Which rear gears?

[Gud T.B. Blown]

Here is what I have:

'73 Mach 1
351C with 4V CC heads
Blue Thunder intake
Holley 700 dp
Moderate cam, exact grind still unkown.
Hooker Long tubes with 2.5" pipe and x-pipe
Tremec TKO with .82 OD
275/60R 15 Rear tires

This is strictly a street car that I will take on occasion Highway trips of up to 300 miles. The two choices for rear gears that I'm looking at are 3.55 and 3.70. Any thoughts or recommendations?

 

[SL 1993 R]

I liked the 3.91's I had in my 71 Boss, nice response getting the big car moving.. but if I were making 300 mile trips regularly, the 3.55's would be my choice... If you can afford the hit on the gas mileage, go for the higher numbered gear..

 

[ratio411]

With your OD, 3.50 gears = 2.87 final drive and 3.70 gears = 3.04 final.

Having used 3.00 gears for a long time, the 3.04 would be liveable on a long trip, but the 2.87 would be a bit nicer...

I'd have to say use the 3.50s for comfort.
However, it is not a huge difference and the 3.70s would run real nice with your combo.
If you didn't travel, I'd say 3.89/3.91 all the way!
That will give you a 3.20 final drive, which is not bad at all. I have driven 10 hours one way with 3.25s without bother. But gas was cheaper then!

Fwiw:
You have some tall tires there. I base my experience on 26" OD tires!
That 2" difference is pretty huge when applied to gearing.
Also, you want to be able to move that 71, it is a little bigger and heavier.

 

[Gud T.B. Blown]

I liked the 3.91's I had in my 71 Boss, nice response getting the big car moving.. but if I were making 300 mile trips regularly, the 3.55's would be my choice... If you can afford the hit on the gas mileage, go for the higher numbered gear..

I'm not so much worried about the gas mileage as the noise in the car. According the speed analyzer on the Keisler site 70 MPH with a 3.89 is at 2700RPM vs. 2400RPM for a 3.5. Now if I want to go to 75MPH the RPM's go up to 2900RPM with the 3.89 gears.
I'm putting dynamat in the doors and on the floors, how much driveline noise can I expect that to reduce?

 

[ratio411]

If you are worried about noise, some important things are making sure the exhaust exits as far back as possible, using a tamer muffler (like a Turbo style), and then the insulation helps...
If the gears are set up proper, they will add no noise.
It's the exhaust drone that will make or break your noise situation.

 

[Gud T.B. Blown]

If you are worried about noise, some important things are making sure the exhaust exits as far back as possible, using a tamer muffler (like a Turbo style), and then the insulation helps...
If the gears are set up proper, they will add no noise.
It's the exhaust drone that will make or break your noise situation.

The drone is the worry.
I don't expect the gears to make any noise because I am going to spend the money to have somebody install them that knows what they are doing. The mufflers are dynomax and I chose them because they made less noise and the exits are at the rear.
I wish there was a way to install them and drive the car around to see the difference without having to pay for it.

Right now looking at the gearing calculator I'm thinking the 3.7 gears.

 

[68RCodeConv]

I've had both 3.89 and 3.50 rear gears behind the 4V Cleveland in my 73 vert. And I've had 3.91 gears on my Boss 351.

Get the 3.70. With the .82 OD they'll be fine on the highway.

And you never said what your first gear ratio is. Makes a big difference when choosing rear end gears.

 

[ratio411]

And you never said what your first gear ratio is. Makes a big difference when choosing rear end gears.

What he said...
And 1st gear also makes a huge difference on the strength of the tranny.
A granny 1st gear mixed with a lower rear gear can strain (and/or break) a tranny.

 

[Gud T.B. Blown]

What he said...
And 1st gear also makes a huge difference on the strength of the tranny.
A granny 1st gear mixed with a lower rear gear can strain (and/or break) a tranny.

The tranny is a TKO 600 and first gear is 2.87:1.

 

[rbohm]

given the OD ratio, and the first gear ratio, either gear will work just fine. i wouldnt go more than the 3.70 gear though.