Axle Ratio - Gas Mileage? - F150online Forums



Axle Ratio - Gas Mileage?

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Old 01-19-2007, 12:02 AM
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Axle Ratio - Gas Mileage?

Alright, So if my meager knowledge on power transfers is accurate, the 3.08 open would give you the best gas mileage and the 4.10 would give you the worst. At the same token, the 4.10 gives the most towing capacity and the 3.08 gives the worst?

Or Am I drawing the axle rations out of proportion? (no intentional play on words..)

Better towing, yes- now what are the negatives?

~B
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Old 01-19-2007, 08:22 AM
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Quote:
Originally Posted by eighteenwheeler
Alright, So if my meager knowledge on power transfers is accurate, the 3.08 open would give you the best gas mileage and the 4.10 would give you the worst. At the same token, the 4.10 gives the most towing capacity and the 3.08 gives the worst?
That is the basics of it, yes. BUT You also have to look at tires, transmission, engine, and the load being hauled to figure out the whole picture and what you wish to accomplish.
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Old 01-19-2007, 01:47 PM
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Fuel ecomony is not proportional to gear ratio. If you change the gearing so that the engine is not running in its efficiency range, then it could use more fuel than a shorter gear.

Examples: People w/ large off road tires that change their gear and increase both performance and fuel economy.
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Old 01-19-2007, 03:08 PM
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Hmm... I'm interested in hearing why. (not doubting you, just interested in more info)

The gear ratio is the final step in the power step process. The transmissions are the same, so it seems to me that it should be the "10 spd bike" idea- Larger ration means drives easier, but at the same speed the engine has to turn more than a smaller ratio. If the engine is turning more revolutions, then your going to use more fuel.

This would be a benefit to towing, as a higher ratio makes it "easier" to pull additional weight. However, this higher ratio seems to me that it would also run the engine at slightly higher revs for the same speed, therefore using more fuel, and losing fuel efficiency.
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Old 01-19-2007, 04:02 PM
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Quote:
Originally Posted by eighteenwheeler
If the engine is turning more revolutions, then your going to use more fuel.
Not necessarily.

As a general rule, engines run very efficiently at or near their peak torque rpm. If you are operating the motor at too low an rpm, you can certainly use more fuel than if you are operating it at a higher, but more efficient rpm. This is especially true under load. High speeds and big tires also place additional load on the motor for any given rpm.

Basically, lugging with a heavy load is bad in many ways, including fuel efficiency. In some applications, going to a higher numerical gear ratio will allow you to operate at a more efficient rpm range. In some caes, towing mileage will not suffer at all, even though towing performance increases.
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Old 01-19-2007, 04:24 PM
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Gas engines operate most efficiently around 25% load at the engine speed with peak torque. Shift too far under peak torque and efficiency can vary a lot. That's part of engine design to define that efficient range and power tradeoff.

Fuel consumption relates to how much air the engine takes in. After the engien gets warmed up and is closed loop control (with the O2 sensors), the air/fuel ratio is pretty much kept at stoich, a constant. So, more air = more fuel used. So, more throttle positon, more air across the MAF sensor and more fuel. Throttle position matters more than engine speed within a 1000rpm window.

Here's an example. If an engine doesn't have enough torque at the (lower) engine speed, the driver compensates by using more pedal, say 75% of avilible torque at whatever rpm. You'll use more fuel than operating at maybe 500rpm higher where you are using only 25% of availible torque.
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Old 01-19-2007, 05:08 PM
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So the real question comes down to, what was the transmission and engine really designed for- 3.55 or 3.7X ratio? The other ratio, given no load beyond truck and driver, and static speed, would use more gas. Correct? Therefore there would be a correlation between axle ratio and gas mileage.

Why does ford have so many axle ratios anyway?
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Old 01-19-2007, 07:14 PM
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3.55 vs. 3.73 is only 5% different, so 5% different rpm at 2000rpm highway cruising speed for example is not going to vary much. F-150's don't tend to have much different real world fuel mileage between the two, all else equal. Your original comparison of 3.08 vs. 4.11 is about 22%, which is significant, plus the gear reduction as a torque multiplier for acceleration.

Why so many gear ratios? Tire sizes. Most 3.08 gears came on 2wd (and ligher) f-150's w/ 29" tires vs. 3.55 and 3.73 w/ 32" tires.
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Old 01-19-2007, 08:31 PM
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Excellent. That makes a lot more sense now. Thanks for your feedback! I didn't know that the tire sizes and the gear ratios were related from the dealer.

Is my rational in line though? like in theory could there be a negligable difference? haha... I know theory doesnt do much, but I guess it's kind of like a riddle to me.
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