10.25" gears

CHKNFKR · Mar 28, 2011

My dd is a 2wd 5 speed '95 psd. As it sits its got 3.55 gears out back and i don't even use first gear. I'm also pulling off an impressive 20 mpg!

My thought is if i went to a higher gearset i'd get even better milage and lose nothing off the line, id just start using 1st gear. Anyone know how high i can go with a 10.25"? Anyone have personal experience with a 5speed psd and say 2.73s?

CHKNFKR · Mar 28, 2011

now that i'm sitting down with a pen and paper, here are some number.

my first and second gear:
1st 5.08
2nd 2.60
my tire height:
31"

my current gear ratio
3.55

I wanted some kind of baseline number to work off of, revs per mile seems like a good candidate. right now taking off in second like i do, it takes 6009 revs to go a mile which is nuts for second gear, i think anyway. to yield the same 6009 revs per mile but in first gear i could run 1.83 gears out back and notice no difference whatsoever, except my 5 speed would be a 5 speed, not a 4 speed. on top of that my highway milage would skyrocket!

now for the reality, the highest gearset i can find for a 10.25 is what i'm already running....

cant find taller gears what do you do? find taller tires!

the magic number to maintain 6009 revs per mile is 60.5"

since this is obviously out of the question where does that leave me? i suppose an overdrive unit would get me closer, but i don't see that being cost effective.... anyone know of a good heavy duty full floater rear end available with 2.73 or higher gears?

done rambling, till one of you pipes in with some good intel

CHKNFKR · Mar 29, 2011

Hmmm.... I've got a 205 laying around wasting space, i suppose i could run it back'rds and get a little more fuel economy. Might be a little more overdrive than i need though.

Someone gonna spitball with me here?

CHKNFKR · Mar 31, 2011

For anyone actually reading the stupid §hit i post, today i realized i overthought a very simple point here. My first gear is 5.08, second is 2.60, divide that out and you get 1.95 between them. My backward np205 idea might actually do the trick, they're geared 1.96:1. That means if i find a divorced 205 from a 73-77 3/4 ton and run it backwards my 5 speed will actually be a 5 speed! And my 20mpg milage i get now will only go up.

Now the only questions left are for those with experience with the case: will it hold up to power going through it backwards, rotating backwards, while in low (i mean overdrive) almost constantly. Running it level should handle any oiling problems it may have, i think...

RangerSVT · Mar 31, 2011

Why not do a Tom and run dual transmissions...this way you would have 1:1 in 4th gear, and 0.72:1( I think) in 5th, while running the second trans as a regular 5 spd trans...and if you ever needed extra pulling power, you got 3 more lower gears to select.... :icon_thumby:

SVT

TheTopher · Apr 1, 2011

Have you considered just taking off in 3rd? That's a taller gear and very easy to install, just push your shifter forward and you're done.

RangerSVT · Apr 1, 2011

I believe he is looking for a double overdrive, so instead of have a 0.72 ish overdrive, it'll be closer to 0.50 overdrive (like a 6th gear)...
SVT

CHKNFKR · Apr 1, 2011

The double tranny idea would hold up better cut the tranny would be doing what its designed to do, but i think i want more overdrive. Running a tcase backwards puts me exactly where i want to be in terms of ratios, the downside is a tcase isnt designed to be a torque splitter. I see it whining just a bit from unusual thrust loads and such. Gonna have to do my homework but i think i can make it work

RangerSVT · Apr 1, 2011

The problem I see with that is if you divide your gears (not multiply as in finding crawl ratio, dividing to find overdrive ratio) so first gear becomes 2.59, and second becomes 1.33, 4th would be 0.51:1 and if 5th was 0.72, its now 0.37:1...Tom used the the 435 backwards and hasn't had any problems with it, I don't see why you would have a problem doing that with the 205...
SVT

CHKNFKR · Apr 1, 2011

RangerSVT said:
The problem I see with that is if you divide your gears (not multiply as in finding crawl ratio, dividing to find overdrive ratio) so first gear becomes 2.59, and second becomes 1.33, 4th would be 0.51:1 and if 5th was 0.72, its now 0.37:1...

Thats the whole idea here though.

Tom used the the 435 backwards and hasn't had any problems with it, I don't see why you would have a problem doing that with the 205...
SVT

Tom still ran power from input to output, the only thing that was different was direction of revolution, and even so he had to tear both trannies apart and rearange things to make them handle thrust loads in the opposite direction.

Whatever happens, it should be a fun little project

RangerSVT · Apr 1, 2011

True, true....so I guess you will be building a reduction box, then flipping it backwards...You might not need to make any changes to it since theoretically it will be like you are driving the reduction box in reverse...
SVT

LOGANSTANFORTH · Apr 11, 2011

Why not use a Gear Vendors...

gwaii · Apr 11, 2011

CHKNFKR said:
For anyone actually reading the stupid §hit i post, today i realized i overthought a very simple point here. My first gear is 5.08, second is 2.60, divide that out and you get 1.95 between them. My backward np205 idea might actually do the trick, they're geared 1.96:1. That means if i find a divorced 205 from a 73-77 3/4 ton and run it backwards my 5 speed will actually be a 5 speed! And my 20mpg milage i get now will only go up.

Now the only questions left are for those with experience with the case: will it hold up to power going through it backwards, rotating backwards, while in low (i mean overdrive) almost constantly. Running it level should handle any oiling problems it may have, i think...

you'll have no oiling or durability problems running the 205 backwards,but to get more advantage out of it you'll want to change the diffs to 4.11s.this will give a final drive ratio of about 2.25 and still keep all the gears useable.

with 3.55s in the diffs you'd end up with around 1.80s.that's a little steep for even the psd.

10.25" gears

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