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3.73s v 4.10s v 4.56s

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ShutUpHippie

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So does anyone know (or can show me where a thread currently is):

1990 K5 Blazer, 350 sm block

RPMs

with 3.73s - in 3rd gear at 75mph
with 4.10s - "
with 4.56s - "

Looking to switch from my 3.73s so I can get a little better fuel mileage and "get up and go" off the line.

Any other + and - for each gearing would be greatly appreciated.
 
You'll have to share your tires size and trans to get a real answer but you can search for "gear ratio calculator" and a bunch of different tools will pop up to let you plug in the numbers.

Numerically higher gears = lower mpg
 
What trans are you running? If its a 350 or 400 then 3rd gear would be a 1:1 ratio , use that same ratio for the t-case as well and just plug in the numbers into the RPM calculator in the link below . :D


http://pitbulltires.com/calculate.php
 
I can tell you from experience , with a lifted K5 on 35's , that mileage is NEVER going to be as good as you want . The gears will help it move , but the gears don't do anything for the air hitting the front diff that is now hanging down a lot more catching the air .

I sometimes got 15-17 mpg when I was stock height , 31's and 3.08's with my 305 and overdrive trans . Now I am lucky to get 14 highway unloaded with 35's and 4.56 gears . Moist of the time its 11-13 .
 
yeah, Ill be able to report a difference hopefully within a few months. It was bitter sweet driving up to jersey for my 38's and 1 tons with 4.10's, knowing that after the swap (including an as yet unpurchased 4in lift), i could kiss my 20+mpg (22 last time) highway miles goodbye :(.

22mpg from a 1986 diesel, TH400 non overdrive, 33in tires, and 10 bolts with 3.08 gears. driving at 55-60mph.
 
If you have 35's and a 700R4 OD trans you will want to do either 4.56's or 4.88's. Don't waste your time with 4.10's. Not enough difference to make it worth a gear change. If you don't run alot of open highway all the time get 4.88's.

Harley
 
So 4.56 seems to be the number that everyone agrees on for my setup. I assume it's a relatively easy procedure replacing those, and they should be easy/cheap to find hopefully.

Thanks everyone for your help. :D
 
i dont know if i would define setting up gears in a differential as easy...... Even if you are well practiced at it, its still an invovled process that requires special tools to do properly.

Ill spare you the details on the assumption that youve done it before and know what youre getting into.

regarding what youll need, to go 4.10's all you need is the install kit, and the gears (ring and pinion of course), the install kit includes the shims youll need to get proper backspacing and backlash. and to adjust the contact pattern of the gears for reliable operation.

For the 4.56 gears, youll need either special (more expensive i believe) gears that are made for the higher ratio carrier, or a lower ratio carrier to go with the 4.56 gears. And youll also of course need the install kit. So the 4.56 will probably be a bit more expensive than the 4.10's.
 
If you are running stock axles for a 90 (gm 8.5/10b) then you will not need new carriers to regear. Just gears and install kits. The carrier break on GM10b axles is 2.56 or 2.73 or something really low. All other gears above those will fit the same carriers.

Harley
 
Hossbaby50 said:
If you are running stock axles for a 90 (gm 8.5/10b) then you will not need new carriers to regear. Just gears and install kits. The carrier break on GM10b axles is 2.56 or 2.73 or something really low. All other gears above those will fit the same carriers.

Harley
Click to expand...

?????

2.73 is the lowest gearing i have ever seen for a 10 bolt. factory options as i was aware of them were 2.73, 3.23, 3.42, 3.73, and 4.11. EDIT forgot 4.56
 
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I ran 4.56s in mine and it was ok but i wanted 4.88s now i have a 4 speed no worries here.
 
Cool, so I have some ideas about which gears to pick. Greatly appreciated help! I'll have to hit up some mechanic friends of mine to get this done I guess, since most likely I won't have the correct tools.

Time for a little :k5: wrenchin.
 
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