Gear ratio with AOD

[Wirenut]

I was reading where fordmanjack posted information on the differential gear ratio on another post on another subject but I do not want to steal someone else's post. Fordmanjack it sounds like you have a lot of knowledge on the gear ratio combination. I have a 1986 AOD referred to as a metric transmission behind a 302. I have not had it out on the open road or pulled any hills with it. I think I have a 2.75 ratio or there abouts. I've been hanging on to another. 9" rear end until I get a chance to do some real driving it the car. I think it's a. 3.12. It looks as though with the AOD I'd be much better off with the lower gear and still not be too low at hiway speeds. Any advice would be appreciated.

[canadian_ranchero]

the aod has a overdrive ratio of .667.i would use a axle ratio of about 3.5.  3.5 x .667 = 2.33.[about 1900 rpm at 60 mph]now if you use a 2.75 ratio. 2.75 x .667=1.83 to high for most engines[about 1400 rpm at 60 mph]hope this helps

[lalessi1]

The overdrive ratio in an AOD is .667. That makes a 3.10 rear end the equivalent of a 2.07 in overdrive. That is 1740 rpm at 70 mph with 28" diameter tires. For me an even lower rear end would be more desirable, maybe a 3.50?

[djfordmanjack]

What they said ?! 
Great idea to start this as a new topic over here in the tech section so the info will be gathered for future use and hopefully some more guys will chime in with their experience.
I did collect and read lots of infos, but have yet to use the AOD on the street to make proof of it. I have many years of experience with 302s and 351s, also C4 and C6 and I just love them. it's just that a 3spd with high rear end ratio gives only limited milage on the highway.
That's why I want to change to an AOD. As mentioned above the OD ratio for a AOD is 0.67.
you will have to multiply that with the actual rear axle ratio to find out your total final drive ratio ( as equivalent to a std trans with said rear axle).with everyday driver SBFs from my experience I think the total ratio should not be under 2.5 ( as in my approx 2700pds 34 Ford). I think it should even be more than that in the heavier 57 (3500pds?).
of course the tire diameter comes into play as well. using big rear hot rod style tires will further lower your total ratio.
keep in mind that the 7.50-14 original tires of a 57 Ford were quite tall to begin with, but for instance I have 235/75-15 on the 57 rear and they will increase diameter a lot.
I think the ca 3.50 std 57 Ford rear would be lowest in a 57 with a factory hp 302 and AOD. exactly what Rich said.
I think that even a 3.96 or optional 4.11 rear (which the Ranchero came if Limey still has the og rearend) would make a good allround ratio with the AOD.
I have driven a 81/2ish Thunderbird V8 before with an AOD and it was a marvelous ride ( as to the drivng and shifting characteristics), need to find out the rear axle of said Bird.
We have to consider that SBFs and Y Blocks do NOT offer abundant torque as opposed to FEs.
It is no use going too low in rear axle ratio, as it puts too much strain on the engine, which according to its characteristics can't handle the power and will become tiring driving experience and getting bad milage.
This is for streetable close to factory engines. as soon as you have a wicked cam or large carburetor(s) its all a different thing. would most probably not work with that low of a ratio, except if the engine had been built especially for low end torque (which I prefer over high end rpms).
Id' be very curious to hear other people opinions. there have been quite some conversions reported with Y blocks and AODs. Wonder what they are using for rear axles?

[djfordmanjack]

I have to add, that we have steap mountain roads AND long straight highways in my home country and no speed limit Autobahn in adjacent Germany. So that's why in my case I need a good combination of both low gears and tall hwy ratio. Someone living in a mostly flat area and using his car as a long distance 75mph cruiser might be pleased with even a lower total ratio maybe.
I'd say in Wirenut's case it depends a lot on the tire diameter whether it will be a good combo or not.

[gasman826]

Several of the Mustangs with AODs had 3.73 rear end ratio and some with 4.11.  Depending on your build and how you drive, but I think 3.50 minimum.  With cam and loose convertor, 3.70 or even 3.90 might be more appropriate.  I'm building an AOD with 4R70 wide ratio gears, wider band, custom valve body and looser convertor for better performance and durability.  Play with the on line conversion software that include tire size and RPMs.  Another thing to consider with the AOD swap is the donor vehicle.  The AOD shift timing and firmness are regulated by line pressure (TV cable properly adjusted) and the governor.  Getting a governor from a similar vehicle will help with drivability.

[djfordmanjack]

looked up the 82 T bird and it says has a 3.08 final drive ratio for a 3466pds car with 4.2L (255) V8
http://www.automobile-catalog.com/auta_details1.php
tires are 195/75-14.

3.08 seems correct. very close to your rearend.
http://www.ebay.com/itm/Ford-Mustang-Mercury-Cougar-Differential-Ring-and-Pinion-Rear-Dorman-697-723-/361705374791?fits=Year%3A1982%7CModel%3AThunderbird&hash=item5437520447:g:d~kAAOSwgZ1XwAL-&vxp=mtr


this is for comparison only. a 57 is heavier and has taller tires, but then a 302 probably has more power than the 255.
interesting what Gary says about fine tuning the governor, which will help in acceleration and downtown drivability, but not at hwy speed, and as the AOD has a lockup and works like a std transmission with no converter slippage in AOD.

[Wirenut]

I am very appreciative of the knowledge you guys have shared. I do have 15" tires and a small cam to perk the 302 up just a bit. I have rolling hills in my area, nothing high, mostly flat to semi flat terrain. I'm anxious to experiment now. I still need to get my car a vehicle inspection so I can get it it on the road. I just lack my windshield wiper motor to get it to pass and then I'll get it on the road for some drive time. I'll post my rpm and final ratio once I decide on which. Thank you again for sharing hour knowledge.

[redshoes57]

I have a 36 Ford 3 window coupe that has a 89 302 alum heads and Tremac 5sp .68 5th gear. I'm got 3.70 rear. If I keep it at 75mph  i get 20 mpg. I tried a 3.25 but it was not happy seemed sluggish. this car weighs 3000 pounds and is not babied. Also i'm running 235/70/15 rear tires. I'm building a 57 2dr custom and it has a 351w with AOD .The trans came from 87 crownvic police car. I'm using a 3.50 pos. Ford used a lot 3.55 gears in the 8.8 rears.

[RICH MUISE]

I don't know offhand what the OD ratio is in my 4R70W (95 Lincoln MK VIII), but I'm running 3.50:1 trac-loc. Mid 20's for gas mileage. Can't imagine a better setup for me. Pretty quick "off the line", and will still get up and move if I hit the gas at 60 or 70 (as long as it's not in OD)

[Wirenut]

I do have a 3.70 but it's doesn't have limited slip or trac loc. I need to see what it'll take to upgrade to that or if locating a 9" already with the limited slip in it. I'm grateful for all of the input guys. Great site with great people, glad to be a part of it. Thanks to all.

[RICH MUISE]

jfyi....I spent 2k on mine. included shortening the housing, changing to big bearing ends, new Yukon trac-loc, new 3.50:1 gears, new 31 spline HD axels, bearings, and a 1" taller yoke.

[djfordmanjack]

very interesting firsthand info ! thanx alot !

I have a 36 Ford 3 window coupe that has a 89 302 alum heads and Tremac 5sp .68 5th gear. I'm got 3.70 rear. If I keep it at 75mph i get 20 mpg. I tried a 3.25 but it was not happy seemed sluggish. this car weighs 3000 pounds and is not babied. Also i'm running 235/70/15 rear tires. I'm building a 57 2dr custom and it has a 351w with AOD .The trans came from 87 crownvic police car. I'm using a 3.50 pos. Ford used a lot 3.55 gears in the 8.8 rears.

[gasman826]

I don't know offhand what the OD ratio is in my 4R70W (95 Lincoln MK VIII)

The 4R70 has a wider gear ratio to help the module motors get going.  The over drive ratio is slightly different from the AOD at .7.  One of the recommended AOD upgrades is to use the 4R70 gear set.

[Raven Rider]

I would suggest calling a couple of shops that specialize in rear ends and get a few quotes.  I'm having an 8.8 Explorer rear diff shortened for $650.00 and they are supplying the rear end.  I comes complete with disk brakes and new bearings and seals.   He is also interested in the 9" that is in my car now so end cost is about $400.00  I know a lot of hot rodders "need" the 9' diff but the 8.8 is plenty strong with 31 spline axles and it is modern cheaper to change ratios etc.