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A lot depends on your axle ratio. With 4.10 it would be a waste of time. With 3.54 and a standard governor spring it would be somewhat desirable but with any taller gearing, specially if you plan to tow heavy, it would be real nice to have ten gears plus there is the durability of the Eaton-Fuller which would be unreal. Remember that as your rear-end gearing gets taller, the gap between the gears gets more troubling and making extra gears more necessary.
When my truck had 4.10 gearing I could easily shift up on steep hills because the jump to the next gear was not great. After I put in the 3.54 I found that the gap was too wide sometimes. The governor spring upgrade gave me the rev's so that I was able to make the shift up ok. The governor spring made up for the downside of the taller rear-end ratio plus it let the cummins keep its rev's up and reduce heat build-up, a critical issue for a non-IC'd cummins.
Also remember that just because you have ten gears doesn't mean you have to use them all of the time. You can make like you have a five speed when you are not pulling hard.
91 (08-90) D250
5sp 2wd 511k VE 'adjusted', 3200 spring, Pyro, boost, 3.54 ,:thumbsup KN, 1\8" timing bump, 25mpg city 30mpg hiway 20mpg towing 6k 5'er
R60/7, Volvo 245 GL 320 k km's, f150 4x4 5 spd, MPG's are in Imp gal's
Last edited by Old 'n Rusty; 12-22-2009 at 05:33 PM.