128 Guests, 1 User
dga57

Author Topic: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?  (Read 1767 times)

0 Members and 1 Guest are viewing this topic.

Offline popbumper

  • Pinto Master
  • *****
  • Posts: 1916
  • FeedBack: +99/-1
  • Gender: Male

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Topic Starter Signature Mobile User Poll Voter Windows User 1000 Posts Fifth year Anniversary
I'm continuing work on my '76 MPG wagon. I am uncertain right now whether I have a 2.73 or 3.18 ratio (it IS a 6.75" rear end), and I'm running the 2.3L with a manual transmission.

I have an 8" rear end that I pulled from a sedan with a 2.8L/automatic, and my understanding is that the ratio on these is 3.4.

QUESTION: WHAT EFFECT WILL GOING TO A HIGHER GEAR RATIO HAVE on my driving? This is a STOCK 2.3L (not putting a lot of power on it), I just wanted to restore the 8" rear end then swap it in - it would be a pretty quick job to swap in and would also allow me to update with more power in the future if I needed.

Can someone explain this in simple terms? I understand ratios, I just don't understand if it will IMPROVE or NEGATIVELY affect my driving, etc. THANK YOU. GOOD OR BAD IDEA?

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline russosborne

  • Pinto Master
  • *****
  • Posts: 2320
  • FeedBack: +47/-0
  • Gender: Male
  • Another Pinto Driver

  • Total Badges: 5
    Badges: (View All)
    Poll Voter Mobile User Topic Starter Signature 1000 Posts
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #1 on: October 03, 2015, 10:23:45 PM »
Quicker off the line (acceleration). More fun to drive.

Slightly worse gas mileage. Higher rpm's than before at same mph. This causes a bit more wear and tear on the engine as well.

I believe speedometer and odometer will be off a bit.

Amount of all the above depends on the amount of the actual ratio change.

Good or bad depends on what you want from the car.

Being an 8 inch rear means you can change the ratio to almost anything you want either now or down the road. Not limited to a couple of ratios like the 6.75 is.

Russ
In Glendale, Arizona

RIP Casey, Mallory, Abby, and Sadie. We miss you.

79 Pinto ESS fully caged fun car. In progress. 8inch 4.10 gears. 351C and a T5 waiting to go in.
74 Pinto Wagon.  To be or Not to be a Pinchero, that is the question.

Offline popbumper

  • Pinto Master
  • *****
  • Posts: 1916
  • FeedBack: +99/-1
  • Gender: Male

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Topic Starter Signature Mobile User Poll Voter Windows User 1000 Posts Fifth year Anniversary
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #2 on: October 03, 2015, 10:47:53 PM »
Fantastic Russ. THANK YOU for the QUICK response. I'm gonna move forward with it :).

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline Wittsend

  • Pinto Master
  • *****
  • Posts: 2042
  • FeedBack: +239/-0

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Mobile User Topic Starter Poll Voter 1000 Posts Linux User Windows User Fifth year Anniversary
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #3 on: October 04, 2015, 05:24:12 PM »
One thing to take into consideration with ratios is that the tire size is a factor too.  This is just a rough example, but you can have a 3.18 ratio with stock 13" tires and go to a 3.40 - and a taller tire and have the exact same effect (meaning no change).  You can even, with a very tall tire, end up with an over all ratio closer to 3.00 than the stock 3.18 even though you have installed the 3.40 rearend.  So, if you are not changing the tire size then everything Russ says is accurate. But, if you ever alter the tire size it becomes a factor.

Since you speak of future modification I'll throw out my example. I have a 2.3 Turbo, T-5 transmission and 175-70-13" tires.  When I first swapped in my 8" it had 3.00 gears.  65 MPH on the freeway was 2,200 RPM. While all that sounds great at all the common street speeds (25, 35, 45 MPH) it was very difficult to find the right gear.  By that I mean if I was revving too much an upshift tended to bog the car. It was never an ideal situation.  I eventually got a 3.40 center section and the car is now far more driveable.  It is happy in a certain gear for a certain speed. There is no more unpleasantness either a gear up - or down.  Freeway 65 MPH is still a decent 2,600 RPM.

With the turbo engine I could probably go down to 3.25 and still be content.  On the other hand if I go to taller tires (and eventually I intend to) the 3.40's are likely the better choice.  Some like the 3.55 gears, but for an all around used I'd personally not go that far unless running very tall tires.  For what is available "As/Is" from Ford I either recommend the 3.40 in the 8" on the numerically high end or if you are not building a high HP engine and can live with the 6-3/4" rear then 3.18 on the numerically low end. But, again tire size is a factor so consider it. There are all kinds of calculator available on the internet to see the engine RPM as it relates to the transmission final drive, rear ratio and tire size..

Offline popbumper

  • Pinto Master
  • *****
  • Posts: 1916
  • FeedBack: +99/-1
  • Gender: Male

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Topic Starter Signature Mobile User Poll Voter Windows User 1000 Posts Fifth year Anniversary
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #4 on: October 05, 2015, 02:39:41 PM »
Wittsend:

  What I am going to end up doing is going to a 14" wheel (either a fox body mustang wheel - four bolt pattern) or may even change out the axles to a five bolt pattern (and do the front conversion as well) - and leaves me MANY options.......

With the lack of quality or variety of available 13" tires nowadays, and the fact that a 14" tire/wheel fills the wheel well nicer, it makes more sense all around.

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline Wittsend

  • Pinto Master
  • *****
  • Posts: 2042
  • FeedBack: +239/-0

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Mobile User Topic Starter Poll Voter 1000 Posts Linux User Windows User Fifth year Anniversary
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #5 on: October 05, 2015, 03:02:26 PM »
I think you will be content with the 3.40 gears assuming the 14" tires are taller than the 13".  I have the added torque of the turbo motor, but even when I had the 3.00 gears the problem wasn't so much the acceleration.. ., it was it was never having the right gear for the speed I mentioned. Then again the overdrive of my T-5 helps support my opinion and with a 4 speed I might not like the 3.40 on the highway.   I'd still hold onto the 3.18 as it is close to the 3.25 that I feel is a good all around ratio. From what I was told Ford only used the 3.25 in a limited aspect of both models and years.  I met the guy who was snagging all the 8" rears from the yards and he said it was almost impossible to find a 3.25, 8" today. That isn't saying you can't buy that ratio aftermarket, but then you are paying a new price and likely a professional to set them up and that adds up to about three or four times the cost over a yard pull.

Offline 65ShelbyClone

  • Pinto Sr. Member
  • ****
  • Posts: 671
  • FeedBack: +28/-0

  • Total Badges: 7
    Badges: (View All)
    Fifth year Anniversary Topic Starter Signature Poll Voter Mobile User Linux User Windows User
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #6 on: October 24, 2015, 03:03:14 PM »
I have an 8" rear end that I pulled from a sedan with a 2.8L/automatic, and my understanding is that the ratio on these is 3.4.

Is the tag still on it? If so, the ratio is stamped on that tag. If not, count how many turns of the yoke it takes to make the axle go one revolution.

If you have 2.73s now, 3.40s will increase your cruising RPM by about 25%.
If you have 3.18s now, 3.40s will increase your cruising RPM by about 7%.

I have 185/75 R13 tires with 3.55s and the RPM @ 65mph was about 3200-3300. With an 0.81 overdrive it's slightly over 2600 now.

My plan is to swap in an 8" with 3.25s because that will closely match the gearing+tire combo of the donor car my T5 trans came from. The 8" I have waiting to go in has 3.00 gears that I'll just make do with for a while.
'72 Runabout - 2.3T, T5, MegaSquirt-II, 8", 5-lugs, big brakes.
'68 Mustang - Built roller 302, Toploader, 9", etc.

Offline Wittsend

  • Pinto Master
  • *****
  • Posts: 2042
  • FeedBack: +239/-0

  • Total Badges: 8
    Badges: (View All)
    Tenth year Anniversary Mobile User Topic Starter Poll Voter 1000 Posts Linux User Windows User Fifth year Anniversary
Re: Effect of going from 6.75" rear axle to 8" rear axle with higher ratio?
« Reply #7 on: October 24, 2015, 03:21:09 PM »
Yep, I'm right about at the 2,600 RPM / 65 MPH with my T-5, 3.40 and 175-70-13" too. It also helps to illustrate how rear ratios AND tire size work hand in hand.  Your 185-75 / 3.55's give a similar result to my 175-70 / 3.40's.

  I just got myself a complete set of the Thunderbird "SSP look a like" wheels though they are 14", not 15" like real SSP's. I've got some marginal 215-60-14" tires I might throw on to test with.  This should get me closer to the (effective) 3.25 ratio that I see as well suited to a Turbo Pinto.

Pics: my set ($15 HF parts washer in background), the car that inspired their consideration (and another), close up of wheel (the caps are hard to find, I've heard Escort caps are similar).