Yesterday there was some confusion about why anyone would install
lower t-case gears, just to turn around and install
higher/taller diff gears. The whole process appears counterproductive. I posted a small explanation
here, but I thought it would be helpful to perform a more thorough explanation, including some numbers and ratios.
First, some basic parameters...
For the purposes of this discussion, there are two scenarios where we are concerned about gearing and tire sizes: The
high-speed highway trip we took to get to our off-road destination, and the
low-speed 4WD capability once we get there. (This roughly equates to: How does the truck behave in 5th gear with the t-case in "High" and how does the truck behave in 1st gear with the t-case in "Low"?).
There are four components that affect these two scenarios: (1) Transmission Gear Ratio, (2) Transfer Case Gear Ratio, (3) Differential Gear Ratio, and (4) Tire Diameter.
Transmission and t-case can shift "on-the-fly" whereas diff gears and tire size is "constant". I will use my truck as an example, but these principles can be applied to any 4WD.
My truck has an H55F transmission, which has 5 gears, plus reverse. 1st gear ratio is 4.843:1 and 5th gear has an overdrive of 0.845:1.
My truck came from Toyota with a stock t-case of 1:1 High ratio and a 1.96:1 Low ratio.
My truck came stock with 4.1:1 diff gears.
The truck came from Toyota with 7.50R16-8 5.50F tubed tires. This is roughly equivalent to the conventional 235/85/16 tires on the truck now. So, for the purposes of this discussion, we will consider the 235/85/16 tires as 1:1 tire ratio (and adjust that ratio based on different tire sizes).
So if we wanted to calculate the Final Drive ratio, the equation would look something like this:
(Transmission Gear Ratio) x (Transfer Case Gear Ratio) x (Differential Gear Ratio) x (Tire Ratio) = Final Drive
Back to our two scenarios above (the "Highway" trip and the "4WD" capability upon arrival) - We are going to examine the truck's Final Drive while the truck's transmission is in 5th gear and the t-case is in "High" (Highway) AND we are going to examine the Final Drive while the truck's transmission is in 1st gear and the t-case is in "Low" (4WD). Obviously there are more combinations than these two scenarios, but we don't want to get overloaded with data.
Stock "Highway" Final Drive
(0.845) x (1) x (4.1) x (1) =
3.46
Stock "4WD" Final Drive
(4.843) x (1.96) x (4.1) x (1) =
38.99
We will use these two final drive ratios as our "baseline" for further comparison. Stock High = 3.46 and Stock Low = 38.99.
As I mentioned earlier, the 1.92:1 Low t-case ratio that the truck came with stock is pretty puny. Most 4WD enthusiasts would consider it a joke. But luckily, the split-transfer case can also fit aftermarket 4:1 Low gears. The 4:1 Low gears come at a price, and reduce High by 10% also. So our High ratio goes from 1:1 to 1.1:1. The Final Drives after 4:1 t-case install are:
"Highway" 1.1:1 t-case
(0.845) x (1.1) x (4.1) x (1) =
3.73
"4WD" 4:1 t-case
(4.843) x (3.98) x (4.1) x (1) =
79.04
Installing the 4:1 t-case gears means that our Final Drive in 4WD/Low
more than doubled! It went from 38.99 to 79.04. Great for low-speed 4WDing. But it came at a cost as our Final Drive for "Highway" got worse. It went from 3.46 to 3.73. We want more overdrive in 5th gear (especially with a diesel), not less overdrive/final drive. We're headed the wrong direction for Highway. (This is why the truck is "miserable" to drive on the highway right now. The engine is revving too high.)
Well, what if we added 3.7 diff gears? We would be making our t-case lower and our diff gears higher. Does that make any sense? Let's look at the numbers.
"Highway" 3.7 Diff Gears
(0.845) x (1.1) x (3.7) x (1) =
3.367
"4WD" 3.7 Diff Gears
(4.843) x (3.98) x (3.7) x (1) =
71.33
In this scenario we have made the final drive slightly taller in both scenarios 3.367 High and 71.33 Low. This is actually the best of both worlds. The 3.367 High ratio is better than the stock ratio of 3.46. And the 71.33 Low ratio is significantly better than stock 38.99. This is where I am headed and why I would like to install the 3.7 diff gears.
But what if I also up-sized to a 255/85/16 tire? A 255 tire is about 4.4% larger than the stock 235 tires, so that equates to a 0.956:1 tire ratio.
"Highway" 255 Tire
(0.845) x (1.1) x (3.7) x (0.956) =
3.22
"4WD" 255 Tire
(4.843) x (3.98) x (3.7) x (0.956) =
68.19
Both the High and Low ratios are still better than stock, but we are slightly sacrificing our low-speed 4WD capability for some Highway efficiency. I might swap to a larger 255 tire after I install 3.7 diff gears if: (1) I am still dissatisfied with the high-speed Highway capability and (2) I don't mind sacrificing a little more low-speed 4WD capability.
I realize this is a lot of data, so I hope everything makes sense.
Overall, the punchline is: Stock High 3.46 and Stock Low 38.99. And with 4:1 t-case and 3.7 diff gears High is 3.367 and Low is 71.33. Both better than the Stock setup.