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Granny Trannies



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These are 4 and 5 speed transmissions that are extremely wide ratio, and have very low "granny gear" 1st gears. Granny trannies can get your crawl ratio down into the 70 to one range still keeping sensible axle ratios. 

Granny 4 Speeds

 Popular granny 4 speed units are the SM420 (7.05:1 1st gear), SM465 (6.58:1 1st gear), NP435 (6.69:1 1st gear), and the T18 (Ford versions and some Jeep versions have a 6.32:1 1st gears.)

** High Impact offers Granny Tranny Conversions **
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The above units all have unsynchronized 1st gears: The transmission is intended only to be shifted into 1st gear at a dead stop. Typically, you would drive them like a three speed using gears 2-3-4 around town, and the granny gear only when you needed that extra low gear. 

Synchronized 4spd: There is a unit available with a fully synchronized 1st granny gear: the T19 ( 5.11:1 1st gear).

Granny 5 speeds: 

The most popular is the NV4500 (5.43 or a less common 6.11 1st gear), which can be adapted to a wide variety of applications. Also frequently used are the "ZF" 5 speed and the Clark 5 speed. These units are fully synchronized in all 5 gears, with 5th gear as an overdrive gear. 

Link: (High Impact specialty) 
New NV4500 Conversions  

Contact High Impact: we will assist you to determine which option would be best for your application and your pocketbook.


All of the above units are plenty strong, most are overkill, for a small block powered short wheelbase 4x4. Most are sufficiently strong for big block full size use. 


 These units will generally not be bolt in replacements. The adapters, replacement bell housings, conversion clutches, throw out bearings, pilot bushings, hardware requirements and such must be researched and considered when planning such a swap. The knowledge base of such mixing and matching parts is available. 

High Impact Free Tech Line

High Impact researches and provides complete kits

Drawbacks to Granny Trannies 

Drawbacks include that the real wide ratio unsynchronized units make a Jeep drive a bit like a dump truck, but are none the less an excellent compromise for a serious 4x4. They are much larger and heavier than the original trans (they were designed for dump trucks and such). These extreme wide ratio units must be wound up in one gear before shifting into the next, and some times the gear you are in is too low, but the next gear up lugs and won't pull.

The later synchronized units tend to cost more, and do not have quite as low 1st gears, but are a somewhat more drivable compromise. 

A fifth gear is a wonderful bonus. This overdrive gear will allow a lower axle ratio retaining excellent highway gears.

If you want it all, read on in the 

"Systems Approach to 4WD gearing"



image12chevy465large.jpg (25183 bytes)

SM 465 4Spd (6.58 1st gear)

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SM 420 4spd (7.05 1st gear)

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NP435 4spd (6.69 1st gear)

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T18 (Ford) 4spd 6.32 1st gear

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Ford ZF 5spd

nv45005spd.jpg (14714 bytes)

NV 4500 5 Speed

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