Gear change on a Ford 9"

MadMaxetc · Post by **MadMaxetc** » Sat Jan 06, 2007 1:00 pm

How had is it to do? Do you need any spetial tools?

I have rebuilt engines and done lots of other mecanical stuff, but never a trans or rear end.

It is a '75 9" from an F-100. I want to put 4.11:1 in it and run a T-5.

fordman · Post by **fordman** » Sat Jan 06, 2007 1:31 pm

its not hard.

72hiboy4x4 · Post by **72hiboy4x4** » Sat Jan 06, 2007 1:48 pm

easy to do.
find a center section with the gears you want
pull the brakes/axles out
unbolt and remove the center section/gearset
install the new center section/gearset in reverse order

its almost that easy! ;lol:

72hiboy4x4 · Post by **72hiboy4x4** » Sat Jan 06, 2007 1:51 pm

But with 29" tires, whats your RPM's going to be on the freeway?

MadMaxetc · Post by **MadMaxetc** » Sat Jan 06, 2007 2:34 pm

Well I know you can just swap the carrier but what about JUST the gears.

How much re-shimming is involved and do the kits come with that stuff?

72hiboy4x4 wrote:But with 29" tires, whats your RPM's going to be on the freeway?

Well I figured it using a T-5.

T-5 has a 0.68:1 overdrive, so with 29" tires and 4.11:1 rear gears thats about 2000 RPM at 60mph and 2400 RPM at 75mph.

The 9" I have has 3.70:1 gears in it now. That might drop the RPMs too low for my 300. It is carberated and I might have to play with the power valves.

I just wanted to know if it is an option.

Jake11 · Post by **Jake11** » Sat Jan 06, 2007 6:45 pm

On a 9" it's a piece of cake. The ring gear is moved side to side with
screw adusters. The pinion has shims. I'll walk you thru it if you want.

MadMaxetc · Post by **MadMaxetc** » Sat Jan 06, 2007 6:51 pm

cool.

I will evaluate what I have and the rought I want to go.

Then I will have you walk me through it.

Thanks guys.

fordman · Post by **fordman** » Sat Jan 06, 2007 7:59 pm

i thought 4:11 was more of a pulling gear than highway.

cdeal28078 · Post by **cdeal28078** » Sat Jan 06, 2007 10:48 pm

I had an AOD behind a 351w with 3.70's before I went to a 460 c6. With the 3.70 and AOD the engine seemed to be almost lugging running 70 mph. I always thought a 4.11" would be better unless I was pulling a load and needed to run in drive.
clint

ladz · Post by **ladz** » Wed Jan 10, 2007 6:49 pm

There has to be a chat to figure out the rpm At mph some where for gear ratio ! I kind of remember that 411 gear with a 28" tire at 60mph was around 2800 0r 3000 rpms i think , that was on a friends nova and always needed gas !

spartman · Post by **spartman** » Wed Jan 10, 2007 7:02 pm

Try this one:

http://www.ring-pinion.com/Calculators.aspx

ladz · Post by **ladz** » Wed Jan 10, 2007 8:13 pm

Spartman , you are the man you just made my day !

MadMaxetc · Post by **MadMaxetc** » Wed Jan 10, 2007 8:36 pm

OK. I think I am going to stick with the 3.70:1 gears.

Here are the numbers...
29" Tire, 3.70:1 Rear Gear
T-5 Trans - 4th 1.00:1 , 5th 0.68:1
So at 60 mph...
4th = 2573 RPM
5th = 1750 RPM
And at 75 mph...
4th = 3216 RPM
5th = 2187 RPM

I think that I should be able to cruize right along normaly and if I ever have to pull a load I can down shift and not blow up the engine.

this stuff is just for referance...

T-5 (well mine is, there are others out there...)
1st = 3.35:1
2nd = 1.99:1
3rd = 1.33:1
4th = 1.00:1
5th = 0.68:1

RPM = (mph * trans gear * rear gear * 336.135) / (Tire Diameter)

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