AOD effect

[superaquarama]

Hi Guys,

I'm considering swapping the C4 on our '68 Sprint 289 for an AOD. Having read the threads & some other stuff, this seems fairly straightforward, BUT I'm a bit concerned about our already high ratio diff, which is a 2.79.

At present 2000 rpm = 50 mph approx, so with a 0.67 4th gear this'll give a 1.87 final drive & 75 mph @ 2000 rpm ! Is this way too tall a ratio ? Maybe I should change the diff as well ? In which case, I may think it's all got a bit too involved and just leave it as it is !

For comparison, in about '82 Mercedes changed their final drive ratio on the 380SL form 3.27 to 2.25. And the T700R4 conversion's used for Jaguars with a 2.88 diff quite happily, apparently, but offhand I don't know what the OD ratio is for that trans.; and the Jag's got bigger wheels, which would make them higher geared.

Any comments would be appreciated.

Thanks

 

[mikemustang289]

I can tell you from experience that a .67 o/d with 2.79 gears doesn't work very well unless you prefer driving at about 75mph on the freeway. Back when I first swapped a T5 in my car I had the stock .68 o/d with the stock 2.79 gears. Freeway cruising was nice as long as I didn't need to slow down, which would frequenlty mean downshifting to 4th. Preferably you'd want to keep a cruising rpm of about 2500, this way if you have to slow down to say 55-60, you can still accelerate back up to speed and have enough torque.

I should add, shortly after I changed my rear gear to 3.55 and freeway driving was much improved. A year later I went to 3.73's and it was even better. Later I went to 4.33's and swapped in a T5z with a .58 which kept the final drive at about the same as with the 3.73 but with the benefit of a lower gear for acceleration.

The 2.79 wouldn't be optimal, but I would still prefer the AOD over the C4 even with the stock rear gear. ~2,000rpm on the freeway is a pretty nice feeling and your gas mileage will improve.

 

[coupe5.0]

My car with 3.25s is almost too high geared. It does cruise really well on the interstate though, keeping up with 80 mph traffic is no problem. It got 21 mpg on a recent trip, I was very happy with that. Keep in mind most fox body mustangs were 3.08 geared with an AOD.

 

[rpk]

I first had 4:10 in my 85GT with the AOD. Great around town but a bit high on highway. I'd hit 3rd gear crossing a 6 lane intersection. Switched to 3:55 and found it lacking around town but decent on highway. The fox guys say for a 5.0 best all round gear is 3:73. Haven't tried yet but sounds about right

 

[rbohm]

fortunately you have a few options when dealing with an AOD coversion. the fox body cars, as indicated, came with 2.79 or 3.08 gears generally, and most fox body guys like to use a 3.73 gear in their daily drivers with a stock geared AOD. another option with the AOD is to use the wide ratio kit that gives you a 2.84 first gear and keeps the stock .67 overdrive ratio. if you do this then run something more like a 3.55 gear. one more option would be to use shorter tires, a 24" tall tire rather than a 25.6" tall tire normally used, and use a 3.55 gear.

 

[rebel65]

on my engine which is really quite torquey, i got a tko600 which has the .64 od i believe, and i have the stock 2.80 rear gear, 1 - 4 are perfect. i wouldn't change them for the world but 5th below 75 is a little sluggish, not quite lugging, till you ~65, then it def lugs, but like ~70+ it's good so if i did switch to a 3.25 or 3.55 it would be good around 65 i believe. and i wouldn't want to go into 5th under 60 - 65 anyway. so bassically.. you will lug in 4th about the same as i lug in 5th, but it's not horrible, you will most likely be fine with it for a while till you can switch the rear gear. i would do it anyway personally even if you kept the c4.

 

[JonK]

I'm in process of installing an AOD but first wanted to upgrade my rear, I ended up piecing together a 9" with 3.50:1 in place of my stock 8" 2.79. Don't be intimidated by a gear change, easiest way is to just get an already rebuilt 3rd member with the gears you want. There are a few builders on fleabay, that's where I found mine.

Changing out the 3rd member was not difficult, you could unbolt the axle shaft retaining plates, slide out the axles a few inches (a few tugs with an axle puller, looks like a dent puller and is a free rental at most parts stores, and they slide free) remove the old 3rd member, clean casing and install new preset 3rd member.

I never trust 40 yr. old bearings so I change them when I redo the rear, this adds a little as you have to get a shop to press off the old bearings and press on the new ones. One of my bearings locked up before I had a chance to pull them anyway.
It wouldn't hurt to do the AOD swap first and then do the rear when you find a deal, that was my original plan but the 9" fell into my lap first.
Jon

 

[rebel65]

that is my plan also, as i still have my 2.80's

 

[tx65coupe]

I would recommend 3.50 or 3.70 gears for an overdrive transmission setup.

 

[superaquarama]

Many thanks for the replies, everyone !

As I rather expected, the general opinion is that a 2.79's too high, so it begs the question of whether I can be bothered to do the conversion at all.

The trans is quite cheap and is a 'known quantity', just having come out for a T5 conversion, so I'll probably just buy it anyway and then think about it, or maybe do the job in two stages as Jon suggests.

Has anyone recorded what difference there is in gas mileage after a conversion ? That, and I guess a more relaxed cruise, is my main reason for thinking of the conversion.

Thanks.

 

[coupe5.0]

Mine was getting about 14 mpg before, and last I checked got 21 averaging about 75 on the highway. The fuel economy jump was great, but overall made the car much nicer to drive at highway speeds.

 

[rpk]

it will probably not really change much, you'll be in the power band a won't need as much throttle. Besides, it will be much more fun to drive,and you'll give 'er a little bit more. After all, in these bad times, if it uses a touch more gas,which I don't think it will, the smile you'll have and enjoy driving it more,will be worth it.

 

[Red5oh]

I have a set of 3.55's with my bone stock AOD and it runs just fine in the 70-75mph range. I have an EFI setup, so I avoid the infamous 2200rpm, which is about 70, so 75 is what I like and its quite a a bird. My last check put it at 21mpg and it is a ton of fun.

 

[mikemustang289]

You might try a higher stall converter as well, that should give you a little more breathing room on the highway. It's more doable with an automatic vs. a manual trans.

 

[superaquarama]

Thanks for the further most useful comments, guys !

 

[j69302]

I have 2.79's in my 69 with a mild 302, a TKO600, 245x60 x15 tires and the gearing is not bad at all.

Overdrive (which is .63 on mine) is pretty high though.. 80MPH is 1800 rpms. cant think about using overdrive unless im going 70 or faster. since the AOD has a torque converter instead of clucth, OD will be easier on your car vs if it had a clutch.

 

[rebel65]

i get about 16 mpg highway now with a overdrive. imagine what i got before : ). before it used to take a whole tank of gas to drive from my parents house in Napa, CA to my house in Santa Clara, CA. or at least i would have to fill up, now i can go back without a fill up. ( with my engine )

 

[cougar_68]

I have 4.11's with my AOD and love it. The only complaint I have is, RPM at cruising speed is right at the place where my "drone-masters" (flowmasters) want to drone you right out of the car.

Probably doesn't help that I don't have any insulation/sound deadener on the floor or between the rear seat and the trunk. Therefore the interior has been removed once again.

Just wanted to let you know though, when I go from Drive to O/D (manual valve body) the revs drop by about 1000 RPM.