Before anyone asks, no I couldn't sleep tonight if you can't tell by looking at the time of my last two posts. I was wondering, this will be the third forum I've asked this question to, does anyone know what the standard gear ratio would be for a 1968 F100 Ranger/390/Automatic? Is there an easy way to find out what the gear ratio is on my truck?? And what is this Colorado trip all about? Do you guys have get together's often???

 

There where several ratios availible for the F-100. Easiest way to fing out put a mark on the inside of tire and a mark on the driveshaft. turn the tire 1 full turn around and count the driveshaft rotations. then do the math 1 turn -- 2 3/4 turns on the driveshaft is 4:11 gears. (2.75 X4 =11)

We go to Colorado to go camping and fourwheeling once a year.

We try to have g2g's at least once a month. This months is in Emporia. THere should be a thread about it.

Don't worry about posting late/early I am the king of the latenight post.

 

some times there is a little flag that is mounted to the differential housing bolts that will say what gear ratio you have, they are hard to read, especially under the truck in the shade. if you see a thin strip of metal with one or two holes on it in that center section of your differential, you might have to scratch the rust/dirt/muck off of it to see if it says.

Chad, quick question, is it when he jacks up one side of the differential and turns the tire by hand, or is it when the tires are on the ground and both tires have made a complete turn? This of course would matter if he had spider gears or some other form of differential.
Please clarify so that I know myself.

 

Here so info on the tag you aer looking for: Ford Rear Axle Information You can just remove it by taking the one fastener loose that hols it in place and then clean it to read it. There are 4 digits for the axle ratio. They are on the bottom row next to the hole where it is attached to the axle center section. The second digit will either be an L for limited slip or blank for open. You can see in the pics posted.

 

On the tags that Jack posted it looks like the ratio is 2.80 with limited slip for the first tag and 2.79 with an open differential for the second one. That's if I remember right.

With a 68 F150 you might have a typical range of axle ratios available from 3.00 to 4.11.

Other common ratios might be in the 3.50 or 3.73 range.

 

Cleaned it up the best I could and here's what I see:

Top Line: WEY - A (don't see this listed on the 9 inch ID tag codes)

Bottom Line: 3 25 7MA 350

I'm assuming from what has already been posted that the ration is 3:25?

What does the rest of it mean and why doesn't WEY-A show up under the ID tag codes? Maybe I better go look again.

 

The link I listed was for a car site not a truck site. So that is why you didn't find the code. You might do a search on it and see if you can find something. And yes if it is the the correct tag for that axle it is 3.25 ratio open.

 

I have lost my link for my gear place. May be on another puter and I'll check and get back. All I can find so far on the 9 3/8 is it is some times call the 9 1/2. That it will bolt into a 9" housing with a little fab work.

Posi is a term that has been used in different ways. I like to use it as the broadest term for non open differentials. And I then break that down into two sections. One is limited slip. The other is lockers. There are many types of both. Clutch and gear biasing limited slips. Air, electrical, and mechanical lockers. And many different designs. And even combinations of the different types.

Used to be certian products were made for certian axles. And still is to a point. So it may take several calls to different companies to find out if anyone makes something for the 9 3/8. Most factory LS were the Trac Loc. It is a clutch type design. The clutches inside the differential apply preload to both axles. The clutches slip and wear when you go around a corner on hard pavement. And they wear out over time and need to be replaced. I would check first to see if a Trac Loc and R & P were available for the 9 3/8. If it is you have somethng to fall back on and can check for maybe a Detroit Locker or something else if you want. If nothing is available for the 9 3/8 and you really want different gears and posi then it looks like you would need to go with the 9". As I said I have read that a 9 3/8 centersection will work in a 9" housing. So a 9" center might work in a 9 3/8 housing? The advantage to that would be the large availability of ratios, posi's and most likely lower cost for parts. Another option would be to check salvage yards for anoter axle assy with the ratio you want.

 

Thanks Jack, good info.

 

Jack has provided good information. I ran across some info a while back on the 9 1/2, but can't remember where. Probably in an issue of Hot Rod or Car Craft. It isn't really any stronger than a 9", so is probably not worth spending much money on if you are changing things. A 9" from an early 60's pick up until '72 should bolt right in. You might have to change the drive shaft, but I don't know on that. That would give you almost unlimited options on ratios and lockers or limited slip. I believe "limited slip" was Ford's term and "Positrac" hence posi was the GM term.

 

The AXLE code is listed on the Warranty Plate for every car from 1949 thru 1979, trucks 1948 thru 1980.

After 1979, or 1980, the AXLE code is listed on the Certification Label.

The 9 3/8" rear axle is classified as a 9 inch. I've never heard of, or read anywhere in any Ford parts catalog as it being refered to as a 9 1/2.

This rear axle has 31 splines, was used in 1968/72 F100 Camper Specials, was optional on other F100's with 360/390 engines.

The rear axle capacity of the 9 3/8" is 3600 lbs, the 9's are rated at 3300 lbs.

Five: That's the number of internal parts that were also used with a 9 inch (housing gasket, one pinion bearing, pilot bearing lock ring, pinion seal, crush sleeve).

Everything else is 9 3/8" only. 3.50-1 was the highest numerically ratio available (F100 only).

The 9 3/8" was also used in 1966/73: Lincolns, LTD's, full sized Merc's and Bird's with 428/429/460's, 1969/73 Lincoln Mark III's & Mark IV's.

WEY-A = Ford 9 3/8" Rear Axle / 3.25-1 / 3600 lb. Rear Axle Capacity / 2 Pinion Conventional non Limited Slip / Large Axle Bearings (9 3/8" only) / XL Differential Bearings (9 3/8" only).