Rumor has it that the DOHC Cobra was originally specified with a 3.55 rear end ratio, but Ford substituted 3.27 gears in order to raise its EPA mileage rating. Whatever the case, it's clear that the car could benefit from having a lower gear (higher ratio) back there -- the 4-cammer likes to rev!

Which Ratio?

Some Stangers choose 4.10 or 4.30 gears for their Cobras and are quite happy -- I can imagine such short ratios would make a huge difference in how the car feels, but since I wanted a good balance between street and highway performance (and in keeping with my philosophy of not getting too radical with the mods), I chose the 3.73 ratio and purchased a set of Ford Motorsport gears for $189 from Summit. This ratio should be plenty to "wake up" the engine around town without being too short for high-speed freeway runs.

Different Diff?

As long as I was changing gears, I figured I'd look into the many options for aftermarket differentials. Many folks have good luck with the Auburn and the Torsen, among others, but I finally decided that a rebuilt stock Ford limited-slip diff, the Traction-Lok, would be the best solution (Tommy Regan helped convince me of this). The rebuild involves replacing clutch plates and springs inside the diff to make it "tighter." The result should be better traction and balance under varying road conditions.

I Didn't Try This At Home

A gear swap is a rather involved process requiring some special tools, so I decided to have the folks at Motion Dynamics (near Austin, Texas) handle the job. These guys aren't what I'd call fast and efficient, but they do take time to do the best job they can. After watching them work for a while, I was very glad I didn't try this at home -- it's a much more difficult and lengthy project than I would've been comfortable with.

The new gears finally were installed along with the rebuilt diff, and the housing was sealed back up and filled with Amsoil 80w90 synthetic gear oil. To keep the speedometer somewhat accurate, a 21-tooth (red) driven gear is required with the 3.73s on a DOHC Cobra (other models vary) -- this little plastic piece, purchased from a local Ford dealer, snapped right into the tranny, replacing the pink 19-tooth factory gear.

Impressions

Back on the road. Wow! The gears make a big difference in the way the car feels -- it's as if it lost a big chunk of weight. Redline comes much quicker than before. I'm not sure if acceleration is actually improved all that much, but it sure seems like it is. The standard Cobra feels a little groggy compared to this. Now it's sprightly and fun. Swapping gears is definitely one of the most effective mods I've tried.

On the highway, there's a bit of an increase in RPMs: in fifth gear, an indicated speed of 75 mph before the swap turned the motor at 2100; now it's 2400. You can barely detect the difference when cruising; it's just that the car feels quicker when you give it some gas. Very high speeds can be attained without much thought -- you have to pay attention now! (The car probably has a lower top speed now, but I don't think I'll miss it. I never tried finding the actual top while it had 3.27s, and I doubt I'll have an occasion to test it now -- I like to stay sane on public roads.)

The 3.73 ratio feels right at home in the Cobra. After living with it for a few days, you'll never think it's been modified -- it just seems natural. Never having driven a Cobra with 4.10s, I can't really say whether that ratio would've been a better choice, but I can say the 3.73s leave nothing to be desired.

The diff rebuild is less noticeable, but it may prove worthwhile under poor traction conditions. However, there's a popping noise and a sensation of tire bind (as if the diff isn't doing as much diffing) when making tight turns. I'd probably skip this mod for a 100% street-use car, but if you autocross or drag race often, it may be worth considering.

Postscript: After changing my differential fluid (to Amsoil Series 2000 75w90) and adding a full 4oz bottle of Ford friction modifier, the noise and binding are pretty much gone. I guess ol' Jim didn't use enough friction modifier when he filled the rear end up. Moral: use the right fluid mix and you should be OK. It is a good idea to change your diff oil a few hundred miles after a gear swap.

How's the Mileage?

The issue of gas mileage is a little tricky since the speedometer (and therefore the odometer) is not quite as accurate as before. Some quick math (and observation) leads me to believe that mine is reading a few percent too high. Taking this into account, my mileage may have dropped about 1 or 2 MPG at most -- not significantly enough to worry about.

Once again I have to commend the job done by the friendly staff at Motion Dynamics. Jim Whelan's rule seems to be, "do it right, no matter how long it takes." I have absolutely no additional noise or vibration from the gears, and he even repainted my diff cover to improve the sloppy factory appearance. He charged me $200 for the whole job including the diff rebuild, plus another $100 for parts, oil, etc. (I supplied the ring & pinion).

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