...go with a 4.11 gear and use a Gear Vendors overdrive behind your existing drive to give you both additional overdrive and split gears between each speed.
http://answers.yahoo.com/question/index?qid=20100106144243AA...

Now, an 18 speed (and to some extent, its close cousin, the 13 spd.) allows you to "split" each gear. On these transmissions there is another switch on the shift lever operated by your left thumb. There is 1L and 1H, 2L,2H, etc... These "split" gears are like a half step between a full gear change. When you use the splitter you do NOT use the clutch (doing so will not accomplish the shift, just a lot of grinding.)
http://www.airliners.net/aviation-forums/non_aviation/read.m...

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Note added at 1 hr (2010-05-23 11:49:05 GMT)
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Można też dosłownie:
- half gears

Almost all heavy truck transmissions are of the "range and split" (double H shift pattern) type, where range change and so-called half gears or splits are air operated and always preselected before the main gear selection.
http://en.wikipedia.org/wiki/Truck