Well... I did some research and some math (And now my head hurts...)
The Caterpillar/D7155/Semi-Automatic trans in the M920 has 16 forward speeds, #15 is 1:1 and #16 is 0.83 (Overdrive)
According to TM 9-2320-273-10 the "Sweet spot" for this truck is between 1,800 and 1,900 rpm. "When driving conditions permit, maintain the legal highway speed in a gear that permits running the engine below governed speed. This is the engine's cruising range and it affords better fuel economy than the higher engine speeds. Recommended normal highway cruising range is 1,800 to 1,900 rpm."
On Randy's Ring & Pinion company website I found a RPM calculator that you input the R/P ratio, tire size, speed and transmission gear and it will calculate the engine RPM for you.
Randy's Ring & Pinion. RPM Calculator.
About 3 posts ago Soni stated that with the stock engine and trans, 4.88 gears and 46" tall tires he was getting 2150rpm at 79mph.
According to the computer, Soni should have been hitting 2337rpm at 79. I doubt he was doing that unless the revlimiter was not working properly and the tach was also off. In order to get the chart to agree with his numbers I had to change the tire size to 50". I'm pretty sure he would have remembered if he had 46 or 50" tires but that is what it took for that computer to have the math come out right.
Between what Soni stated and the results the chart gives there is a 187rpm difference. Since the "Sweet spot" is only a range of 100 it is going to be difficult to nail it unless somebody better at math than I am can explain and correct the inconsistency. In the mean time, I did some math just to see what it looked like.
In order to compare apple to apples And just for general info, I did 2 sets of calculations. One set using a 50" tire and one set using a 46" tire. I also did 2 speed ranges, 65 and 70mph. Reason I did this is as Soni noted the M920 is about as aerodynamic as a 3 story house. The faster you try to push it through the air the more your fighting air resistance. He stated that after 70mph he saw a notable decrease in mpg. In my flat front class A motor home I noticed about the same thing. Going from 65 to 70 there was about a 3.5mpg drop, at 75 the drop was 6+ mpg. That extra 5 mph cost quite a bit. I was comfortable in the right lane at 65 most of the time. So I did these calculations to see what gear would get me closest to the right (according to the book) RPM at both speeds. I also calculated the red line speed for each combination.... Anybody wanna go 94mph in a M920?
Using the book number of 1,800 to 1,900 to arrive at 1,850 as "The" desired rpm and my guess that 65mph is probably going to be the point of "No return" for mpg then a gear that gives 1,850 at 65 would be ideal.
50" Tire
4.88 - 79mph = 2150rpm
4.88 - 70mph = 1905rpm, 65mph = 1769rpm
4.56 - 70mph = 1780rpm, 65mph = 1653rpm, 85mph = 2162rpm
4.11 - 70mph = 1605rpm, 65mph = 1490rpm, 94mph = 2155rpm
46" Tire
4.88 - 79mph = 2337rpm
4.88 - 70mph = 2071rpm, 65mph = 1923rpm
4.56 - 70mph = 1935rpm, 65mph = 1797rpm, 78mph = 2156
4.11 - 70mph = 1744rpm, 65mph = 1620rpm, 85mph = 2118rpm
Given the roughly 200rpm discrepancy between Soni's number and the "uncorrected" calculator number if you try to compensate by taking 1/2 of that error off the 46" figures and adding 1/2 of it to the 50" numbers (100rpm correction factor) You can come up with a whole different set of results... (However, even when you do that, the numbers still line up to give what looks like the same final decision)
I did input the factory numbers. 6.17 gears, 46" tires at the book stated max mph of 59 (The book states that this will give you the red line rpm of 2100) the calculator said 2207rpm. I changed to a 50" tire and then it said 2030. Real darn close to the book number, (only 70rpm less).
So...
At this point all I can do is guess that since the calculator showed almost the same RPM as the book when using a 50" tire and the 50" tire numbers match Soni's statement of his performance that until the local math whiz comes along and fixes the calculator, the 50" results are going to be pretty close to what we should see in the trucks in real life.
Given that, it looks like 4.88 may not be to far off the mark of "perfect" as it is 1905rpm at 70 and 1769 at 65.
If the engine does not mind running just a tad below the book number then 4.56 gives 1780 at 70mph - Since the calculator showed the 6.17 (Factory) gear 70rpm less than the book, if you add 70rpm to the 1780 then your dead nuts on 1850 which should be the absolute perfect rpm according to the book. At 65 1653 (1723 corrected). This is a tad bit low. So maybe you have to go faster to get better mpg?? lol... (down shifting form 16th to 15th gear will not correct that as you will end up with 2165 (2235 corrected) at 65 because your moving from over drive to 1:1 direct drive in 15th gear.
Of course if your going to be correcting numbers... The 4.88 with the 70 rpm correction, 65mph = 1839rpm, pretty close to the "Ideal 1850".
So it looks like if you can go to the 4.88 and not have to change the carrier and to go to the 4.56 you do have to change the carrier, (at something like $1,200 each if I remember correctly) I guess the 4.88 wins.... Unless somebody more smarter than I can point out some flaw in my rithmatik.
So, can anybody find a major flaw in the above that would cause me to rethink the 4.88 selection? If so, please speak up!
Now all we have to do is figure out where to get the gears from!
