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Author Topic: Rear End Swap - Ratio Change Question  (Read 11967 times)

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Offline dave1987

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Rear End Swap - Ratio Change Question
« on: March 20, 2010, 12:39:26 AM »
I picked up a 3.40 ratio 6.75" rear today off craigslist. The car only had 27,061 verified miles on it, and I would like to swap it out for the one I have on my 78 Sedan since the ring gear and pinion on it's stock axle arn't matched up right and resetting it is something I do not have the skills to do right now.


My question is....

How much and what difference will I notice by changing from the stock 2.73 ratio axle to the 3.40 ratio axle?


From my understanding I will take a hit on fuel economy, but gain more drive ability. Am I translating my research wrong?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline 71pintoracer

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Re: Rear End Swap - Ratio Change Question
« Reply #1 on: March 20, 2010, 07:55:33 AM »
That is going to make a HUGE difference, if you have an automatic (with that tall gear you have now I'm guessing auto) that 3:40 will be too low. You'll have graet low end power though!! ;D
If you don't have time to do it right, when will you have time to do it over?

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #2 on: March 20, 2010, 11:24:47 PM »
The 78 has the stock 4 speed in it actually. Here are the gear ratios for it:

1st - 3.98
2nd - 2.14
3rd - 1.42
4th - 1.00
Reverse - 3.99


From your response, I'm guessing that I will be loosing power at higher RPMs (compared to the 2.73 rear I have now), but gain more acceleration? Does this also mean that I will be loosing power while driving up hill and have less of a top speed?

I want to swap the rear ends out because I know mine isn't in the greatest shape, but at the same time I don't want to change my driveabilty so much that the car seems completely different around town, on the freeway or on road tips/driving up mountains.
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline blink77

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Re: Rear End Swap - Ratio Change Question
« Reply #3 on: March 21, 2010, 06:04:22 AM »
Dave
I really think you'll love the change. You'll have more
power at all speeds. You'll have more RPM at the top
end and you'll have to change the speedo gear, probably
by 1 or 2 teeth.     DO IT!!!!!
Bill

Offline 71pintoracer

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Re: Rear End Swap - Ratio Change Question
« Reply #4 on: March 21, 2010, 08:22:38 AM »
Wow, a 2:73 with a 4 speed? Seems like all of the 4 speed cars I've had were 3:18, 3:40 or 3:55. The auto's were 2:73 or 3:00.
I've always run a 3:55. The 3:40 is a good swap. You will be higher up in the rpm range which should be a good thing, get the engine in the power band. These little motors like to rev! You won't be gaining or losing power, but the first time you drive it with the 3:40 you'll think you gained about 50 horses!!  :fastcar:
If you don't have time to do it right, when will you have time to do it over?

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #5 on: March 21, 2010, 06:52:38 PM »
Awesome! I'm down for it! :D

The car is a PONY model, the later version of the MPG cars from what I have heard. Still looking for some PONY decals that I can put on the fenders if anyone knows where to get them!
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline pintowagon77

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Re: Rear End Swap - Ratio Change Question
« Reply #6 on: March 21, 2010, 11:01:23 PM »
I run a 8.8 with 3.89 gears, I also have a t-5 tranny but have done this with a 4 speed, and its a little low but still manageable. It lets you launch the car at red lights and climb hills with consistent power. Dosn't like to do over 75 but its more fun at low speed. Do it!
Contact any time for info or parts.

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #7 on: March 21, 2010, 11:08:08 PM »
Glad to hear such positive things about it! I'm happy I didn't waste my money on it!

My speedometer tops out at 85. The fastest I ever take the car is 75 MPH on I-84 down to Salt Lake City. I have noticed climbing hills is rater treacherous and the car really slows down by the time I get to the top, especially when passing over the Blue's! Should be nicer to drive on road trips now! :)


Now, where do I get a new speedometer gear from?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline TIGGER

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Re: Rear End Swap - Ratio Change Question
« Reply #8 on: March 22, 2010, 12:30:02 AM »
When I fixed my speedo on the crusing wagon a did a search and found a formula that spit out the number of teeth speedo gear needed with my setup.  I don't remember the web site so you will have to google one.  It seemed to work as my speedo reads pretty much dead on.  I have a bunch of spare speedo gears so the swap was easy for me.  I imagine your local Ford dealer should be able to order you what you need.
79 4cyl Wagon
73 Turbo HB
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Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #9 on: March 22, 2010, 06:31:03 PM »
The dash I got when I picked up the rear end came with the speedometer cable still attached. The car was an automatic, no a 4-speed like I have right now.

The gear that went to that transmission was an 18 tooth gear. Does this mean I need to find an 18 tooth gear for the manual transmission to?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Srt

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Re: Rear End Swap - Ratio Change Question
« Reply #10 on: March 22, 2010, 08:34:22 PM »
You're gonna love it. More usable power, better all around and you probably won't lose in the mileage dept either
the only substitute for cubic inches is BOOST!!!

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #11 on: March 23, 2010, 02:46:05 PM »
Some questions on speedometer gears:

1) can I use an automatic transmission speedometer gear with my 4-speed manual transmission, or will I have to get a T-5 speedo gear?

2) how many teeth does the 4-speed manual transmission's speedometer drive gear have? 6 or 7?

3) for calculating the teeth needed on a new speedometer gear, I am saying that I am using 24.7" tire diameter 3.40 axle ration and 6 or 7 drive teeth, is this correct?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #12 on: March 25, 2010, 12:20:04 AM »
I figured everything out with my speedometer gear issue! :D

I pulled the existing gear out of my transmission today to really SEE what it looked like.

The manual transmission gears have a shorter shaft on them than the automatic transmission gears do and will not interchange. They might work the other way around, but not in my situation. Not a problem though!

I went to the Ford Dealership with my gear and showed them what I needed but with 18 and 19 teeth on them (I have 17 teeth on mine). They pulled out a sheet with gear teeth counts and part numbers on it and looked up the numbers to find out what stores had them in stock. I had to drive about five miles out to another dealership out in Nampa to get them, but I have two new gears now! :D

I purchased a 18-tooth gear and a 19-tooth gear, and I will see which one is most accurate.

18R gear - $4.40 - P/N CODD-17271-B
19R gear - $3.37 - P/N CODZ-17271-B

Might have some fun with the new axle starting Sunday! :D


Thanks for all your help guys, hope some of this info provides useful to current and future members!
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Wittsend

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Re: Rear End Swap - Ratio Change Question
« Reply #13 on: March 25, 2010, 11:45:42 AM »
While on the speedo-gear subject, just a word of caution.  I used the C-4 cable gear in my T-5 when I did the swap.  The C-4 gear was longer than the T-5 by a small amount. Unfortunately this caused the cable gear to bind on its end when clamped down.  The end result was when I backed up the gear on the tail shaft "walked" out of place as it "rode" the teeth of the bound cable gear.

  I was about to pull the tailshaft when I noticed a lampstand (those halogen torch lamps that were popular 10 or so years ago) had a pipe that just fit over the tail shaft, but was thin enough to get past the seal.  Anyway, I was able to push the gear back into place with a hammer and the pipe.  So, be careful what cable gear you use!!!

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #14 on: March 25, 2010, 10:59:27 PM »
I will keep that in mind! I will be sure to measure everything before installation!

Is there anything I should know about installation of the replacement axle? Do I need to do anything specific to ensure perfect straightness before securing it to the leaf springs?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Wittsend

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Re: Rear End Swap - Ratio Change Question
« Reply #15 on: March 25, 2010, 11:30:06 PM »
If you have a good plum line and are sure the car won't move you can at least set it to the original location.  Drop the plum line at the center of the axles and put marks on the floor.  Assuming nothing moved check again with the new rear end.

That said, I just bolted mine in. There really isn't much wiggle room for adjustment.  I was able to get mine out/in  (6-3/4" to 8") without dropping the springs.

Make sure your car is well supported and stable.

Tom

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #16 on: March 25, 2010, 11:36:12 PM »
What exactly do you mean by "Drop the plum line at the center of the axles"?
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline hellfirejim

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Re: Rear End Swap - Ratio Change Question
« Reply #17 on: March 26, 2010, 05:28:39 AM »
A plum bob line is a way to mark a vertical center line.  Raise your vehicle as high as you safely can support it without it moving around.  Take off both rear wheels.   Visualy put a piece of tape on the wheel opening above the center of the rear axle. 

Now you make up your 2 plum bob lines.  Take a piece of string and tie a large nut on the end of it.  Do this twice.  Take one of them and hold the string allowing the nut to dangle and pull the string tight.  Move the string around at the fenderwell untill the the string falls on the center line of the existing rear axle.  Tape it to the wheel well and also mark it on the tape already put on the wheel well incase the string comes loose.  Repeat on the other side.

Pull out old rear.  Put in new rear. align using both plum bobs down the center of the new axle.  tighten down and clean up mess.
jim
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Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #18 on: March 26, 2010, 12:52:17 PM »
Thank you Jim! I will be taking a copy of this thread with me to ensure I don't forget anything! :)
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline dave1987

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Re: Rear End Swap - Ratio Change Question
« Reply #19 on: April 01, 2010, 03:24:47 AM »
Got the swap done! :D Found out that the correct gear I needed was a 21 tooth (red, gear, P/N CODD-17271-B). It leaves my speedometer 1-2 mph slower than actual, but I can live without the speeding tickets! :)

You can check out my project log (below) for details about issues I ran into during the swap.

http://www.fordpinto.com/index.php/topic,7356.260.html
1978 Ford Pinto Sedan - Family owned since new

Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!