Best Ratio/Rear for an Auto-6

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Best Ratio/Rear for an Auto-6

January 09, 2009, 10:00:35 AM

Gonna start hitting up the junkyard again within in the next couple months~and i have a large transport vehicle now, So want to keep an eye out for a nice "upgrade" rear axle swap, As it would only cost about $80.00 at this yard.

I'm thinking 8.8 3.73 Trak loc, But is this just too much on an Auto/3.8?
4.10 seems like overkill.

Or maybe im just better off just keeping the 7.5 and changing the ratio...
Whats the better performance choice here while not going overkill for a street car?

What do you guys think?

**Currently have stock 7.5/2.73 standard

Best Ratio/Rear for an Auto-6

Reply #1 – January 09, 2009, 10:13:42 AM

All things being equal, if you could find a locking center section for your 7.5" rear and simply change out the gears, then that would be your most economical swap for several reasons:

- Not having to remove the rear end.
- Not having to mess with a brake swap (you have unique rear drums).
- Lighter weight vs. an 8.8" rear.
- You're not putting out that much power to the rear end, so a 7.5" is going to be plenty strong enough.

The base curb weight for an '84 V6 was right under 3100 lbs. It's a very lightweight car compared to later versions. So the 7.5" plan makes the most sense on paper.

Far as gears go, a 3.45 gear will still give you pep while saving a little on fuel. It's a relatively odd gear ratio that is surprisingly easy to find. However, if you want the most performance and the fuel economy, 3.73's are perfect.

Now if you can get an entire rear end for $80...that's a different story. It may just be worth the hassle to swap in something else. Are you planning to keep the car 4-lug or are you entertaining thoughts about upgrading to 5-lug? I suppose that's the big question...from there you can make the plans.

Best Ratio/Rear for an Auto-6

Reply #2 – January 09, 2009, 10:38:05 AM

we did the swap from 2.75 to 3.73 on a 5.0/auto over drive and we gain 3 miles per gallon on the high way. thats because the 3.73 let the tranny stay in over-drive.(which also cause less wear on the tranny). where the 2.75 was allwise switching in and out of over drive.

go with the 3.73.

Best Ratio/Rear for an Auto-6

Reply #3 – January 09, 2009, 12:50:12 PM

That works good with an AOD...but he has a C5 3-speed auto. His torque converter, if it's working properly, should lock up in third gear and stay that way even with the stock 2.73 gears. Mine does in my '84 V6/C5. So he can step up to 3.45 or 3.73 gears easily, it's just a matter of fuel economy needs at that point. Without an OD band to make up for the fuel economy, he has a tougher decision to make for gear selection.

Best Ratio/Rear for an Auto-6

Reply #4 – January 09, 2009, 04:10:28 PM

With the C5 I'd personally go with the 3.45's or 3.27's....
I'm running 3.45's with my AOD currently FWIW.

Best Ratio/Rear for an Auto-6

Reply #5 – January 09, 2009, 06:58:32 PM

With overdrive, a 4.56 rear gear is nearly same as 3.08s without overdrive... This is assuming both are lockup transmissions...

The eight passenger Areostars have 7.5" rears with either 3.73 or 4.10 gears...

Best Ratio/Rear for an Auto-6

Reply #6 – January 10, 2009, 03:20:49 PM

the gear from a aerostar fit prefect in cougars and thunderbird 7.5 rear ends.

Best Ratio/Rear for an Auto-6

Reply #7 – January 11, 2009, 02:48:05 AM

3.73 gears and higher are all traction lock and require a traction lock rear end.

Best Ratio/Rear for an Auto-6

Reply #8 – January 11, 2009, 11:06:10 PM

No they don't my dad's 90 2.3 Mustang convertible has open 3.73s, and my 93 2.3 Mustang convertible had open 3.73s. It's code 6. It doesn't matter though since he should put it in a trac-lok since he'll have it apart anyways. :hick:

But without o/d I wouldn't go over 3.27s if you do any interstate driving.

Best Ratio/Rear for an Auto-6

Reply #9 – January 11, 2009, 11:22:49 PM

Quote from: 5.0 bird;251309

No they don't my dad's 90 2.3 Mustang convertible has open 3.73s, and my 93 2.3 Mustang convertible had open 3.73s. It's code 6. It doesn't matter though since he should put it in a trac-lok since he'll have it apart anyways. :hick:

I'm sure that is correct, had a '89 Stang Hatch that was 2.3 & A4LD, and had the peg leg 3.73... Sold that rear to a guy who wanted it for a SVO...

I'll have to double check, but I'm 99% sure the 4.10 in the '93 Aerostar I have is a open diff...

Best Ratio/Rear for an Auto-6

Reply #10 – January 12, 2009, 01:33:05 AM

hmm.. my bad. I got one out of a turbo diesel rear, and its a locker...

Best Ratio/Rear for an Auto-6

Reply #11 – January 12, 2009, 03:29:20 PM

yeah the some of the aerostar van are locking. the explorers with the 7.5 are locking some have 3:45-4:10 depending on the v6.

Best Ratio/Rear for an Auto-6

Reply #12 – January 14, 2009, 03:04:57 PM

My moms '89 XLT Aerostar has a 3:45 standard 7.5 as i recall.

How much does an 8.8 weigh versus a 7.5?

And thanks for all the info guys.
Im leaning towards a locking 3.73, Now the big decision is whether to stay 7.5 or just go 8.8.

At some point i'll be putting 5 lug and disc brake rears as well.
And im sure i'll get a C4 when the opportunity arises.

Will i be needing to change any speedo cables/gears to accomodate the ratio change?

Best Ratio/Rear for an Auto-6

Reply #13 – January 14, 2009, 03:19:17 PM

yes. you will have to match the speedo gear to the gear set you install.

i had 3.73 and c-4 in my 86. ran great around town.

if your staying with aV-6 there no need to go to 8.8.

Best Ratio/Rear for an Auto-6

Reply #14 – January 14, 2009, 07:51:51 PM

...and they're much heavier. There has been some argument as to exactly how much heavier, and I don't know the exact amount, but I do know I could easily pick the 7.5 I took out of my car up and carry it around the yard, but I could hardly lift the 8.8 (much less carry it about)