Rear End Gears 3.49 to 3.73

[SD5805]

Has any one changed or enhanced there rear end gears. If so was it positive or a negative change. I love my cobra, but we always can improve. I'm thinking this may be a cost effective way to get a bit more punch rather than doing engine work. I have a 351 W crate putting about 400 horses.

I like an overall performance. low and high end
Thanks

[blykins]

Rearend gear swaps are an easy way to make the car feel a lot stouter. I don't think you'd really notice a lot of difference going from a 3.50 to a 3.70 though. The big differences are where you go from like a 3.08 to a 3.73 or a 3.08 to a 4.10. The 3.70/3.73 is a good overall gear in my opinion.

[Cashburn]

You need 3.91.

[1985 CCX]

Depends upon your driving. If you are on back roads and car shows taller gears ore ok. I had 3.54 in my CCX and found them a touch high for regular hwy use. I will now be using 3.31 with 4spd, I am told it will still pull nice and lower the rpm's on the back roads.....

[SD5805]

what will the 3.91 do for me. will that be a noticeable change. I think I have 3.49 if im not mistaken

[1985 CCX]

The increase in ratio will add take off 0-60 however also will increase cruising rpm when on the open road. This is especially the case with a 4spd. Jay does know best, transmission also plays a big roll.

[B.M.B.C]

what are the lowest ratio diffs for a BDR, is there anything about 2.8 - 3.0

[Roush #1]

To make a meaningful recommendation for you, we need to know what transmission (with the gear ratio) you are using, the tire size, as well as the RPM you wish to achieve at a specific speed.

Don't mean to be anal but without this information it is all a guess of what you want and what you have to use to get there.

J

[DocDirk]

Google "gear calculator" and you'll find a lot of options for checking out the ratios based on the above recommended stats. It's a fun game, but I've not yet found the best of both worlds - launch and cruise.

Regards!

Dirk

[Cashburn]

He has a T-5 and 295 15" BFGs in rear. Engine is a Ford Racing 351W so I think he would love the 3.91 based on that info from my crystal ball. And currently is running 3.46 rear.

[vector1]

have a 3.55 with .68 od and 17" wheels with 351/385 crate engine, real nice on the hiway. changed from a 3.27 and much better.

[Roush #1]

With the information provided by Cashburn, I agree with his crystal ball

J

[6t8stang]

I have 3.91's behind a T5 with 315 Nittos on 17" wheels with a Performance Engineering 351W and love it!

[Excaliber]

I recently went with a lower gear ratio trans, and currently I am now SLOWER at the the 1/4 mile track than I was before!!! I will be faster, eventually, I hope, as soon as figure out the new variables for the hole shot and shifting into 2nd without going up in smoke.

If you already have trouble hooking up, lower gear ratio's will make it worse. Thats why a lot of folks, with low gear ratio's, get a better ET starting out in 2nd gear instead of first. But that does tend to fry your clutch, rather than your tires.

One of the worst gear ratio combo's that is fairly common is a T5 trans with a 3.27 (or is it a 3.35?) first gear and a 3.5 rear gear. Great for smokin' tires, looking good, playing around. Fairly worthless on the strip without some serious slicks to hook it up.

T5 with a 3.91? What??? Top speed in first is what, 25 mph??

[SD5805]

what would be the performance difference from 3.46 gears to 3.91 For example in 5th gear at 2000-2200 rpm I'm cruising at approx 80mph.
thanks

[CobraEd]

Originally Posted by Excaliber

I recently went with a lower gear ratio trans, and currently I am now SLOWER at the the 1/4 mile track than I was before!!! I will be faster, eventually, I hope, as soon as figure out the new variables for the hole shot and shifting into 2nd without going up in smoke.

If you already have trouble hooking up, lower gear ratio's will make it worse. Thats why a lot of folks, with low gear ratio's, get a better ET starting out in 2nd gear instead of first. But that does tend to fry your clutch, rather than your tires.

One of the worst gear ratio combo's that is fairly common is a T5 trans with a 3.27 (or is it a 3.35?) first gear and a 3.5 rear gear. Great for smokin' tires, looking good, playing around. Fairly worthless on the strip without some serious slicks to hook it up.

T5 with a 3.91? What??? Top speed in first is what, 25 mph??

If he has the T5Z from Ford racing, it has a 2.95 first gear.


.

[Excaliber]

2.95 is better than being in the 3's for sure. I think my TKO is 2.87 with my 3.31 rear gear. It's in the ball park, but I would prefer something like a 2.7 first gear, just a touch higher but not much.

Of course a lot of the final gear ratio selections have to do with a couple of factors. What cruise rpm do you want at a typical freeway speed, 70-75 mph. 80 will get you in ticket territory for many states. And torque and horse power play a roll as well. Lower gears can be more fun, BUT, if your running a bunch of horses/torque it aint so fun when it's so hard to control wheel spin and the car is more likely to get sideways when you least expect it.

A 2.95 with a 3.9 rear gear would be a disaster for my engine. First would be so low I'd quickly get tired of having to shift so soon (20 mph maybe) around town and would be tempted to just use 2nd to take off. On the drag strip there is no way I could hook that up. I AM running slicks already and can barely control wheel spin outta the hole with my current gearing!

2000-2200 at 80 mph wouldn't work for me. To fast for cruising in Oregon!!! Yet, I HAVE to keep my engine in the 2,000 rpm range minimum or it "bucks" and tends to load up the plugs. So I went with a .82 rather than the .6 over drive.