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Topic: Question on going from 2.73 to 3.55 (Read 5714 times) previous topic - next topic

Question on going from 2.73 to 3.55

A guy at work is telling me that if I go from the 2.73 rear end to the 3.55 then it will throw off the speedometer and that the transmission won't shift gears at the same speeds that it does with the 2.73.  Is that true?  Will I need to change a lot with the transmission if I go to the 3.55 rear?

87 Cougar XR7

Question on going from 2.73 to 3.55

Reply #1 –
You should only have to change the speedo gear to get it to read right. I'm not into AOD automatics much but I'd think that after the speedo gear change, the trans would shift like it should. I'm quite sure someone with more knowledge with the AODs will chime in.


86' T/C 4.6L DOHC
16' Chebby Cruze 1.4L Turbo
17’ Peterbilt 389 600hp 1850ftlb Trq 18spd

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Question on going from 2.73 to 3.55

Reply #2 –
The new ratio will cause the governer on the output shaft to be spinning faster at any given road speed, since it does it's job by centrifugal force, your shift points WILL be affected, but if you're anything like me you won't mind once you've driven it.  They're generally be a few hundred rpm higher, especially on WOT upshifts. Ever had your foot on the floor and your AOD shifts to 2nd at like 4800rpm?  That will go bye bye.  I've swapped my 2.73 with a 3.73 and if I had to do it again, I'd go 3.55, additionally, I replaced the mechanical internals with those of a 4R70W, giving me the wide-ratio kit you can't get anymore (used the thinnest available forward clutch piston, too so that I could run 2 more steels and clutches) which also got me lighter stronger stamped steel parts to replace all the brittle and heavy cast iron parts, a 2 inch OD band, and more steels and clutches in the direct clutch. I've still got the AOD valve body, but I pulled the lower 1-2 acspoogeulator spring, and enlarged a couple holes in the separator plate. The wide-ratio work brought my upshifts a little higher still. I can hit 2nd at 6500rpm on the WOT upshift, and I chirp the tires almost every time.

Question on going from 2.73 to 3.55

Reply #3 –
The transmission is affected by engine speed....these cars don't care what speed your wheels are turning.

In Short, if you change the drive gear in your trans to match the new ratio, you'll not notice anything...except the easier acceleration.
'84 Mustang
'98 Explorer 5.0
'03 Focus, dropped a valve seat. yay. freakin' split port engines...
'06 Explorer EB 4.6

Question on going from 2.73 to 3.55

Reply #4 –
Accelerating faster than you could before will affect the governor, which will affect the shift point. Try and see. Output shaft speed correlates statically to engine speed, but when you change the ratio of the final drive, you cause the engine and, by proxy, the output shaft to be spinning faster at any given road speed, faster output shaft speed, more centrifugal force acting on the governor, higher shift point. This is a desirable thing anyway, at least with an aod, so it's not a problem.

Question on going from 2.73 to 3.55

Reply #5 –
This could explain why my TV cable would not work. The original vehicle my trans came out of is unknown. I got it out of an 89 mustang that was originaly a standard. Had to use a Locar cable to get the shift points right. My speedo is about 10 miles an hour off.

Question on going from 2.73 to 3.55

Reply #6 –
My crownvic with 3.55's and aod shirts from first at 35mph, with the 2.73's in my bird, it shifted at 42. You will gain a couple hundred rpm's on freeway crusing. Should get better city, worse freeway. Best I can squueze out of my wagon is 22 freeway.
Quote from: jcassity
I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
Hooligans! 
1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
1986 cougar.
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Question on going from 2.73 to 3.55

Reply #7 –
When I went from 2.73 to 3.73 and was still auto, it did not negatively affect the AOD transmission in any way.  As others have stated your RPM's in overdrive will obviously be higher than before.  Your speedo will be off by quite a bit but all that requires is a speedo gear change.

Just swap to a 5 speed and you won't have to worry about it!  ;)
'88 'bird, 10.9:1 306 w/TFS top end, forged rods/pistons, T-5 swap & bunch of other stuff, 1-family owned, had it since ‘98, 5.0tbrd88 on Instagram and YouTube

Question on going from 2.73 to 3.55

Reply #8 –
Quote from: 50tbrd88;399857
When I went from 2.73 to 3.73 and was still auto, it did not negatively affect the AOD transmission in any way.  As others have stated your RPM's in overdrive will obviously be higher than before.  Your speedo will be off by quite a bit but all that requires is a speedo gear change.

Just swap to a 5 speed and you won't have to worry about it!  ;)

What RPM is your 1-2 upshift at WOT?  What was it with your 2.73?

Question on going from 2.73 to 3.55

Reply #9 –
My shift point is where ever I want, lol.  I have a 5 speed now...I honestly can't remember how the shift points were affected on the AOD.

The point I was trying to make with the OP is that if he re-gears there will be no harm done to his AOD.  I drove my car for 7-8 years with 3.73's and the stock 180,000 mile AOD with no problems at all.
'88 'bird, 10.9:1 306 w/TFS top end, forged rods/pistons, T-5 swap & bunch of other stuff, 1-family owned, had it since ‘98, 5.0tbrd88 on Instagram and YouTube

Question on going from 2.73 to 3.55

Reply #10 –
I didn't say there would be problems, just that lowering the final drive would lift the max rpm where the trans will upshift at Wide Open Throttle.  This is a good thing, because stock, most of them WOT upshift at less than 5000 rpm.

Question on going from 2.73 to 3.55

Reply #11 –
I have about the same set up. An 1986 Cougar with a AOD transmission and a 5.0 engine and 2.73 gears. How much will it effect your top end when you went from 2.73 to 3.55? Our freeways are set at 75 mph. Which means that half the traffic goes at 85 mph. I don't have an rpm gauge on my Cougar, so I have no idea what my rpm's are at 85 mph. If I were to go to 3.55 would that change about 5-10 mph?

Question on going from 2.73 to 3.55

Reply #12 –
Quote from: 86cougar;399963
I have about the same set up. An 1986 Cougar with a AOD transmission and a 5.0 engine and 2.73 gears. How much will it effect your top end when you went from 2.73 to 3.55? Our freeways are set at 75 mph. Which means that half the traffic goes at 85 mph. I don't have an rpm gauge on my Cougar, so I have no idea what my rpm's are at 85 mph. If I were to go to 3.55 would that change about 5-10 mph?

This is kind of an interesting question.  The answer may not be as straight forward as you might expect.

It's highly unlikely there would be a difference in "top end" with such a mild gear change with this combination. 

Top end is a function of horse power as well as gearing:

There is a difference between being drag limited (the amount of horsepower is not adequate to overcome aerodynamic drag) and RPM limited (having enough horsepower to over come aerodynamic drag and mechanically top out the engine RPM).

A stock S/O V8 Cougar is horsepower limited on the top end with 2.73 gearing (we're talking a whopping 140 hp here).  This combination will likely reach it's max MPH in 3rd gear (maybe even 2nd, depending on the current power of the engine as well as the AOD's acceleration killing 3rd gear lock up feature, which eliminates the torque multiplication of the converter).

The car may even slow down after pulling to it's top speed and shifting into the next higher gear (either 3rd or O/D) as it may fall so far out of the power band it no longer has enough torque to accelerate the car against the aerodynamic drag. 

The 3.55 gear will increase cruise RPM (there are plenty of calculators on line to get an approximate RPM per mile an hour change), but it won't limit this combinations ability reach it's max MPH or to maintain highway speeds, particularly in OD.  It'll still pull enough MPH to get a healthy ticket on the highway!

A 3.55 gear will provide greater leverage to utilize the torque and HP the engine does have (shifts will occur quicker, putting the engine back in the "meaty" part of the torque band ), which provides quicker acceleration.

Guys have been swapping 3.55 & 3.73 gears into Fox bodied cars for three plus decades as a first modification because it offers two of the biggest clichés in hot rodding in just one step!: "seat of the pants" & "bang for the buck"

Question on going from 2.73 to 3.55

Reply #13 –
The 3.55 and 3.73 is a perfect street gear with an AUTO. Ford thought the same thing as they equipped the 3.55 with a 5 speeder on the Turbo Coupe. And the 3.73 on the auto Turbo Coupe. Hay who can argue with those BLUE OVAL ENGINEERS!!!
I spend money I don't have, To build  cars I don't need, To impress people I don't know

HAVE YOU DRIVEN A FORD LATELY!!

Question on going from 2.73 to 3.55

Reply #14 –
Sounds like 3.73 rear gears w/351 Cleveland, AOD transmission might be a good set up for me. Who knows, I might go with a 5 speed.