Correct gear ratio

[67 GT350]

I have a 67 GT 350 with 3:89 gears. Correct for a 4 speed, but my car is a auto with air. Seems crazy on the highway, but I love the around town driving. What would be a nice gear ratio for overall driving? What did the car come with if equipped with auto and air?

[shelbydoug]

The 68 350s with auto and air were 3.50s. The 69s were 3.00 I believe.

I don't remember what the 67s were. I would recommend though that 3.25s would be a nice all round ratio.

[67 GT350]

I don't want to to be a dog though....

[GT350DAVE]

The car came with 350's. It is a nice gear. Tire diameter also enters in the equation. If a lot of highway driving is done, you could consider 3:25's . I run them in my automatic Fairlane but it is a big block with lots of torque.
Dave

[Side-Oilers]

You could try your 3.89s with a Gear Vendors overdrive. That has an 0.78:1 ratio, and makes a big difference in cruising rpms (a 22% drop.)

I have that setup in my 4-speed KR (3.50 rear) and love it on the highway.  But you still keep the regular non-overdrive performance everywhere else.

Pretty straightforward to install. You will need a shorter driveshaft, though.

[greekz]

I am running stock 3.50's in my '67 GT-350.  It has plenty of pull from a dead stop and nice cruising on the freeway.  Very satisfied for the type of driving I do.

[Brandon]

You could try your 3.89s with a Gear Vendors overdrive. That has an 0.78:1 ratio, and makes a big difference in cruising rpms (a 22% drop.)

I have that setup in my 4-speed KR (3.50 rear) and love it on the highway.  But you still keep the regular non-overdrive performance everywhere else.

Pretty straightforward to install. You will need a shorter driveshaft, though.

Did you shorten your original driveshaft or get a new one made?  How do you control it with a stick?

[shelbydoug]

If you are looking for alternatives using a Ford AOD transmission you could leave the rear alone and get a 30% overdrive that way. That would give you a 2.7 final drive in fourth and still have a good first gear pull since that wouldn't change or not much as well as second and third.

You would be going from a three speed automatic to a four speed automatic.

There is a difference in the look of the case. it is one piece like an FMX is, not a separate bell like the C4 is but other then that it is a clean swap.



I was actually just looking at the Ford 6 speed automatic. it has a 4.16 first gear and 5th and 6th are overdrives. The issue is that the drive shaft tunnels in the early cars narrow down too quickly. The six speed is a fatter transmission and you would have to alter the tunnel.

They virtually bolt into a Fox body with a couple of spots you have to beat with a lumping hammer, but just a little, not a lot.



I've seen two 68's done with them and performance wise they are much faster then a manual.

I think it was Mustangs Monthly did a test at the dragstrip and they couldn't match the manual to the automatic's time by over a half a second?

They give the car a crazy sound and the transmission shifts faster then you can even think of downshifting a manual. The two cars will just spin the tires at will. The shift points are programmable with an inexpensive aftermarket add on mini-computer. It's a VERY trick set up.

IF I was building a Street Rod that is definitely the way I would go since the entire transmission in the door is under $1400...but, that's not what you asked about. Just a thought.

Good for us "older" guys that drag our left legs around now.

[67 GT350]

A while back I did the AOD trick to a 66 and it was OK. My 67 has a nice built auto and with the 389's it is great out of the hole and grabs the gears perfect, the only thing is the highway, (65 and sometimes 70) WOW that is revving! I thought of a AOD, but I think I rather change the gears. So now that I think the stock gears are 3:50's I will go with that, just a thought though, it does not seem to me like 389's to 350's are a big difference.

[Shelby_r_b]

My 67 GT350 has AC and is an auto, too. I love the stock setup. I can can still spin the tires in first and chirp going into second and third. And, truly, being able to cruise at a higher speed with low RPM is a blessing!

I'd definitely go with the 3.50s, IMO.

[shelbydoug]

A while back I did the AOD trick to a 66 and it was OK. My 67 has a nice built auto and with the 389's it is great out of the hole and grabs the gears perfect, the only thing is the highway, (65 and sometimes 70) WOW that is revving! I thought of a AOD, but I think I rather change the gears. So now that I think the stock gears are 3:50's I will go with that, just a thought though, it does not seem to me like 389's to 350's are a big difference.

They don't. My observation is that "most" folks are looking for 3.00:1's. Liska is down to 2.50's in his 69 Convertible. He was mumbling something about 1,800 rpm at 70mph or something?

The Lincoln Versalles were using 2.75's with the AOD and a 302.

Your delema is really that you need more forward gears. If you saw the 68s converted to the 6 speeds it would change your thinking. All of a sudden you get 20 mpg and run 12 second quarters. It's an epiphany. Virtually life changing and will haunt you.

[427heaven]

The key here is to find your tolerance for RPMS or lack of it. My small block auto cars seem to fit the 3.25 3.50 ratio for that perfect sweet spot. If you want more cruisability the 3.25 is perfect, more pep around town the 3.50 will fill that void. Good Luck!

[JD]

The key here is to find your tolerance for RPMS or lack of it. My small block auto cars seem to fit the 3.25 3.50 ratio for that perfect sweet spot. If you want more cruisability the 3.25 is perfect, more pep around town the 3.50 will fill that void. Good Luck!

+1

[Side-Oilers]

You could try your 3.89s with a Gear Vendors overdrive. That has an 0.78:1 ratio, and makes a big difference in cruising rpms (a 22% drop.)

I have that setup in my 4-speed KR (3.50 rear) and love it on the highway.  But you still keep the regular non-overdrive performance everywhere else.

Pretty straightforward to install. You will need a shorter driveshaft, though.

Did you shorten your original driveshaft or get a new one made?  How do you control it with a stick?

You could do either, but I had a new one made. 

I have a Hurst t-handle shifter, with the button on the t-handle.  I put the control box in the glove compartment. It has red and green lights to indicate whether you are in O/D or not.  When up to the mph that I want O/D, I push in the clutch, push the button on the t-handle, let out the clutch and you're in O/D.  Simple.  There's also ways to split gears, 1st low, 1st high, and so on, but I never do that.  I just wanted reduced revs at highway speeds. Works great for me.  Check out their website for all the info.

Here's tach and speedo photos of my car in regular 4th and 4th O/D, at 75 mph.

[OldGuy]

A while back I did the AOD trick to a 66 and it was OK. My 67 has a nice built auto and with the 389's it is great out of the hole and grabs the gears perfect, the only thing is the highway, (65 and sometimes 70) WOW that is revving! I thought of a AOD, but I think I rather change the gears. So now that I think the stock gears are 3:50's I will go with that, just a thought though, it does not seem to me like 389's to 350's are a big difference.

You are correct in that the difference between the 3.89 and the 3.50 is essentially 10% in reduced engine RPM, i.e.. "not much". Another way of saying this is to say that if you were to replace your 3.89's with 3.50's your "new" engine RPM for what used to be 3000 would now be 2700. Its difficult to discern the difference in both the sound of the engine, overall drivability, and/or increased fuel milage.

As what was previously stated, 3.00 is a more ideal highway gear.

Frank