Well, the first thing to do is to go to the Roadranger page (
www.roadranger.com) and follow the links for transmissions. The letters have to do with the basic class of the tranny (overdrive or not, etc); the first two digits are roughly the torque rating/1000, the third digit is the series (upgrades, so a "9" has some level of improvement over a "6", and the last 2 digits are the number of forward gears (more or less, since the hardcore off road trannies have some creeper gears that aren't counted in that). So, let's say what you see is an RTO18913 - that will be a 13 speed overdrive with an 1850 foot pound capacity, of pretty much the latest technology. (This is a CRUDE description - use the website and look at the various descriptions if you want to be correct!) The letter prefixes are important in terms of the general nature of the technology (such as differentiating the Lightnings from the standard OD trannies; the numbers in terms of capacity and # of forward gears).