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Hemling's avatar
Hemling
Explorer
Jun 14, 2021

How critical is axle ratio?

In the modern day of 10 speed automatics, how necessary are 'towing' gears? By 'towing' I mean anything over 4.xx. These new transmissions have such low first gears compared to just a decade ago, is it really necessary to get deep gears anymore? I understand it makes getting going easier on all the moving parts, if you have a 4.30 compared to say a 3.55 or something, but doesn't it all come out in the wash? What I mean is, a truck with lower numeric gears will just tow in a lower gear, while a truck with deeper gears will allow the transmission to hold a higher gear. Is there a tradeoff or 'sweet spot' that anyone can speak to? I am still towing with an Excursion, so everything is a trade off. I could install 4.56 gears, but only have 4 gears. I could keep 3.73 gears, but know I'll always be in 3rd or 2nd to pull. I know that the towing guides always list bigger weights the deeper the gears get, but with a modern 10 speed automatic, I'm just not seeing the big deal anymore.
Tow Vehicles

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  • Groover's avatar
    Groover
    Explorer II
    Jun 14, 2021
    Bottom line is that drive shaft RPM is irrelevant. What really matters is whether the engine is in its power band when you need it to be. Back when truck transmissions only had a 2.5 to 1 spread between high and low axle rations where much more important than they are with the newer transmissions that have a 7 or 8 to 1 spread.

    The op is correct, unless you find yourself in first gear wishing that you had a deeper gear or in 10th gear wishing that you had a higher gear you get the same effect by simply changing gears.

    I do suspect though that axles built with higher ratios may also be built to handle higher torque to the wheels, at least in some cases.
  • MitchF150's avatar
    MitchF150
    Explorer III
    Jun 14, 2021
    No idea on the new 10 speeds... I have a 3.73 with a six speed and I still lock out 6th on the flats..

    Engine stays cooler, gets better mpgs than trying to hold 6th..

    Up the grades, 4th works great.

    In short, love the 3.73's.. :)

    Mitch
  • dodge_guy's avatar
    dodge_guy
    Explorer II
    Jun 14, 2021
    If you are going to tow any significant weight always go for the lower gear ratio (numerically higher). Towing mileage will be better and the truck will work less harder.
  • BenK's avatar
    BenK
    Explorer
    Jun 14, 2021
    noteven wrote:
    A person would think that torque delivered through a shaft turning at a higher rpm would deliver more power to do the work.


    That is torque multiplication. Note not HP, just torque multiplied by the ratio and is basically from the engine crank shaft to the tires.

    The tire's rev's per mile also a factor and use the OEM tire's rev's per mile as basis to figure

    Then all of the gear boxes between the engine and diff add their gear ratio multipliers to the mix.

    If the tranny's first gear is, say a 4.0:1, then multiply the engine crank torque by 4 and then multiply that by the diff's ratio.

    And so forth for all of the gears in each gear box between the engine crank and diff

    Yes, having a higher numeric ratio will spread the torque loading over the resultants of the above math
  • noteven's avatar
    noteven
    Explorer III
    Jun 14, 2021
    A person would think that torque delivered through a shaft turning at a higher rpm would deliver more power to do the work.
  • Terryallan's avatar
    Terryallan
    Explorer II
    Jun 14, 2021
    Hemling wrote:
    In the modern day of 10 speed automatics, how necessary are 'towing' gears? By 'towing' I mean anything over 4.xx. These new transmissions have such low first gears compared to just a decade ago, is it really necessary to get deep gears anymore? I understand it makes getting going easier on all the moving parts, if you have a 4.30 compared to say a 3.55 or something, but doesn't it all come out in the wash? What I mean is, a truck with lower numeric gears will just tow in a lower gear, while a truck with deeper gears will allow the transmission to hold a higher gear. Is there a tradeoff or 'sweet spot' that anyone can speak to? I am still towing with an Excursion, so everything is a trade off. I could install 4.56 gears, but only have 4 gears. I could keep 3.73 gears, but know I'll always be in 3rd or 2nd to pull. I know that the towing guides always list bigger weights the deeper the gears get, but with a modern 10 speed automatic, I'm just not seeing the big deal anymore.


    The big deal is in getting the load moving. The 3.56 will start a load from 0 much better than a 3.15 will start a load from 0.. Yeah once you get moving you may be able to find a gear it will be happy in. But moving it to begin with will be much harder, as the 3.73 provides much more torque, and torque is what moves the load.

    Go out and put the selector in 2nd, or 3rd gear, and try to move the load. Is it harder than first? That is the difference. The ratio not only changes the gears while running down the road. It also changes the starting gear power.
  • theoldwizard1's avatar
    theoldwizard1
    Explorer II
    Jun 14, 2021
    What you are missing is "ratio RANGE".

    Take your 1st great ratio and multiply it by the final (rear) axle ratio. Write this down. Do the same for your top gear ratio. Repeat for a ten speed transmission.

    Ford still offers a 10 speed with a final drive of over 4. This gives you tremendous pulling power when trying to get a very large load moving. If 10th gear times the final ratio is significantly different then your current transmission then you are FB correct. You can comfortable cruise in 7th, 8th or 9th.

    The primary reason for 10 speed was to make the steps between ratios smaller so that the engine can operate in more optimal speed.

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