Rear Gear Ratio Question with C4

[GeotechDuck]

Hi Everyone,
We have a 66 GT350H - original 289 HiPo & C4 transmission.  However, at some point the rear end gears were switched out to 4.11s.  As you can imagine, freeway driving is pretty brutal.  I don't have it up on the freeway that often, and when I do, it is usually only for a few miles.   However, we may start looking to drive it on some longer day trips and want freeway driving to be a little more reasonable. 

Right now I am 3500+ RPMs on the freeway at around 65 mph.  Also, it usually shifts out of 1st almost immediately off the line unless you are really standing on it.   

What are others running with a C4? 3.50, 3.25, 3.00? 

Not tracking the car.  It is a weekend driver, mostly cruising on weekends for coffee and shows.

Looking to be able to cruise at 75mph, but also want to keep some get up and go off the line.  Is 3.25 a good middle ground here?  Calculator shows RPMs will be under 3500 at 75mph, which is good, but not sure how much off the line acceleration I am giving up to get there. 

 

[Bob Gaines]

Hi Everyone,
We have a 66 GT350H - original 289 HiPo & C4 transmission.  However, at some point the rear end gears were switched out to 4.11s.  As you can imagine, freeway driving is pretty brutal.  I don't have it up on the freeway that often, and when I do, it is usually only for a few miles.   However, we may start looking to drive it on some longer day trips and want freeway driving to be a little more reasonable. 

Right now I am 3500+ RPMs on the freeway at around 65 mph.  Also, it usually shifts out of 1st almost immediately off the line unless you are really standing on it.   

What are others running with a C4? 3.50, 3.25, 3.00? 

Not tracking the car.  It is a weekend driver, mostly cruising on weekends for coffee and shows.

Looking to be able to cruise at 75mph, but also want to keep some get up and go off the line.  Is 3.25 a good middle ground here?  Calculator shows RPMs will be under 3500 at 75mph, which is good, but not sure how much off the line acceleration I am giving up to get there. 

 

3:50 was the factory gear for the C4 in your GT350. You will give up a little acceleration if you go to the more rpm friendly 3:25 gears but I doubt given what you described as your driving habits you will notice a difference between the 3:50 and the 3:25 IMO.

[FL SAAC Team Leader]

On the 1966 GTH 3.5 gear was the factory choice for the C4

Would not change it to anything else on ours

Best of luck

[shelbydoug]

There is a "C4" overdrive transission which is essentially a C4 with a 4th gear. Going from memory on this , the overdrive would effectively give you 2.80 rear when in overdrive.

Since that transmission is essentially a bolt in, you can put the original aside and run the od trans.

I'd have to find the gear ratio chart which is not at hand, but if you were considering doing something to make it cruise better, the OD trans should be a serious consideration.

[pbf777]

There is a "C4" overdrive transission which is essentially a C4 with a 4th gear.

Since that transmission is essentially a bolt in, you can put the original aside and run the od trans.

    I'm assuming that this "C4 overdrive transmission" you might be referencing is the "A.O.D.", this as found, in as of others, the FOX Mustangs? 

    If so, just a note: The A.O.D. utilizes the 164 tooth flex-plate (which is of the wrong 50 in. oz. balance application for the "5.0" engine, but the proper 28 in. oz. application is available) and hence the resultant larger area required for this vs. the 157T earlier C4 applications.  Unlike the C4 where the bell housings are a separate component of the assembly, this allowing swapping for different flywheels, the A.O.D.'s is cast a one incorporated with the gear case. 

    I do consider, for those not so concerned with originality, but rather wish to enjoy the vehicle to a greater extent, the conversion to the A.O.D. a good choice; and we have executed many a transformation with no regrets (that I can remember anyway  ) for any of those customers. 

    Scott.

   

[shelbydoug]

When I ran that by Randy, i.e., "how about using a 6R80?", he said he had no experience with them but the AOD would work very well.

I'd need to look up the details of that discussion but my recollection was that it wasn't a difficult switch over.



My initial investigation showed the same regular gears as a C4 and just the forth gear was the overdrive.

In my constant, self annoying, self analyzation, I liked the super low first gear (4.17, 2.34 second, 1.52 third, ) and the double overdrive of the 6R80 (1.14 forth, 0.87 fifth, and 0.69:1 sixth).

The mere fact of it needing a computer sub plug in device was not daunting but the mere size of the tail shaft is, particularly in that few, if any had put one in an "early Mustang" but in thinking of a contingency plan in which an issue arrives by which the left leg no longer is capable of operating a high performance clutch, re-installing an automatic trans could in some way or form be acceptable to me? Thus the AOD OR more desirably, the 6R80.

Being in sixth would make your final drive a 2.14:1. That in fact could be too tall for just a 289? First gear 4.17 would make it pull like it had 6.20 ears in first. You could probably almost pull a wheelie with that?

Us "old folk" need to have a contingency plan other then a new Corvette with an "automatic only" transmission option? 

[68stangcjfb]

Not exactly Apples to Apples comparison as I have a four-speed in my Fairlane station wagon so you have to account for some torque converter slippage and I am running a 28 inch tall rear tire (275/60/15). In 4th gear, 3.50 gears give me 3000 rpm at 70 mph. 3.25 gears gave me 3000 rpm at 75 mph. 4.11 gears gave me 3000 rpm at 55 mph. 3.89 gears gave me 3000 rpm at 60. Mph Figures were gathered using Waze give or take a mile an hour or two. I've tried all these gears and for my car the 3.50s were the best compromise. If your car has lower height tires the 3.25s might make a better choice. The overdrive transmission would be the best choice if you would like to keep the 4.11s but also the most expensive one.

[GeotechDuck]

Thank you all for the quick replies.  I am running a 205/70R14 tire, which I believe the height is just a hair over 25 inches.  I checked it and I should be able to reduce the RPMs by about 500 at 75 mph on the freeway.  If I can get to 3,500 at 75mph, that is probably good enough. 

I actually have a set of 3.50 gears in my garage, so maybe that is a good place to start.

[shelbydoug]

Hi Everyone,
We have a 66 GT350H - original 289 HiPo & C4 transmission.  However, at some point the rear end gears were switched out to 4.11s.  As you can imagine, freeway driving is pretty brutal.  I don't have it up on the freeway that often, and when I do, it is usually only for a few miles.   However, we may start looking to drive it on some longer day trips and want freeway driving to be a little more reasonable. 

Right now I am 3500+ RPMs on the freeway at around 65 mph.  Also, it usually shifts out of 1st almost immediately off the line unless you are really standing on it.   

What are others running with a C4? 3.50, 3.25, 3.00? 

Not tracking the car.  It is a weekend driver, mostly cruising on weekends for coffee and shows.

Looking to be able to cruise at 75mph, but also want to keep some get up and go off the line.  Is 3.25 a good middle ground here?  Calculator shows RPMs will be under 3500 at 75mph, which is good, but not sure how much off the line acceleration I am giving up to get there. 

 

3:50 was the factory gear for the C4 in your GT350. You will give up a little acceleration if you go to the more rpm friendly 3:25 gears but I doubt given what you described as your driving habits you will notice a difference between the 3:50 and the 3:25 IMO.

I agree with Bob in the sense that there isn't a big enough difference between the 325s and the 350s.

To quite down the beast you are looking for a final drive ratio in the 2.50 to 2.80 vicinity which is where the AOD will put you without loosing the first gear pickup.

[NC TRACKRAT]

...OR...You could install the 3.50 rear, then pull out that rare C-4 C-code for safekeeping and install a T5 5-speed. Easy conversion kits out there. That OD 5th gear will give you just what you need on the highway.

[6s1640]

When I drove my GT350 to SAAC 35 in San Rafael California, I pulled the 3.89 and put in 3.00 gears.  It was a 2000 mile round trip.  IIRC, the car would do 75 MPH around 3100 rpm.  It made the trip much more doable.  I had no issues starting from a stand still with the close ration T-10M1.  It also helped on the mileage for such a long trip.  The car got 18 mpg.

Best of luck

Cory

[Bob Gaines]

The 9 inch rear end set up makes it relatively easy to switch out the center section gear setup. I know of numerous people besides myself that have a extra center section setup with higher or lower gears depending on if highway or drags.

[GeotechDuck]

...OR...You could install the 3.50 rear, then pull out that rare C-4 C-code for safekeeping and install a T5 5-speed. Easy conversion kits out there. That OD 5th gear will give you just what you need on the highway.

This is something I have seriously considered in the past, but was always worried about impacting the original car.  From what I have read, I believe this is a very straight forward conversion and can be done without modifying the car (cutting / welding)?  If I could do this and still get it back to original without too much hassle, then maybe it is worth considering?

[GeotechDuck]

When I drove my GT350 to SAAC 35 in San Rafael California, I pulled the 3.89 and put in 3.00 gears.  It was a 2000 mile round trip.  IIRC, the car would do 75 MPH around 3100 rpm.  It made the trip much more doable.  I had no issues starting from a stand still with the close ration T-10M1.  It also helped on the mileage for such a long trip.  The car got 18 mpg.

Best of luck

Cory

Thanks Cory!  I was probably there for that event.  I grew up in San Rafael. 

[GeotechDuck]

The 9 inch rear end set up makes it relatively easy to switch out the center section gear setup. I know of numerous people besides myself that have a extra center section setup with higher or lower gears depending on if highway or drags.

Hi Bob,
Thank you for the response. I have a complete center section with 3.50s sitting in my garage that I picked up a while back.  Would be an easy swap, so I could probably start there and see how things go.