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Old 06-27-2008, 12:20 AM   #1 (permalink)
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Hi all,

Finally got my EFI/AOD project on the road this week. Runs and handles great, but one thing I did notice and am wondering about is the ideal rear end ratio with the AOD conversion. I think my car still has the original 2.79's in it, so I would imagine that the slight bog I feel at higher speeds is due to the new gearing of the AOD.

I'm curious to hear the experiences of others. Would it be a good investment to move up to a set of 3:50's or something higher?

Trevor
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Old 06-27-2008, 09:03 AM   #2 (permalink)
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With the ever rising cost of fuel, I wouldn't go more than the 3.50 ratio. I used a 3.55 with my GearVendor overdrive.
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Old 06-27-2008, 10:10 AM   #3 (permalink)
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I have 3.0 in Red and have considered going 3.25 but seems like too much work for just a .25 difference.

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Old 06-27-2008, 12:16 PM   #4 (permalink)
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I have a link to a ratio calculator on my home computer, you enter your tire height, 1,2,3 and OD ratios and it will give you RPM's at any speed. Find out what the ratios are for your tranny, my AOD 1-3 will be close to my C4's but haven't installed the AOD yet.
After doing numerous combinations I decided 3:55 would be the most useful for my driving. I plan 90% cruising/highway and 10% drag strip/autocross/whatever I want to try.
I'll be home this weekend and can post then.
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Old 06-27-2008, 12:29 PM   #5 (permalink)
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Quote:
Originally Posted by 66HertzClone
With the ever rising cost of fuel, I wouldn't go more than the 3.50 ratio. I used a 3.55 with my GearVendor overdrive.
Unless yours is a daily driver, who cares about mileage? These are play cars we drive for the sheer enjoyment.

Modern Mustangs have anywhere from 3.2_ to 3.7_ gears in them from the factory. The overdrive gears let you have your cake and eat it too.

I believe an AOD has a final drive ratio of .67. So, if you had a 3.0 gear, you could bump that to a 4.50 and end up with the same engine rpm at a given speed! It would seem that any ratio in the 3.5 - 3.8 range could still be used while cutting engine rpms at cruise.

It depends on how the car will be used and how important the cost of operation really is.

FWIW, I went with a 3.80, but have yet to drive the car. It's going behind a high-revving 408 and I couldn't care less about how much gas I burn on my Sunday drives. I'm confident it will be wonderful.
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Old 06-27-2008, 01:15 PM   #6 (permalink)
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I was curious about the same question for my AOD/EFI conversion. I didn't want to be turning 5 grand while cruising down the interstate. The rearend I got had a new 4.11 set in it, though I have a spare 3.00 pumpkin. I found this online calculator to help me decide: Gear/Speed Calculator Link

Bashed is correct on the final drive ratio on the AOD, it's 0.675. Don't forget, tire size also has an impact on your final speed/rpm. The calculator is easy to use and let's you play around with different combos of rear end and tire size.

BTW, I ended up settling on using my 3.00 gears with 195/75R-14 tires. It's intended as a cruise car though, not an asphalt ripper. I agree with Craig though, a 3.25 rear is what I'd prefer.
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Old 06-27-2008, 01:32 PM   #7 (permalink)
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i used 3.89 with my AOD and it ran great. nice power off the line. around 2300 rpm at freeway speeds. AOD sucks a lock of power from system. 3.89 compensated nicely for this....
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Old 06-27-2008, 01:45 PM   #8 (permalink)
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Old 06-27-2008, 04:58 PM   #9 (permalink)
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Im going with 3.89 behind my 408.I havent drove it yet .Not worried about milage to much
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Old 06-28-2008, 03:47 AM   #10 (permalink)
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Quote:
Originally Posted by Bashed68

Modern Mustangs have anywhere from 3.2_ to 3.7_ gears in them from the factory. The overdrive gears let you have your cake and eat it too.

I believe an AOD has a final drive ratio of .67. So, if you had a 3.0 gear, you could bump that to a 4.50 and end up with the same engine rpm at a given speed! It would seem that any ratio in the 3.5 - 3.8 range could still be used while cutting engine rpms at cruise.

It depends on how the car will be used and how important the cost of operation really is.
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Old 06-28-2008, 08:55 AM   #11 (permalink)
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In my late model mustang, I just installed an AOD with .67 final drive. I have 3.55 rear gears. At 45mph, I am at 1500rpm, at 55mph - 1800rpm, at 70-75mph - 2500-2800rpm. I plan on using the same drive-train set-up in my conv. project.

Before my conversion of tranny & gears, the factory setting was 1500rpm at 55mph and about 2000-2200rpm at 70mph (this was 3.08 gears & T-5 with .68 final dive ratio)

I would 'guess' that if you used 3.27's, the rpm at 70-75 would top out at about 2300-2500rpm

My C-4 with 3.55 gears does about 3400-3500rpm at 70mph. This shows the benefit of the AOD - I think my next project will be converting my C-4 to AOD.
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Old 06-28-2008, 12:21 PM   #12 (permalink)
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For 8 years now I have run 3.50 gears in my cougar with AOD and 351W. It has been a good compromise in performance and economical highway use.

If you purchase a new converter, consider one that operates like the original and still allows bypass of the hydraulic coupling...if you want a little more efficiency and less heat generated from the constant slippage of the hydraulic coupling.

John
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Old 06-30-2008, 02:01 AM   #13 (permalink)
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The aod is geared a little differently than the t5, and slightly more gear is benificial. anything below 3.55's is a waste of time for the money spent. 3.73's-4.11's at the most are perfect, with 3.73's being my first choice. Rpm's shouldn't be much more than 2100 at 60-65 mph or so...
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Old 06-30-2008, 10:26 AM   #14 (permalink)
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Quote:
Originally Posted by v8only
The aod is geared a little differently than the t5, and slightly more gear is benificial. anything below 3.55's is a waste of time for the money spent. 3.73's-4.11's at the most are perfect, with 3.73's being my first choice. Rpm's shouldn't be much more than 2100 at 60-65 mph or so...

All the 5.0 magazines say the same, either 3.73's or 4.10's to put an AOD on eqaul footing with a T5 with 3.27's.
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Old 06-30-2008, 11:10 AM   #15 (permalink)
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Quote:
Originally Posted by v8only
The aod is geared a little differently than the t5, and slightly more gear is benificial. anything below 3.55's is a waste of time for the money spent. 3.73's-4.11's at the most are perfect, with 3.73's being my first choice. Rpm's shouldn't be much more than 2100 at 60-65 mph or so...
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