two stick is another name for the 4+4,5+4,6+4 etc. Those are the boxes folks are generally referring to. These days truckers will put in a second stick that is just an air switch for the under/direct to be "cool". There are also heavy heavy haul trucks that still run auxillaries behind the roadrangers we know today.
Basic roadranger nomenclature will help you better understand the transmission model number, and from there you will know the shifting pattern, torque rating and ratios.
Generally from getting in the truck you can see if there is a splitter button on the side of the stick. Grey for 18spd, whitish grey with no high/low for super 10, red for 13spd, blue for deep-reduction transmissions 8LL, 9ALL and 15spd. no side splitter and just a high/low means 10spd or less. Now these are for late model trucks. Back in the day they used to have 13 speeds with blue buttons, and a lot of other stuff that led people to confusion between the 15 and 13 back in the day. They also used to put deep reduction on the dash sometimes. There is also the old school 13 speeds as well with low, direct and over on the round knob. Then there is the 6613 which was a deep reduction 15 speed but had 13 progressive gears and was labeled as a 13 speed. It also had the three position switch on the round knob but was labeled low, int and direct or over.
Best to find the model number though or you could be confused, especially if you see a blue button. Could be one of 3 transmissions.
here is a PDF on all eaton HD products nomenclature http://www.google.com/url?sa=t&rct=j...EfmHaHH-HtSLww
It is important to note that the O used to stand for overdrive and X used to stand for over drive with a direct shift pattern as the overdrive patterns used to have the top two gears flipped so the big hole would be at the dash. The rumor I always heard was that it was easier for the co-driver to crawl in and out of the sleeper this way in narrow cab trucks. Dont know if that story holds any water though. Times have changed and the flipped top gears have been discontinued. But if you find yourself in an older truck and can't figure out whats going on, this could be your issue.
I like the 15spds. They aren't rated for the torque the 18spds are rated for and don't give you as many progressions (only 12 progressive gears) but they do have a nice spread for decent powered motors to pull out of a field then do 90mph.
As for true double overdrives. They don't exist anymore. They were discontinued in the 80s. The roadranger double over drives were labeled RTOO or RTOX. People sometimes refer to them a "triple overdrives" because there were technically 3 overdrive ratios because instead of the underdrive splitter we are all used to on the newer trucks (13spd and 18spd) eaton added an overdrive splitter section to a 9spd overdrive. This gave the trans the following ratios.
As mentioned above, these transmissions have since been discontinued. Traditional super 13 and 18 speeds we know today have two overdrive ratios however they are not referred to as double over drive transmissions because there is not an overdrive gear being overdriven. The top 0.73 gear is an overdrive gear but the gear before it 0.86 is the 0.73 being underdriven by the aux underdrive section. Whereas the older true double over drive transmissions had the 0.73 in the transmission with the auxillary overdrive section being used to overdrive the 0.73 to 0.62.
Contrary to what some believe, the side splitter (red or grey) on the 13spd and 18spd is an underdrive when back and direct when forward. However many people still refer to it as direct when it is under and over when it is direct from the old school days.
If you want to calculate your speed or rpm at any speed all you need to do is some basic math.
First you need to know tire diameter. To calculate this by hand you need to know that standard profile tubless truck tires have an aspect ratio of 88%. (11R22.5,11R24.5) The other tires are all labeled in the metric sizing we are used to on car tires. For correct math you need to remember there are 25.4mm in one inch.
Ill save you some math with a few of the common sizes all are done with 22.5" rims, obviously add 2" for 24.5s.
Now that you have the tire diameter you just need the differential ratio and transmission gear ratio. The Diff ratio is easy to find stamped on the truck. To find the proper gear ratios for the transmission you need the full model number. Then just google it or if it is a 13 speed for example google "eaton 13 speed gear ratios" and a website called vibrate software will show up as a result, click it and it will take you to the charts of all of the ratios.
If you are just looking for cruising speeds you will basically just need to know if it is an overdrive or direct model. Direct is always 1.00. Most all of the roadrangers top overdrive gear is a 0.73 or 0.74. There are two 9 speeds that are different, one has a 0.80 and the other has a 0.86 top gear ratio just like the 18 speed and 13speeds have when the splitter is back in the "underdrive" position. The top gear in the 18 and 13s is still 0.73 or 0.74. The 15 speed also has a different overdrive ratio of 0.79. Now keep in mind these numbers are for most of the later model roadrangers. Yours may vary, best to get the numbers and call the dealer or fuller to verify the ratios if you want to be perfect.
Now that we have the tire diameter, transmission ratio and rear end ratio all you need is a target speed or target RPM then you can do the math.
To find RPMs at a target speed this is the formula.
Say you want to find the right rear end ratio for a certain RPM and speed you just need to change it up a little bit.
3.43 is the ratio you would be looking for.
Its really all pretty simple once you get the hang of it.
Generally speaking the taller the rear end is the weaker its gears are. This can be an issue when pulling out of fields if you are not a smooth operator. However with that said it is more efficient to have a taller rear end versus multiple overdrives.
Good luck either way. Lots of confusion and options out there when spec'ing trucks and it can become a nightmare to try and find the perfect one. Just figure out what you need it for and whats most important first.