Gearing change - Ford Powerstroke Diesel Forum
General 6.7 Discussion (2011-2016) General 6.7 Discussion

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post #1 of 14 Old 01-08-2020, 05:53 PM Thread Starter
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Gearing change

Hey all just looking for some input on a possible gear swap. Currently have the 3.31 if I remember correctly and currently I average around 11.5 mpg. It’s a farm work truck/weekend adventure rig so it weighs in around 10k daily and currently have 36” Toyo mts on it. Was just wondering in my case if going up to 4.30 gears would help improve my mpgs and overall driving experience and towing experience?


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post #2 of 14 Old 01-08-2020, 07:34 PM
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You would really only notice that it won’t downshift into 5th as often because it won’t lug as much. 6.7s get poor fuel economy generally and if it weighs 10k and you tow with it, averaging 11.5 is totally normal. Most normal unloaded trucks get 13-14. It wouldn’t be worth the money in my opinion.

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post #3 of 14 Old 01-08-2020, 08:06 PM
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I've regeared a few rigs (keeps). And honestly could never tell a difference after the gear swap.

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post #4 of 14 Old 01-09-2020, 04:02 AM
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For the costs of the gear swap that's a lot of fuel to buy. 4.30s aren't going to gain you any significant MPGs.
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post #5 of 14 Old 01-09-2020, 06:25 AM
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Gearing change

Going up in gear ratio will increase the engine RPMs in all transmission gears so it will further decrease your fuel mileage not help it. Going from the lowest gear ratio offered to the 2nd highest gear ratio offered will increase your engine RPMs by a few hundred at least. As others have said the engine will lug less and you’ll be able to pull any longer/steeper hills easier but fuel mileage will be worse. My dually has 4.30 gears in it and with 33” tires on it now it turns 1750RPMs @ 60MPH. With the stock 31.5” tires it used to turn 1850RPM @ 60MPH.


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post #6 of 14 Old 01-09-2020, 08:38 AM Thread Starter
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Thanks for all the info looks like gear change won’t be happening then. Ha


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post #7 of 14 Old 01-09-2020, 08:40 AM Thread Starter
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Quote:
Originally Posted by 18M_RWR View Post
Going up in gear ratio will increase the engine RPMs in all transmission gears so it will further decrease your fuel mileage not help it. Going from the lowest gear ratio offered to the 2nd highest gear ratio offered will increase your engine RPMs by a few hundred at least. As others have said the engine will lug less and you’ll be able to pull any longer/steeper hills easier but fuel mileage will be worse. My dually has 4.30 gears in it and with 33” tires on it now it turns 1750RPMs @ 60MPH. With the stock 31.5” tires it used to turn 1850RPM @ 60MPH.


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Thanks for the info. Sometimes I feel like my truck is sitting at too low of an rpm in 6th gear is another reason I was thinking of swapping gears. Currently it’ll be around 1200ish rpm at 60 mph.


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post #8 of 14 Old 01-09-2020, 10:40 AM
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Originally Posted by Joek6.5 View Post
Thanks for the info. Sometimes I feel like my truck is sitting at too low of an rpm in 6th gear is another reason I was thinking of swapping gears. Currently it’ll be around 1200ish rpm at 60 mph.


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A 36" tire is approximately 113" in circumference and will make 561 revolutions/mile.
Your 3.31 gears have a final axle/tire ratio to engine RPMs of 2.23 in 6th gear [3.31x0.674(6th gear ratio of 6R140 transmission)] so going 60MPH on your 36" tires you should be turning roughly 1250RPMs [561x2.23=1251]
So if you put 4.30 gears in it your final axle/tire ratio to engine RPMs would be 2.89 in 6th gear (4.30x0.674) and going 60MPH on your 36" tires you should turn roughly 1625RPMs. Those roughly 400RPMs will make the engine thirstier for certain. But I understand why you are looking to change your gearing.

So here are the different rear-end gear ratios and roughly what the engine RPMs would be on 36" tires going 60MPH in 6th gear:
3.31 gears = 1250 +/-
3.55 gears = 1342 +/-
3.73 gears = 1410 +/-
4.10 gears = 1550 +/-
4.30 gears = 1625 +/-

Hope that helps some.

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post #9 of 14 Old 01-09-2020, 03:03 PM Thread Starter
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Quote:
Originally Posted by 18M_RWR View Post
A 36" tire is approximately 113" in circumference and will make 561 revolutions/mile.
Your 3.31 gears have a final axle/tire ratio to engine RPMs of 2.23 in 6th gear [3.31x0.674(6th gear ratio of 6R140 transmission)] so going 60MPH on your 36" tires you should be turning roughly 1250RPMs [561x2.23=1251]
So if you put 4.30 gears in it your final axle/tire ratio to engine RPMs would be 2.89 in 6th gear (4.30x0.674) and going 60MPH on your 36" tires you should turn roughly 1625RPMs. Those roughly 400RPMs will make the engine thirstier for certain. But I understand why you are looking to change your gearing.

So here are the different rear-end gear ratios and roughly what the engine RPMs would be on 36" tires going 60MPH in 6th gear:
3.31 gears = 1250 +/-
3.55 gears = 1342 +/-
3.73 gears = 1410 +/-
4.10 gears = 1550 +/-
4.30 gears = 1625 +/-

Hope that helps some.


Thanks a ton that’s actually extremely helpful!


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post #10 of 14 Old 01-09-2020, 03:43 PM
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One other thing that need to be considered in axle ratios, is engine load
The more load that is on a diesel the more fuel it uses

so, even tho the 4.30 gears cause more RPM, they also cause less load
less load = less fuel
so both factors need to be considered when choosing a axle ratio

The 3.73 may be a more middle of the road choice
another thing, is body height, ...lifted trucks have more air turbulence under the body
you see these little rice burners with a 2 inch road clearance, ...that is part of how they get good mileage
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