Once again member Agnem better known as Mel has given us some useful tips and info on how to better understand what a gear splitting device does.
There are several pieces of hardware on the market currently, and from the past, which allow you to get more gears or "speeds" out of your truck. You may have heard of a 2 speed rear. This is the same concept. Gear ratios in transmissions are fixed. That is, for one revolution of one gear, it's mate which is larger or smaller, will turn more or less revolutions according to it's size difference. This is how you get the standard 1st, 2nd, 3rd, etc. In first, a smaller gear turns a larger gear, and as the gear ratio moves up towards the final drive ratio of 1:1, at which point the two mating gears are the same size, the smaller gear gets bigger, and the bigger gear gets smaller. Once 1:1 is achieved, any gear beyond that would be an overdrive gear, in which case the driving gear is larger than the driven gear. That would be 5th in a ZF-5. A T-19 only has 4th, with 4th gear being 1:1. The same would be true of the E40D and C6. What the overdrive (sometimes called underdrives, depending on configuration, and hereafter refered to in general as "auxiliary") transmissions do, is they have 2 drive ratio's. 1:1 and something to something. The "Something" depends on the unit. For overdrive transmissions, that Something to Something is a bigger gear turning a smaller gear. For an Underdrive transmission, it would be a smaller gear turning a bigger gear. In the 1:1 drive ratio, nothing happens. That is, what you see in the way of driveshaft revolutions would be the same as if the auxiliary transmission was not even there. The main transmission controls the final drive ratio. However, once the auxiliary transmission is engauged, it takes whatever the final drive ratio of the main transmission is, and either increases or decreases it by a fixed amount. A typical amount is 21%. This has the effect of producing a drive ratio that is exactly half way between the difference in ratios of the other gears in the main transmission. For example, let's say 3rd gear is 2:1 and 4th gear is 1:1. When the overdrive transmission is activated, the final ratio might become something like 1.5:1 and 1:1.5. That would be more like a 50% increase, but I'm bad at math so I stuck to round numbers for this example. Real ratio's are not usually that rounded. So, when you have an overdrive, you can shift "between" gears, by activating the overdrive transmission. Let's say your in 3rd, and want to "split" the difference between 3rd and 4th. This would be what we call "3rd over". 3rd over on a T-19 equipped truck is a sweet gear, because it is often just what the doctor ordered to get over that hill that was too steep for 4th, but too easy for 3rd. Now that we're in 3rd over, and we want to go to 4th, we shift the shifter lever from 3rd to 4th, while at the same time, turning the overdrive off (making it go back to 1:1) so that when we land in 4th, our final drive is now 1:1 all the way through. The crankshaft is now turning at the same RPM's as the driveshaft. Now we want overdrive, so we activate the overdrive, so we are at 1:1.5 as in our example. Now we've got lots of speed and low RPM's for good fuel efficiency. When I equip the Lady Moose with a GV unit this spring, I'll have "double overdrive" because the ZF-5 will give me overdrive, that I will then run through the GV unit which will increase that final drive ratio even more. With the 4.10 rear, this should make it more like a ZF-5 without a GV turning 3.54 gears. Why only 7 speeds instead of 8, you might be asking? That is a "feature" of the GV unit. It uses an electric over hydraulic system to activate the internal gear changes. There is insufficient hydralic presure at the speeds obtained in 1st gear in order to get it to shift, so I cannot split 1st and 2nd. I don't see this as a problem, as I can't imagine ever needing to do that. If I did, I bet I could put the transfer case in 4lo and find a ratio that would fall in that region. But then the GV is 2WD only, so it would be unusable at that point. As for the other types of overdrives and underdrives, they work the same mechanically, but may be placed differently in the drive train, and activated by different means. The 2spd road ranger goes in front of the main transmission and has a stick, while the U.S. Gear unit shifts electrically and goes between the main tranny and transfer case if you have one, so both of those units can be used in 4X4. Honestly, I've never needed the OD when I'm in 4X4, so that is something you have to think about if your getting one.
Here are two links that JPR dug up to add to this article.
US Gear
Gear Vendor
There are several pieces of hardware on the market currently, and from the past, which allow you to get more gears or "speeds" out of your truck. You may have heard of a 2 speed rear. This is the same concept. Gear ratios in transmissions are fixed. That is, for one revolution of one gear, it's mate which is larger or smaller, will turn more or less revolutions according to it's size difference. This is how you get the standard 1st, 2nd, 3rd, etc. In first, a smaller gear turns a larger gear, and as the gear ratio moves up towards the final drive ratio of 1:1, at which point the two mating gears are the same size, the smaller gear gets bigger, and the bigger gear gets smaller. Once 1:1 is achieved, any gear beyond that would be an overdrive gear, in which case the driving gear is larger than the driven gear. That would be 5th in a ZF-5. A T-19 only has 4th, with 4th gear being 1:1. The same would be true of the E40D and C6. What the overdrive (sometimes called underdrives, depending on configuration, and hereafter refered to in general as "auxiliary") transmissions do, is they have 2 drive ratio's. 1:1 and something to something. The "Something" depends on the unit. For overdrive transmissions, that Something to Something is a bigger gear turning a smaller gear. For an Underdrive transmission, it would be a smaller gear turning a bigger gear. In the 1:1 drive ratio, nothing happens. That is, what you see in the way of driveshaft revolutions would be the same as if the auxiliary transmission was not even there. The main transmission controls the final drive ratio. However, once the auxiliary transmission is engauged, it takes whatever the final drive ratio of the main transmission is, and either increases or decreases it by a fixed amount. A typical amount is 21%. This has the effect of producing a drive ratio that is exactly half way between the difference in ratios of the other gears in the main transmission. For example, let's say 3rd gear is 2:1 and 4th gear is 1:1. When the overdrive transmission is activated, the final ratio might become something like 1.5:1 and 1:1.5. That would be more like a 50% increase, but I'm bad at math so I stuck to round numbers for this example. Real ratio's are not usually that rounded. So, when you have an overdrive, you can shift "between" gears, by activating the overdrive transmission. Let's say your in 3rd, and want to "split" the difference between 3rd and 4th. This would be what we call "3rd over". 3rd over on a T-19 equipped truck is a sweet gear, because it is often just what the doctor ordered to get over that hill that was too steep for 4th, but too easy for 3rd. Now that we're in 3rd over, and we want to go to 4th, we shift the shifter lever from 3rd to 4th, while at the same time, turning the overdrive off (making it go back to 1:1) so that when we land in 4th, our final drive is now 1:1 all the way through. The crankshaft is now turning at the same RPM's as the driveshaft. Now we want overdrive, so we activate the overdrive, so we are at 1:1.5 as in our example. Now we've got lots of speed and low RPM's for good fuel efficiency. When I equip the Lady Moose with a GV unit this spring, I'll have "double overdrive" because the ZF-5 will give me overdrive, that I will then run through the GV unit which will increase that final drive ratio even more. With the 4.10 rear, this should make it more like a ZF-5 without a GV turning 3.54 gears. Why only 7 speeds instead of 8, you might be asking? That is a "feature" of the GV unit. It uses an electric over hydraulic system to activate the internal gear changes. There is insufficient hydralic presure at the speeds obtained in 1st gear in order to get it to shift, so I cannot split 1st and 2nd. I don't see this as a problem, as I can't imagine ever needing to do that. If I did, I bet I could put the transfer case in 4lo and find a ratio that would fall in that region. But then the GV is 2WD only, so it would be unusable at that point. As for the other types of overdrives and underdrives, they work the same mechanically, but may be placed differently in the drive train, and activated by different means. The 2spd road ranger goes in front of the main transmission and has a stick, while the U.S. Gear unit shifts electrically and goes between the main tranny and transfer case if you have one, so both of those units can be used in 4X4. Honestly, I've never needed the OD when I'm in 4X4, so that is something you have to think about if your getting one.
Here are two links that JPR dug up to add to this article.
US Gear
Gear Vendor
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