1989-1993 Supercoupes with AOD: What rpm do you hit when in overdrive at 55mph? - TCCoA Forums
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post #1 of 25 (permalink) Old 07-16-2019, 03:20 PM Thread Starter
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1989-1993 Supercoupes with AOD: What rpm do you hit when in overdrive at 55mph?

I'm curious to see what rpm is to compare with the 2.73 and 3.08
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post #2 of 25 (permalink) Old 07-16-2019, 04:32 PM
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All AOD equipped SC's had 3.27 gears and 225/60/16 tires. 4th gear in an AOD is 0.67:1, so 55mph in OD with 3.27s would be just over 1500rpm. 3.08s would be about 1450rpm, and 2.73s would be about 1300rpm.

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post #3 of 25 (permalink) Old 07-16-2019, 04:36 PM
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The biggest problem is that there's not enough torque to run 2.73s and the 1st gear in the AOD.

Well, there is, but you'll lose out on the first part of the 1/4 mile due to having to get up into the torque curve.

Why are you looking for longer legged gearing? *is curious*

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post #4 of 25 (permalink) Old 07-16-2019, 05:37 PM Thread Starter
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I have 2.73s on my car and it really kills the acceleration. I'm thinking about going with the 3.27 to increase 0 to 60, but I also want the good mpg. Also, i dont have a super coupe, I have the 5.0. The reason I'm asking is because it's the "best" thing to compare it to 😕
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post #5 of 25 (permalink) Old 07-16-2019, 09:17 PM
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Actually, no, the SC isn't the "best" thing to compare it with; the best thing is a 5.0HO.

The 3.08 was the stock issue most years; and it does tolerable at best. A 2.73? I bet that car feels like a dead pig off the line.

I've got some 3.55s to go into Ruby as I redo her drivetrain; 3.73 is about as stiff as I'd go for a highway-also car, but YMMV natch.

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post #6 of 25 (permalink) Old 07-17-2019, 08:29 AM
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I think this is a trick question because who would actually drive 55?

They put 2.73's in V8 T-Birds for years, pretty much all years in the 70's and 80's. In the 60's they used 2.75's with the 429's. Talk about a waste of big block power.

I am curious which year the OP has, and if he actually has 2.73's. If it's an 88 or older, definitely!

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post #7 of 25 (permalink) Old 07-17-2019, 09:37 AM Thread Starter
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Not a trick question. I own a 91 5.0 and I'm using it for daily driving. The roads I'm getting on are 55 mph. The 5.0s in 1991 were equipped with either a 2.73 or a 3.08
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post #8 of 25 (permalink) Old 07-17-2019, 01:43 PM
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That’s not true. Unless you or someone before you changed the rear gears, you have either 3.08s or 3.27s. The only MN12s that came factory with 2.73s were the 5-speed SCs. The fox body cars could be had with the AOD and 2.73s, but they also had shorter tires, and were at least 400lbs lighter.

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-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
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post #9 of 25 (permalink) Old 07-18-2019, 12:29 PM
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Quote:
Originally Posted by MadMikeyL View Post
That’s not true. Unless you or someone before you changed the rear gears, you have either 3.08s or 3.27s. The only MN12s that came factory with 2.73s were the 5-speed SCs. The fox body cars could be had with the AOD and 2.73s, but they also had shorter tires, and were at least 400lbs lighter.
This is incorrect -I have a 91 Cougar LS, 5.0HO, stock 2.73 rears switched to 3.73's about a 1 second improvement in 1/4 mile times, LOL. 5000RPM shift out of first was at 60 mph or so
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post #10 of 25 (permalink) Old 07-18-2019, 12:31 PM
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And yes, horribly slow until about 3800 RPM. Down shifting at 35MPH was fun though, before I knew how slow it still was...
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post #11 of 25 (permalink) Old 07-18-2019, 04:30 PM
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What does the door tag list under axle?

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad
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post #12 of 25 (permalink) Old 07-18-2019, 06:18 PM
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My SC RPM at 55 is 1600. I never heard of 2.73 gears on a auto MN12, I wonder if Matt knew about that.
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post #13 of 25 (permalink) Old 07-18-2019, 07:57 PM
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I'm about an hour away from the car unfortunately. I'll look at the tag when I get a chance. Was under 1400 rpm at 55, just over 1600 at 70. I worked at a Ford dealership during college when I did the gear swap. Discussing options with the Ford tech we decided to find a junkyard rear with the trac-lok to build while I was still driving the car with the 2.73 open diff. I've been mistaken before but don't think I am here....
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post #14 of 25 (permalink) Old 07-18-2019, 08:11 PM
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It could be that the junkyard diff you got had 2.73s, since all the early SCs got a trak-lok, and nobody wanted the 2.73s so they were always cheaper than the 3.27 ones out of the autos. The door tag will tell for sure what was originally in there though.

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
-"Your buddy Mike is INSANE!" -ClintD's dad
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post #15 of 25 (permalink) Old 07-19-2019, 09:06 PM Thread Starter
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It could just be that I have a 5.0 with 2.73s -_-
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post #16 of 25 (permalink) Old 07-20-2019, 12:16 AM
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I'll take a pic of my tag tomorrow. Fairly certain mine was 2.73 as well.
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post #17 of 25 (permalink) Old 07-20-2019, 01:27 AM
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Where does it say what gears it has on the tag?
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post #18 of 25 (permalink) Old 07-20-2019, 05:34 AM
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Kevin,

3.27 traction-lok.

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post #19 of 25 (permalink) Old 07-20-2019, 11:40 AM
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Axle code is 8. 2.73 open diff, 91 5.0 LS. And yes it was a dog off the line.
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post #20 of 25 (permalink) Old 07-21-2019, 12:46 PM
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I wonder if they made any 3.8 n/a with 2.73 gears, that would be so slow. Whats your freeway mpg?
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post #21 of 25 (permalink) Old 07-21-2019, 06:15 PM
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I'd get 27-28 mpg on 2 Lane roads going 62 in a 55mph. 72 on the freeway around 24, if I remember correctly. Switched to 3.73s 13 years ago. Been in the garage most of those years after trans went out.
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post #22 of 25 (permalink) Old 07-21-2019, 06:23 PM
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What was your mpg with 2.73
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post #23 of 25 (permalink) Old 07-21-2019, 06:56 PM
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That was with the 2.73. can't say I really cared to check after the switch to 3.73. maybe 20 highway? Rpm was closer to 1700 at 70 iirc.
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post #24 of 25 (permalink) Old 07-24-2019, 06:30 PM
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A big FWIW

I recently switched to 3.73 from my stock 3.08 5.0 lx and mpg went from 24.5 mpg at 74 mph to maybe 21 or 22 (any long drive involves hills in my area). I don't do long trips with it since doing mods but it was a 21% gear reduction using my math so a fuel economy reduction is expected/guaranteed when you are in high gear and past about 1500 rpm.

As far as 0-60mph went for me. I switched to 3.73's in part to improve the time since it had to shift to second before it would hit 60mph. I remember not being able to hit 60 mph in first without hitting the 5000 rpm red line and I won't try it now with 3.73's. My tach does have a 500 rpm echo or something that can't really be trusted at high or WOT. Nobody has offered to time me so 0-60 is still a mystery.

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post #25 of 25 (permalink) Old 07-24-2019, 06:52 PM
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Put an acceleration app on your phone, and you can see the shifts.

Should be able to see the timing as well.

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