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Old 09-09-2019, 11:43 PM   #1 (permalink)
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Exclamation Help with gearing

I'm having a hard time deciding what to do.

I am finally making my truck 4wd, but the donor has 3.55 gears and I have 4.10's after getting a new axle several years ago.(4.10 was the only thing available) Do I swap in the 3.55 gears or buy a matching 4.10 for the new front end?

I currently have the m5odr2 which has a fairly high low end and not much overdrive. The 4.10 helped with the low end, but hurt me on the top end. It pulls like a dream and it's nice to not down shift for most hills when loaded, but it just feels like it's screaming along at cruise.

The donor has the zf5 which has a deeper low end and higher top hole. I think it would be fine with the 3.55 gears because it has the deeper gears on the bottom to make up the difference. The extra overdrive would be awesome too.

It's going to be ~20% reduction in wheel torque in the top hole with the 3.55/zf combo by my math. About 14% with the rear end and another 5% with the overdrive step. I'd be shifting down on hills for sure.

I normally weigh ~12,000 lbs loaded, so that lost torque will be noticeable.
Shifting down to 4th for hills will put my rpm's higher than they would have been with the 4.10 in 5th gear.
M5od/4.10 is 2100 in 5th
Zf5/3.55 is 2270 in 4th

I'd like to get my rpm's down and get a few mpg back, but I don't want to lose my grade pulling ability.

What would you guys do? Any advice? Feel free to toss ideas around. I'm not buying two new 3.73 gear sets, so that is out.


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Old 09-10-2019, 07:26 AM   #2 (permalink)
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I can't speak for towing, but in most vehicles, going with taller gearing improves economy all the way up to the point that you can't maintain speed in that gear anymore - and then you just downshift.

In my car I picked the tallest 6th available from any model (usually paired with shorter final drives) and had a custom final drive cut which was 30% taller than anything OEM. I also upsized my tires slightly. Going even taller would help even more, but my low gears might need to be changed so they're a little closer together to keep it fun when I want to waste fuel.
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Old 09-10-2019, 12:12 PM   #3 (permalink)
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You are in the position to test what you need. Continue with your conversion, install the ZF and drive with your load. I think you will like the 4:10s loaded with the small engine. I have 4:56 gears in my NA Diesel pickup with a 30% od and it is fine on the highway towing. I have driven this same engine/trans with 3:73 towing and it was in direct all the time. So no fuel saving loaded.
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Old 09-10-2019, 07:02 PM   #4 (permalink)
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Ford went standard with 3.55 gear sets a few years ago.
But I think 4.10 is still the ultimate towing package.
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Old 09-10-2019, 07:52 PM   #5 (permalink)
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I'd always prefer to get front an rear differentials matching the same ratio, but some wider gear spread is also desirable. Either a 6-speed with a wider ratio, or a supplemental overdrive, could be a good measure to balance both priorities.
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Old 09-10-2019, 09:09 PM   #6 (permalink)
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Quote:
Originally Posted by Ecky View Post
I can't speak for towing, but in most vehicles, going with taller gearing improves economy all the way up to the point that you can't maintain speed in that gear anymore - and then you just downshift.
If you have an Insight, sure :P

When I had my FR-S with an instantaneous mpg gauge (which I used to mentally calculate the load from), I realized that it is really easy to step too hard on gas in 6th gear and run rich.

In a heavier car (for the gearing/torque), this can be a huge problem. Cruise control would make the car run rich up hills!

I still like tall gearing, but you definitely have to be on top of your shifting game and have a careful foot to get the best out of it.
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Old 09-11-2019, 09:09 AM   #7 (permalink)
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might be a crazy thought, but can you match rpm by having two different o.d. size tires from front to back?
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Old 09-11-2019, 11:02 AM   #8 (permalink)
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Quote:
Originally Posted by serialk11r View Post
If you have an Insight, sure :P

When I had my FR-S with an instantaneous mpg gauge (which I used to mentally calculate the load from), I realized that it is really easy to step too hard on gas in 6th gear and run rich.

In a heavier car (for the gearing/torque), this can be a huge problem. Cruise control would make the car run rich up hills!

I still like tall gearing, but you definitely have to be on top of your shifting game and have a careful foot to get the best out of it.
I found something quite interesting when poking about with Honda's K series. Turns out the engine in cars like the older Type R, TSX, Civic Si, don't actually go rich even at WOT. That's true of most stock engine maps. Honda chose instead to pull timing to prevent knock. Now, admittedly this also has a fuel economy impact, since your peak cylinder pressure is farther from optimal crank angle.

I have no idea how Ford chose to handle WOT with the straight 6, but food for thought, you want to be in the engine's most efficient range as much as possible, and its lease efficient range as little as possible. If there's an issue with enrichment or getting at a high enough load that timing is seriously retarded, there's always a downshift.

My 2 cents.
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Old 09-11-2019, 01:11 PM   #9 (permalink)
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You gotta match ratios axle-to-axle. Otherwise you destroy the transfer case within a few minutes of engagement.
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Old 09-11-2019, 03:58 PM   #10 (permalink)
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You can't talk rear end gear ratios without talking transmission gear ratios. The internet is telling me your transmission has 3.9:1 first and 0.8:1 fifth. Picking your rear end ratio to me is really about a trade-off in first and fifth, would you rather have a lower first or a taller fifth. For any other gear, you could just shift to alter the ratio but you can't go lower than first or higher than fifth.

In my experience I would prefer the 3.55 to the 4:10, as long as you don't think you will have a problem in first taking off loaded. With 3.9:1 first I don't think you will, but you know your engine/torque curve better than me. The fact that your donor vehicle is set up that way from the factory tells me it can't be that bad.

I would much rather have to downshift on the hills than to be downshifted all the time. To me a truck that drives around all the time in OD and never needs to downshift for steep hills is over-geared IMHO. It also seems you already own the matching 3.55 and would have to purchase the 4.10 front, so I would try it out before spending additional cash.

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