With a 292 Y-Block connected to a stock 3 speed overdrive transmission, what difference in performance, if any, can I expect between 4:10 rear end and  4:27 rear end gear ratios.  I can go either way so I am curious as to which would be my best choice.  Thanks, Alan

The answer depends on what has been done to the engine such as camshaft, valve springs, intake manifold, carburetion, distributor, headers, heads, valve sizes, tire sizes, vehicle weight, and intended usage.  For street, you certainly don't need 4.27 gears for normal driving, and unless you have a good valvetrain, you won't be able to "wind it out" very high.  The difference in those two gears in minimal at best.  For the street always err on the side of 3.50-3.91 for pep and fair mileage.  Joe-JDC

JDC

Having run both 4:11 and 4:27 gears in my birds, I agree with Joe. Either way, in 1st and 2nd you’re shifting fairly soon if you’re not going over 4000 rpm or so. I haven’t done the math, but it’s probably less than the difference in 15” and 14” wheels on a 55-56 or 57 bird. But with a good cam, etc. those 4:27’s are really a kick in the a- - when you’re in it and headed for 6000 rpm or so.

If it’s applicable for you, the 3.7 down and the 3.9 (IIRC) up ratios use a different carrier in the Dana. So you kind of lock yourself in if you’re thinking of adding a limited slip.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

My 55 Tbird has a 4.10 gear with a stock manual overdrive transmission and 15" radial tires. My O/D works and I have experimented with driving the car with and without O/D.  When driving around town with the O/D engaged, I find the 4.10 gears a little high as I rarely get into third gear.  Most of my around town driving is done in 2nd gear O/D. I also never rev my engine over 5,500 rpm.   If I had 4.27 gears, I would get into third more often. It's at highway speeds that the O/D is most valuable. If you don't drive much at highway speeds, I would consider the 4.27.

If I decide to go with the 4:27 does anyone have any idea how many RPM I would be turning with the OD engaged at freeway speeds?  (i.e.65-70 MPH)  Would be RPM be higher or lower at freeway speeds with the 4:10 vs. 4:27?

The RPM will be lower with the 4.10 but we're only talking about a difference of about 90rpm at cruise speed.  It won't make any practical difference unless perhaps the engine is built such that that 90rpm puts the operating speed in a better place.  For the average street engine, you probably won't notice any difference between the two.


Lawrenceville, GA

I have the 4.27 rear in my truck, I like it. Changed it from the 3.92. I have overdrive that’s not working right now, need to drop the tranny. Where I live, roads are twisty turny so I’m not worried about top end. Response is good when I step on it.

At 60 mph I’m turning around 2700 rpm.

  Dennis in Lititz PA

4.10x / 4.27x = .960187% difference.  That is an indiscernible difference to most folks.  I have 3.92 gears behind my Y, and I wish they were at least 3.73 or 3.55.  When you are talking rpm difference at 5500 with 4.1 gears, you have 5728 rpm with 4.27 gears at same speed.   Divide 5500 by 5728, and you get .96% again.   Joe-JDC 

JDC

I really need to get the overdrive working again. The output shaft bearing is loose and needs replacing. I had to replace the input shaft bearing and didn’t realize the output shaft bearing needed replacing until reassembly.



  Dennis in Lititz PA

Either you have very tall tires or your speedometer is off. Without overdrive/w 3.92 gears, you should be running more than 2700 rpm @ 60 mph.