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2013 Ford F150 XLT 3.5 EgoBeast SuperCrew 4X4, 3.55 axles, 9.75" w/elocker
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There is an axle code on the white sticker on Driver’s door or door jamb.
 
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What Ark said is a sure bet. If you happen to also have your window sticker, it's on there also. Mine is in my glove box.

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equating tire Size with rear Axle ratios is apples and oranges. Using a taller tire means a heavier tire with the weight further from the center of rotation which means it requires more energy just to accelerate the rotational speed and the linear speed. Plus they are most likely a wider tire with more frictional loss in contact with the road surface.
the best rear axle ratio is the one that lets the motor run at the lowest rpm that it can to maintain a steady speed over the terrain and load conditions. Since load and road are infinitely variable picking the most economical ratio for all applications is impossible. enter The transmission.
Heavier wheel is no harder to accelerate mounted on axle than if carried in the bed of the truck.

If engine could grunt at low RPM and still pull the truck and get best MPG then the 10 speed would never downshift for the slightest hill. But it does. Ford knows better than you do, the 10 speed is there to raise RPM to improve fuel efficiency not to have the power to get over the hill, a 3 speed can do that. The effort expended to produce a 10 speed is to select the optimal RPM.

In years past the company I kept found a Powerstroke with 3.73 got better MPG with modest trailer than same trailer behind a 3.55. Running empty the 3.55 was just a touch better. 4.10 did poorer with the smaller trailers but better with the large 5th wheels than 3.73.
 

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Heavier wheel is no harder to accelerate mounted on axle than if carried in the bed of the truck.
Your wrong. Rotational mass and static mass are two different animals that effect how an engine can apply torque to a situation

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The rotational mass has a much larger impact!

My dad's 4.10 geared 7.3 powerstroke did better in town than my 3.73 geared powerstroke, but mine did better on the open road unless he kept it very slow. With a trailer we didn't see much difference.

I suspect it's due to needing less skinny pedal to get going.

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2016 Ford F-150 XLT 2.7 Ecoboost 3.55 Axle
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Discussion Starter #20
Before we continue..

OP, did your question get answered?
Yes, you guys are fine, I’m just enjoying watching the conversation. It seems to me there’s a lot of controversy on what gear ratio is good? I rarely pull a trailer and when I do it’s a 8’x10’ little single axle trailer. So I don’t care about towing, I’m more into acceleration... so what’s the best ratio for that? 3.55?
 
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