Rear end

[unfrozen1969 11]

If I am thinking of putting a 351 or a 390 in my convertible, what rear end should I look at. What difference do the ratios mean. This is going to be a nice cruiser, not a racer, but I want some umph on th eroad.

 

9" Pose Rear Ends: 2.75:1(ratio), 3.00:1, 3.25:1, 3.50:1,

3.7:1, 3.9:1 & 4.11:1

Bill from Ottawa

[Grabber70Mach 107]

Try this out I think it will answer most of your questions.

 

http://www.differentials.com/faq.html

[jmlay 80]

Allot really depends on what you are planning for the car. Is it going to be a cruiser, drag car, etc... 3.50:1 & 3.7:1 are a good compromise between mileage & performance. Also your trans gearing & tire size will play a part.

 

Mike

[unfrozen1969 11]

I never realized that one thing affected the other so much. As for wheel size, I have no idea what tire size I would go with. I was anticipating using Magnum 500's, so I would imagine that I would be somewhat limited. The car is a convertible that used to have a 6 cylinder. I know where I can get a 390 block, but I have a 351. It maybe more affordable and easier to use the 351. As for a transmission, the car I am parting out has one, what it is I am not sure. I also have a coupe with an original 302 with a C-4 Cruise-O-Matic. It also has a 2.79 standard axle ratio if that helps. The 302 has not been turned for 12-15 years, where the 351 appears to be fresher.

 

I don't think that I have a 9 inch rear end, does tghis make sense if the ratio is 2.79. Learning from the link I was given;

 

"With a lower gear ratio the drive shaft (and thus the engine) turns more for each revolution of the wheel, delivering more power and torque to the wheel for any given speed. Lower ratios are generally desirable when going off-road. Higher ratios are better for freeway driving since they run at lower RPM’s and offer better fuel economy."

 

Bill from Ottawa

[R4K 10]

Just to make sure you're clear, a 2.79 is a highway gear.

 

This means that the engine has to turn the driveshaft 2.79 times to get the rear wheel to rotate once.

 

Let's say that you run 70 MPH in your 2.79, and 3,000 RPMs is needed to sustain that.

 

With a 3.50 rear, those 3,000 RPMs would only put you at 56 MPH, because your engine has to turn the driveshaft more times to achieve a complete revolution of the rear wheel. You'd actually have to run the engine at ~3,750 RPMs to achieve 70 MPH.

 

I just had a 2.79 in my Gal, and swapped it for a 3.25. The off-the-line is much better now, highway isn't bad at all, and both will be better when I get a the engine hopped up and a nice quiet exhaust on it.

[bnickel 10,004]

i have a 3.25 gear in my car as wel and plan on putting a 3.25 in the cougar too. i like the 3.25 for a good all around gear in a cruiser.

[6T9Mach1 12]

The 3.25's would be my suggestion for a cruiser.

[Pakrat 1,043]

Transmission will make a big difference too in your gear choice. I currently have 2:79 with my 3spd but when I go to a 5spd I will likely step up to 3:50 at least since I will now have overdrive for the highway.

[unfrozen1969 11]

What automatic transmission do you recommend if I have a 3:25 gear. Also is there anything that needs to be done to the tranmission if I am running a 3:25 gear ratio.

 

Bill

[bnickel 10,004]

What automatic transmission do you recommend if I have a 3:25 gear. Also is there anything that needs to be done to the tranmission if I am running a 3:25 gear ratio.

 

Bill

 

 

any auto trans will work fine for the 3.25, it can be a regular 3 speed auto and it will also wrok well with an AOD trans as well. nothing needs to be done to the trans but you may need a new speedometer gear depending on what gear the car originally had and what size tire you plan on running

[d.reese 13]

I got an fmx auto, tried the 3:50 possie and yanked it after a week. Thought I was really twisting the motor for a crusier. Put the stock 2:75 and highway no problems, more to what I was looking for! One thing for sure the 3:50 put some teeth in her!