Hey guys!

I just was wondering, I'm doing my first restoration project on a 73 f100 and I need to know what rear end gears to use. I will be rebuilding a ford 9" and my truck will be a daily driver. I want to use a good gear ratio for in town and mild highway driving (15 miles or less,40 to 65 mph).

Also, the rear end I'm gonna be using does not have posi. Should I try to get one to put in? And, are there any tips I need to know about rebuilding a 9"? I've heard that they are a great rear end to learn on.

Thanks!

 

well alot depends on how you use your truck , what size tires you'll be using, and how important fuel milage is to you. personally, i prefer 3.73s with a limited-slip. you get resonable milage but you can still haul/tow quite a bit. 4.10 or lower will give you more low end power , but will decrease top speed and gas milage, while 3.08 will give you good fuel milage but will be more difficult to get a heavy load moving or start uphill with a std tranny. keep in mind this is with stock tires (27-29 inch) like a 235-75-15. if you run bigger tires you efeectivly raise youre ratio, such as 235-75-15 with 3.55s will drive like 3.08 when you go up to a 32-11.50 tire. (not exact-just a example) if you see any snow at all and you are running a 2wd i would recommend a limited-slip. with 4wd it's nice also but not really nessesary. i've never rebuilt a rear end. get a good shop manual on it and the proper tools and it can be done. my current truck is a '93 ranger 4x4 with a 4.0 (245 c.i. ) 5-spd and 3.73s i get 17 mpg back and forth to work and turn just under 2,000 rpm in 5th @ 55 m.p.h. (about 2,300 @ 55 in 4th gear) with a auto tranny you can get away with a little lower ratio than a std. hope this helps.

 

The ratio will be determined from tire size and engine. If you put a 3.50 ration behind a 390, 302, and 300, each will give difference perform off the line and at 2500 rpms.


Need more info:

engine size
tire size
transmission

If the truck is not 4x4, I would put a limited slip in the rear end.

 

You guys are awesome!

Here's some more info:

Engine: 390 (.030 bore)
Tire size: 29.74 inches (diameter)
Tranny: C4 manual (considering c6?)

Thanks again for the help!

 

I like the 3:73 ratio. I have a 460 4X4 with the 3.55, which is ok with this size motor. If you go into the rear, spend the extra and get a limited slip. There are many different types for the 9in, so do your homework. Four-wheel magazines would be a place to start. Might be cheaper to just buy a carrier set up with every thing new. Here is a link that will help with the effective ratio. Someone else posted it if I come across it again I will give the name. Good stuff

http://www.tyres1.com/tech_calcs.html

 

> 15 miles or less,40 to 65 mph

The Ford 9 inch in trucks sort of came standard with a 3.50 ratio because with a 1:1 transmission ratio you end up at about 2500 rpms at 55-65 mph. It was the standard ratio under later model Broncos (80s) if you need to go shopping for an axle or center section.

3,000 rpms is all about where you want to be with the bigger V-8s, imo. 4000 rpms is about your red line on a 390 and 3,000 is about where you want to max it for any length of time. I try to keep my 390 as close to 2300 rpms as as possible when cruising. If I keep my foot into it with a load I can practically watch the fuel needle drop by the minute. With my hubs unlocked I get almost 10 mpg at 60 mph.

15 miles per day, especially if it is mostly low speed city driving, is not a lot of miles and I doubt you will see any real difference between 3.50 and 3.73, so I would go with either. As someone noted, 3.73 would be better for hauling. Though you will be sucking down the gas either way at 65 and 2500-2700 rpms.

I have found 60 mph to be the sweet spot for my 390/np435 (1:1 ratio in fourth)/3.50 rear end/31s (measure 30 inches). It is not the RPMs that really kill the gas mileage, it is the drag. In GA this means the slow lane because traffic in most 55 mph zones travel at least 80 mph

If you have a four barrel I would shoot lower on the RPMs so the primaries are not wide open. On the 2 bbls I do not think a few 100 rpms either way will matter much at all.

If you have a 390, I highly doubt you have a C-4. On your truck it should say C-6 right on the side of the transmission case.

Limited slip and 3.50 to 3.73 would probably work best and the 3.50 would be easy to find in a junkyard axle under a Bronco. You might even luck out and get a factory limited slip. The later model Broncos had 31 spline axles too.

If you want some rebuild tips, look at the articles section where people have installed lockers (not something I would put in a daily driver). One guy did a pretty good article with pictures and everything.