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Discussion Starter #1
I don’t really like how the gear ratios are spaced out in the ZF-6, it’s not very nice for towing in the mountains. Shifting from 4th to 5th (1.00 to 0.72) is a massive RPM drop when you’re going uphill with a heavy trailer. Neither a rear end swap nor even a gear vendors unit can change that.

I’m planning to rebuild the transmission soon, and I thought I might change some of the internal ratios while I’m working on it. I know you can buy replacement gears in the original ratios, but what about different ratios? Any place to buy those ready-made or would I need to get a custom fabrication shop to do this?

Also, is the 1.00:1 ratio a direct drive or is there a countershaft gear for it?
 

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I have 2 trucks with ZF sticks. From what I found out form an authorized ZF shop is you need a special fixture to put them together. You can have gears made and you can probably figure several thousand dollars per gear. you would need to design the gears to work together to get the ratio you want. You have to figure how much rpm drop and the what rpm and what speed. I rebuild Ford Toploader 4 speed transmissions for old mustangs. A friend has gears made at a company in Florida. It is very expensive to have one gear made. He orders 2000 of each gear at a time. You would be better off with 4.56 or 4.88 rear gear and a gear vendors. You can split the gears with it. U.S. Gear makes or made a 2 speed gear splitter for the 99-02 ZF 6 with 4WD. It works the same as a 10 speed Semi transmission.
 

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I didn't think that sounded right.

ZF-6 Ratios

Low 5.79:1
First 3.30:1
Second 2.10:1
Third 1.31:1
Fourth 1.00:1
Overdrive 0.72:1
Reverse 5.23:1

I have an NV5600 in my dodge, similar ratios for direct and overdrive. I hardly towed uphill in overdrive. If I wanted speed uphill I'd leave it in direct drive and let the engine sing with the rpm's just a few hundred below recline.

The way I was taught was, don't lug it under load and don't get stupid with towing in overdrive. That was the gear to be respected above all else.

Is the ZF-6 that different? Is your truck not making enough power and you're mistakenly trying to hit the next gear for speed instead of using your rpms? Are you afraid of the rpm's?

How are you setup, year of pickup, tire size, diff ratios, power level? When up shifting are you above peak torque (rpm wise) and then ending up right on top of peak torque in the next gear?

Custom gears in your trans sounds flat out crazy to me. How much weight are you pulling?
 

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Discussion Starter #4
This particular truck is a 2000 F-450 with 4.88 gears. 225/70R19.5 tires. The only engine modification is a Banks exhaust brake, I have stock engine power, the other mods are for suspension and brakes.

When I’m loaded I try to catch the next gear around 1,800. More if we’re going up a grade. This usually means I shift between 2,500 to 2,800. That seems excessive though, and not very good for fuel economy. On some grades I get stuck at 56 mph screaming in 4th because the .72 overdrive gear is too tall to use.

If you look at the 6R140, you have 1.15, 0.86, and 0.67. Very evenly spaced ratios, and you only drop 550 or so RPMs.

I have a tridem gooseneck rated for 40k gross weight. I’ve been using the F-450 for anything up to about 22k, I have a bigger truck for hauling more but it’s only a regular cab. I would like to haul more with the F-450 as it’s a crew cab and much more comfortable for long trips.
 

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Sounds like you're doing it right. Might throw a tuner on it to get a little bump in power and be happy with what you got at this point.

This new 2020 6.7L and 10 speed combo is pretty dang sweet if you don't mind dropping some coin! Go drive one!
 

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The 4.88 gear would work better with a gear vendors. Leave the gear vendors on and downshift and bring the rpms down. Also 2800 rpm is not excessive for the 7.3. 56 mph is low but The F450 is a commercial truck and not made to go fast.
 

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Discussion Starter #7
Honestly if I was going to drop $70k I would just do some major mods to this truck and get some nicer seats for my other bigger one. I’ll go out of my way to avoid automatics.
 

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Shifting at 2500-2800 isn't excessive at all if your loaded. You can wind it out past 3000 if you need to.
 
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