rear end

[joshryan2]

i have a 12 bolt rear end in my 84 chevy short box, engine is pumpin out around 400 horse.  which gear ratio should i use?

[Blazin]

Depneds if you want neck snapping power off the line or top end speed? off the line 4:10s or 3:73s. top end speed the higher the better, air plane gears 2:73 or 2:93s.

[Lt.Del]

Quote:which gear ratio should i use?

If there was one best ratio, why would GM create so many?  As Blazin said, it is whatever suits your taste.

Are you going to pull anything with it, race it, do you want hight mpg, drive lots of highway miles or city miles????  

There is no one best answer to this--which is why GM made so many rears.  The lower the  number (say 2.73) the better on top end highway miles.  The higer numbers (say 4.10) the quicker off the line but it suffers doing 70 mph.

So, the answer is........I dont know your cicumstances.    

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[VileZambonie]

www.csgnetwork.com/multirpmcalc.html

[joshryan2]

whats in it right now is 3.07.  i am going to be using it as a cruiser and the odd street race...i guess what i am trying to say is there any way i can put a posi gear setup in the housing i already have.

[VileZambonie]

You can put a nice 3.73 posi in that differential case.

[joshryan2]

i took the numbers of the data tag and they are 3966163 does anyone know the specs to this rear end.  acording to my research gm never used the rear end on an 1984 chevy short box silverado

[roundedline]

If it indeed is a 12bolt and the truck is an 1984 model, it was put in there.  Last year for the 12bolt was 1981.

If you don't have overdrive, the highest I would go for ratio is 3.42.  As for putting in a new limited slip and gears, any ratio will work if you are getting new internals.

Chris Lucas
www.73-87chevytrucks.com
www.captkaoscustoms.com
Project Su
Jimmy 2WD Project

[joshryan2]

what if i switch it over to manual 5 speed?

[VileZambonie]

You can easily calculate your final ratio plus your transmission ratio in each gear range. If you want to fry tire go with a 3.73 or 4.11 with that 5 speed  Just take into consideration tire size

[SUX2BU99]

Depends too how torquey your engine is. Good HP and a tall rear gear will make it fly on the freeway but around town stoplight-stoplight racing in a motor that produces little torque down low will feel really soft. But if you have a 400 HP 350, I'm going to presume it has some decent grunt down low (unless you have a tunnel ram and dual 4's with 2" headers or something silly  lol). 3.42 is nice all-around ratio for a motor without overdrive. My truck has 3.08's I'm pretty sure and it sure doesn't have 400 HP but I'd love 3.73s......but with a TH700R4 to back it up or a Gear Vendors overdrive on the TH350 it has.  With the 295/50-15 or 275/60-15 tires I run on it, it revs fairly high at freeway speeds (2500ish at 70ish). Not sure if available for your diff but 3.23 could be a more rev-friendly gear to use as well.

My two Dakota R/Ts I had came factory with a 3.92 limited slip rear end but had an overdrive automatic. Even still, doing 65 on the freeway meant 2500 rpm. No wonder gas mileage in it sucked! But yes, what a great tire-fryer