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  •  » FYI Ford, Classic Mustang Tech Discussion
  •  » 66 289 With. C4 Auto Best Rear End Ratio?

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    9/22/2013 10:05 PM  #1

    Fastbackbhoy
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    From: St. Catharines Ontario Canada
    Registered: 4/01/2013
    Posts: 156

    66 289 With. C4 Auto Best Rear End Ratio?

    Here's my Mild Build

    C4 Auto Transmission
    66 orig 289 bored to 30"over
    Flat Top Pistons
    Comp cams #21-432-8 .512 Lift
    Alum Heads
    1.6roller Rockers
    7121 Edelbrock Intake manifold
    Summit racing S.S Carb
    Headman Elite Long Tube headers,Flowmasters  40s.
    MSD Ign

    Orig Rear end from Build sheet  2.80.1  ( May have been changed No Tag on Rear Diff)

    I plan on checking out what my rear diff ratio is over the winter and rebuilding completely.

    What Rear Axle Ratio would work best with my Engine Trans Combo.
    Bearing in Mind this Car is Just a Cruizer for around the City and the Odd Hwy trip .

    Jimmy O

    Just looking for suggestions,and or what works for you Guys

    Last edited by Fastbackbhoy (9/23/2013 5:46 PM)

     

    9/23/2013 3:55 AM  #2

    cougrnut
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    From: hershey
    Registered: 4/03/2013
    Posts: 194

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    If you are gonna do any combination of city and freeway driving, 3.25:1 is a good all around gear. I wouldn't go any lower than 3.50 without overdrive, or it'll drive you crazy on freeway.  If yours is an 8" rear, sometimes you can find a 3.33:1 and 3.40:1 gears. I had those in our 65 mustangs and they work great, but hard to find, as they are Chevy/mopar ratios.  Not sure if any of the aftermarket guys offer these for a ford, but worth a look.

     

    9/23/2013 5:33 AM  #3

    Novi Rick
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    From: Novi, Michigan
    Registered: 4/01/2013
    Posts: 148

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    I would first determine the long range plan.  If you are fairly certain that you are going to change to a 5 speed manual or a AOD at some point down the road, I would build it to suit those transmissions.

     

    9/23/2013 6:39 AM  #4

    MS
    Forum Originator

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    From: Canyon Lake, Texas
    Registered: 1/24/2013
    Posts: 12,082

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    Like cougrnut stated, if you go lower than a 3.25 ratio, the car will be really revving going down the freeway at 60 MPH.  I used to have 3.25 with a C-4 and really hated it.

    Bottom line:  What is the intended use for the car?  You cannot have a race car that works well in street trim, and you cannot have a street car that works well in race trim.  Pick one, or prepare for alot of compromise in desired performance, whether that be cruising in comfort or First On Race Day.

    Money you enjoy wasting is NOT wasted money... unless your wife finds out.
     

    9/23/2013 6:44 AM  #5

    Don
    Senior 24-7 FYI Addict

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    From: Coppell, Texas
    Registered: 4/02/2013
    Posts: 1,218

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    Fastbackbhoy wrote:

    Here's my Mild Build

    C4 Auto Transmission
    66 orig 289 bored to 30"over
    Flat Top Pistons
    Comp cams #21-432-8 .512 Lift
    Alum Heads
    1.6roller Rockers
    7121 Edelbrock Intake manifold
    Summit racing S.S Carb
    Headman Elite Long Tube headers,Flowmasters 40s.
    MSD Ign

    Orig Rear end from Build sheet 2.80.1 ( May have been changed No Tag on Rear Diff)

    I plan on checking out what my rear diff ratio is over the winter and rebuilding completely.

    What Rear Axle Ratio would work best with my Engine Trans Combo.

    Jimmy O

    Just looking for suggestions,and or what works for you Guys

    A trans update to a AOD or 5speed might be the first issue.  I went thru similar updates and had a 3.0 rear gear.  After all my updates of: new engine, ac, ps, rear change to 3.55 and 5 speed. the one that completly changed the atitude of the car was the over drive tranny.    (my car is also a emberglo gt fastback)  
     

     

    9/23/2013 7:44 AM  #6

    Rudi
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    From: Stuck in the 50's Welland Ca
    Registered: 4/01/2013
    Posts: 3,920

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    Jimmy, use this chart to see what kind of cruise rpms you can live with on the C-4
    http://www.summitracing.com/expertadviceandnews/chartsandguides/rpm-vs-gear-ratio-and-tire-height

    Knowing you I think you will more than likely go to an O/D tranny somewhere down the road, it is the answer for performance and gas mileage.
    I'm running 3.25 gears with my AOD and seeing 22-24 MP/USG with my 331.  My set up gets me 1950 RPM at 70MPH, very nice for highway cruising.
    I could have lived with 3,5 gears but I am more interested in the fuel saving over a bit of extra low end perfrormance.

    Good work ain't cheap, Cheap work ain't good!   Simple Man
     

    9/23/2013 10:55 AM  #7

    Jeff in GA
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    From: San Luis Obispo, CA
    Registered: 4/01/2013
    Posts: 209

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    My brother's '66 vert has a warmed over '85 5.0L with the C-4 and the 3.00:1 gears are about perfect, all things considered. I have 3.25:1 in my car with a T-5 and I'm into 5th gear without much effort. I had 3.50:1 with a different Mustang and a Toploader and it got a bit tiring.

    1965 Mustang 2+2 EFI
     

    9/23/2013 2:47 PM  #8

    6sally6
    Hyper FYIFORD ADDICT

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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,687

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    Rudi wrote:

    Jimmy, use this chart to see what kind of cruise rpms you can live with on the C-4
    http://www.summitracing.com/expertadviceandnews/chartsandguides/rpm-vs-gear-ratio-and-tire-height

    Knowing you I think you will more than likely go to an O/D tranny somewhere down the road, it is the answer for performance and gas mileage.
    I'm running 3.25 gears with my AOD and seeing 22-24 MP/USG with my 331. My set up gets me 1950 RPM at 70MPH, very nice for highway cruising.
    I could have lived with 3,5 gears but I am more interested in the fuel saving over a bit of extra low end perfrormance.

    X-2...I'm with you Rudi.......3&1/4 would suit me just fine.
    6s6
     

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    10/01/2013 12:53 AM  #9

    Jon Richard
    FYI Addict

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    Registered: 8/23/2013
    Posts: 199

    Re: 66 289 With. C4 Auto Best Rear End Ratio?

    According to Mustang Steves FAQ section on rear ratio selection you need at least a 9:1 torque multiplication ratio for a good street performer and perhaps should take this into consideration as well as highway driving.

    A stock C-4 first gear is 2.46:1. multiply the first gear ratio by the rear end's ring & pinion ratio to arrive at the TQ X ratio

    I went with the Broader Performance wide ratio gearset for my C-4 2.92:1 first/ 1.68:1 second/ 1:1 third and a 2.79:1 rear. Still putting the car together so I can't give a proper testimony but it seems about the best option for running the C-4 on the street.

    http://www.broaderperformance.com/ford-c4/46-c4-wide-ratio-gear-kit.html
     

     

     

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