Stock diff Gears

[AC Trust]

I looked around online but all I get is adds to sell me gears lol.

What would the stock gear ratio be on a 1981 I6? I know it's 7.5" diameter but would they be 2:73?

And what gear ratio would increase fuel mileage if any?

Thanks once again!

[SRM351]

I can't tell you exactly what ratio you might have but there should be a little metal tag on one of the inspection cover bolts that would have the ratio on it. A lot of times when the unit gets serviced the tag gets tossed though so it might not be there.

You can also try placing a mark on the driveshaft/pinion and another mark on the brake drum or wheel. Counting the number of times the driveshaft makes a complete rotation for each rotation of the wheel will give you a ballpark estimate i.e. 2 3/4 turns of the driveshaft to one turn of the wheel equals a 2.73 and so on.

FWIW my '79 originally equipped with a 2.3l had 2.60 gears in the 7.5.

[IDMooseMan]

See my signature for my rear gear info. You can look at the door tag for the transmission code. Post that code and someone will be able to tell you what gear ratio you have as well.

If those items, tranny tag and door tag, are missing, do like SRM351 suggested. IIRC, if you have a non-locking rear differential, only the right-rear wheel will turn. If you have a Trac-Lock, both wheels should turn.

[AC Trust]

wow thanks guys!

[FoxChassis]

1981 3.3L w/ 4-speed manual = 2.47:1 (Traction-Lok optional exc w/ high altitude emission system)

1981 3.3L w/ automatic = 2.73:1 (Traction-Lok optional)

[ethangsmith]

As for fuel economy, there's a bit of science there. You want a ratio that won't be so high (Numerically low) that the engine is lugging all the time and the car is slow as a snail, but you don't want something so low (Numerically high) that it revs the engine like mad either. With the superior torque output of the inline six, but the lack of overdrive gears, a 2.73 is about right- which is probably why Ford used it. My 86 2.3L auto came with 3.27:1 rear gearing. If I go higher in number, the car will wind out by 65mph, but if I go lower, it won't have enough oomph to get moving nicely. It's a tough balance. If you don't do much highway driving, go to a 3.08:1 or a 3.27:1 for some more excitement and better acceleration- though there will be a slight penalty to fuel economy for it.

[xctasy]

Beware, not 3.3 I6 had 7.5", so ratio swaps can be a little bit dicey unless you can pull the codes.


From 1979 to 1983, quite a large portion of Fox body I6's, even the bigger ones like the Cougar wagaons were the 6.7 " Sterling 24 spline diff from the Pinto/Mustang II 2.3 and 2.8's. Ratio was 2.73, or 3.08, but there was a 6.875" 3.45 in the Ranger that bolts right in to the Mustang/Capri/Fox housing.

No LSD, no real strength, and I'm pretty sure the common 7.5" 2.47;1 ratio wasn't a 6.7" ratio option, it was the 3.08 with the 0.81:1 SROD top gear factored in. Ford called it a 2.49:1 in the brocure, but it wasn't.

The 1984-1987 Ranger 2.0 and some 2.3's used the same diff centre but in leaf sprung, five stud form, code 74 allows you to lower the ratio right down to 3.45:1 with factory parts and get some strength.There was a 72 code 3.08:1 in the Ranger as well. I don't think there was an LSD option even with that. I think the Fox body had no LSD option in the first years, a major problem when you consider how horriable a BR78 13 tired car could be in the snow or dirt. Even myC3/2.73/ 3.3 can smoke its BMW 528i P200HR390's in short order on any road, any time.

The Pinto and Mustang II ratios were listed as 2.73 and 3.18, 3.40, but although the diff was similar, I'm not 100% certain if those leaf sprung diff internals are interchangable. in 1974, the spline count was raised, and the Sterling diffed Fox and Ranger use the same spline count.

Ford probably didn't change anything, as the cost of certifying a different axle ratio was in the millions, so it was cheaper to make an new ring gear or pinion than use the earlier Bobcat based diff ratios in the Fox or Ranger

[AC Trust]

As for fuel economy, there's a bit of science there. You want a ratio that won't be so high (Numerically low) that the engine is lugging all the time and the car is slow as a snail, but you don't want something so low (Numerically high) that it revs the engine like mad either. With the superior torque output of the inline six, but the lack of overdrive gears, a 2.73 is about right- which is probably why Ford used it. My 86 2.3L auto came with 3.27:1 rear gearing. If I go higher in number, the car will wind out by 65mph, but if I go lower, it won't have enough oomph to get moving nicely. It's a tough balance. If you don't do much highway driving, go to a 3.08:1 or a 3.27:1 for some more excitement and better acceleration- though there will be a slight penalty to fuel economy for it.

awesome thanks!!!

[AC Trust]

I found a set of new 4.10 gears for $60.00 here in my city. I would like to hear some opinions on that ratio. Keep in mind I have a 3 speed auto transmission.

The gears are 7.5" so I will have to look and confirm that is what my vehicle uses. 1981 Mustang I6.

Will there be a tag on the diff or can I simply just measure it?

Thank you.

[ethangsmith]

There should be a tag on your rear diff cover. However, it won't be anywhere near 4.10:1.

With a 3 speed automatic and an inline 6, you'll have insane acceleration, but by about 55 mph, your engine will be screaming (Probably like 4000rpms). My 3.27:1 gears in my Capri will take the engine to about 2700-2900rpms at 55 mph and around 3000rpms at 60mph. So if you're into big burnouts and hard acceleration and don't plan to go much over 65mph, those may be the gears for you. Otherwise, I'd maybe skip on them for something a bit lower in number, like 3.27:1, 3.45:1, or 3.73:1.

[blake4591]

From 1979 to 1983, quite a large portion of Fox body I6's, even the bigger ones like the Cougar wagaons were the 6.7 " Sterling 24 spline diff from the Pinto/Mustang II 2.3 and 2.8's.

Is there any easy way to identify the 6.7 axle? I know the 7.5s have smaller axle tubes than the 8.8 and that's how you tell them apart.

[CapriGT]

Is there any easy way to identify the 6.7 axle? I know the 7.5s have smaller axle tubes than the 8.8 and that's how you tell them apart.

A quick look at the cover will be the easiest way to identify the 6.75. It's very small and square looking.

[xctasy]

This is a carefully edited repeat.

Foxes were only 2.73,3.08,3.45. Rangers were 3.08,3.45. Mustang II's were 2.73,3.18,3.27,3.4,3.55. Info I have is that the diff centre section was all the same from 1974 to 1988, when the last Ranger 2.0 was made.

The actual 2.73 ratio came out in 1977 in the Mustang II 2.8 V6. Then a 3.27 ratio came out in 1978 on 2.8 V6's. No 6.7 diff ever got Limited Slip diffs

Common early 1974 on Mustang II tags for those were

WGF-H 3.40:1 6 3/4 C2 Integral
WGF-K 3.40:1 6 3/4 C2 Integral
WGF-M 3.55:1 6 3/4 C2 Integral
WFZ-B 3.55:1 6 3/4 C2 Integral

From 1977
WGF-AJ 2.73:1 6 3/4 C2 Integral
WGF-K1 3.18:1 6 3/4 C2 Integral
WGF-U1 3.18:1 6 3/4 C2 Integral
WGF-AA 3.18:1 6 3/4 C2 Integral

From 1978
WGF-AJ 3.27:1 6 3/4 C2 Integral

See http://www.mustangii.org/decoder/axle/



See http://i1215.photobucket.com/albums/...6/IMG_6984.jpg.

On my 6.7 corporate diff, the code was:-

WGG-C 0G24
2 73 6 7 S258C

WGG-C OG24
2 73 6 7 S258C

Std for a ‘79-‘82 Open 6.75” diff with a 2.73:1 ratio.

Brown is Fords code for the ratio
Red is the crown wheel diameter
Blue is the Plant.

All 6.7 (6.75, 6.875", but called6.7 on the tagm, but also 6.5, 6 3/4 in on line literature) and 7.5"s are Sterling built Salisbury integral carrier diffs.

A generic 7.5 would be

WFC-BY 4F13
3 08A 7 5 S27IF


If its limited slip, its 7.5 V8 or one of the Turbo LSD 7.5" from 1981 to 1984.


Not everything 3.3 had a 7.5" diff/C4 combo there, 6.7/C3 was very common.

The numbers are simple for the Sterling plant made 6.7 diffs, very common.

Based on pinion seal part number, the US 6.75" diff came out in 1957, but was used in early XK 2000 Falcons (1960 year), then got replaced when early round bodies went to short pinion 7" diffs, then long pinion 7.25" diffs. The 6.75" lookes quite different even though it has 8 bolts. The diff then appeared to continue as the weedy short pinion 6.75" diff used in 23 spline form in 1971 to 1973 Pintos.

The diff was based on the upgraded long pinion, big bearing and 24 spline item from the 1974-1978 PintoMustang II's, and the 1974 to 1980 Pinto/Bobcats. The ratio choices themeslves had been expanded to suit V6's, so the 2.73 gear I have in my 1982 first came out in 1977 in those leaf sprung diffs used in 2.8's. As I've said, it looks ilke Each emissions package required a certified diff ratio, so it was cheeper for Ford to tool up for a 2.73:1 ratio in the weedy 6.7 " diff than add a 2.73 ratio to the 8" diff. Part of the confusion with Ford diffs is that the 6.7, 7.5 and 8.8 are integral carrier diffs, with the same interchangable axles in the 7.5 and 8.8, and most people assume if its a Fox six, the diff in it will -be a 7.5 incher. The other point, is the 2.73, 3.08, 3.27 and 3.45, although proven to be inagural 6.7" ratios, they are 7.5 and 8.8 inch diff ratios too because cost of CAFE/EPA/FMVS certification ment they had to use the same ratio, despite the diff type. That's why the C3, C4 , 6.7 or 7.5" diff, transmissions and grey low mount or blue high mount 3.3 I6 engines where inserted willy nilly on the line...it cost Ford nothing, because each combo individually and as a package had 49 States certification.

90% of the Fox I6's were 2.73. A lot of the Fox 2.3's had the 6.7" in 3.08 ratio.


If 3.3, and SROD or T4 four speed I6, they were 3.08. Again, it was listed in the brochure as 2.49, but that was with the 0.80 overdrive fourth ratio applied.

Codes were 72 and 74 respectively in the Rangers with the 3.08 and 3.45, the only easy way to find 6.7" diff parts was the entirely forgettable 80 hp to 73hp 1984-1988 2.0 liter base model 1-bbl Lima engine.

[blake4591]

Well after reading this a couple days ago it had me mildly concerned that I might have a 6.75 since it was an 82 cougar with a 3.3 just pulled the tag of my axle and managed to make out

WCX Z 2F25
_73 7 5 T842B

so that's a relief

[roush235]

Here is mine




WGX-NX 9C12
2.73 75V 275A