Want lower rpm at freeway speed
 

My '65 Valiant Droptop with 273 V8 looks and runs great, but the auto tranny drops into third gear about 30 mph, and then by sixty it's really winding out. The car has headers and dual pipes, and I wonder if "performance" gears were put into the differential for low-end torque. Or is this typical of the OEM rear end. Whatever ... can I engage a mechanic to change the diff-gears ... and if so ... to what? Obviously I know next to nothing (except how I wish the car to drive), and I am grateful for any help.

2 -

Chock the front wheels, jack the rear of the car up, so the wheels are off the ground.
With the car in neutral and the parking break off, rotate one of the tires 360* and see how many time the drive shaft rotates. (Might help to mark the tire and drive shaft to better see the rotation. Pieces of masking tape work.)

BTW are you reading a tachometer or just hearing the engine "winding out".

The gears are change-able, but you or the mechanic will need to know the size of the diff housing.

Archer

here is my idea.
1. which rear end do you have? if you have a 7 1/4 change it for a 8 1/4. cause if you dont do it now you will soon. here is very good spotters guide. http://www.crankshaftcoalition.com/w...er's_guide
2. if you are not racing you can go to a 3.23 gear or 3.01 or 2.73. any lower number with that engine your power will not be good.
3. changing the gear in the housing is for experts ONLY.

As was said above.......

1.....Google for an online gear / speed / RPM calculator, and get your rear tire size

2.....Take your GPS, or borrow one, and make sure the speedo is correct

3....Borrow a tach and see what engine RPM is VS speed

4....Archer's destructions were incomplete. Determining rear gear ratio depends on whether you have a "non" limited slip rear end, sometimes called "open" or "single track" or whether you have a limited slip, often called "Sure Grip" in Mopar parlance

A.........Chances are you have a NON limited slip. Jack up only ONE rear tire, blocking at least one other tire securely. Put the trans in neutral, parking brake off. See if you can turn the tire. If so, you do in fact NOT have a limited slip. Watch the driveshaft and turn the tire TWO (not one) full turns. Count driveshaft turns. If you have say, a 3.23, the shaft will turn just under 3 1/4 turns. You can easily estimate that.

B.........If you can NOT turn your one jacked up tire, you have some form of limited slip. Or, you have not released the brake, or gotten the trans into neutral. After checking, jack up the second rear tire. Now turn one of them and watch.........the other side should follow exactly in the same direction. This time, turn one tire only ONE turn while counting driveshaft turns.

C........You can look on the rear for a tag, but frankly as old as these girls are, I would not go by that. The gears could have been changed over ? how many ? years.

Your replies contain a wealth of sage advice. I will print them and begin to work through the process, then report back what I have found. I'm also thinking that my first project ought to be front wheel conversion to disc brakes. Safety first?

"A mind shaped to a new dimension can never return to it's original shape."
(G.B. Shaw ..?)

Depends You may have a 7 1/4 or an 8 3/4. Hopefully an 8 3/4 which makes changing the gears very easy... Look for something like a 2.91 or 3.23. Also you probably have 13" wheels So a change to 14" wheels will help some