Hello there. I am new to the site and am trying to find some information about a transmission swap. I just recently bought a 1986 F250 with a 351W HO and what I believe is a C6 transmission. I have read that swapping out to an AOD is possible. Is it worth it to do the swap for milage savings? Right now I stay at 55 on the highway and get about 10 mpg. What I am wondering is how many more MPG's can I realistically achieve? I also plan on purchasing a travel trailer to do some camping with in the future, should this also be a concern? I am not sure if the cost of doing the swap is worth 1 to 2 more MPG's or transmission issues in the future. Thanks and great site, awesome info!!

 

tranny swap aint worth it mpgs wont change but 1 or 2 and then with an aod and pullin a camper its not gonna pull as well and them aod dont hold up as good as a c6 i had an 86 with a c6 one of the only autos i have ever owned and i ran it hard everyday pulled alot with it. and never had a problem i know a guy thats got an 87 with an aod and he has problems with the tranny gettin hot while pullin or runnin it hard and he has put a bigger aftermarket tranny cooler on it. to me the cost and the headache wouldnt be worth it

 

I had an AOD in my F250 for three years then swapped it for a C6, with no change in gas mileage - but I didn't use overdrive with the AOD (60mph local limit, so rpm's were too low in od).

If you intend to still travel at 55ish, & your axle/engine combination is suited to that, the AOD won't be any advantage; & with planning to tow in the future, it would be a liability IMHO.
A C6 is much better suited to that role. AOD's can be made more durable but not cheaply.

 

What he said x 2. But, if you are getting 10 MPG at 55 MPH with the 351HO and a C6 there is some kind of a problem. I had an 82 351W with a C6 and 3.50 gears that got 13 on the highway at 62 MPH, and would have done better at 55. So, long before swapping to an AOD you should figure out what the problem is. Low MPG may be a fuel problem, like a blown power valve on the Holley or the choke being stuck on, or it may be that the ignition timing is off, as in the initial lead or the vacuum advance may be blown.

 

Thank you Ken, that is exactly what I was looking for. The local speed limit here is 70, but if that would get me about the same milage with the trans swap as keeping it at 55 it hardly seems worth the cost or hassle for the swap.

Gary, I was wondering the same thing. It seems to me like it is running a little rich. My dad and I are planning on going through it and checking the timing and the adjustment on the carb, just have not had the chance yet. Hopefully we can get the done soon if it is affecting my milage that much. Honestly, when I bought the truck all I was figuring on was about 8 mpg, so 10 to me seemed like a good deal.

 

I've been contemplating the same swap for mine, but i also will be towing a camper. I wonder if a hughes "fuel miser" or TCI "towing" torque converter and shift kit wouldn't be a better choice. They advertise as lowering highway RPM ~ 20% for better gas mileage, reduced slipping, & lower operating temps but i'm a bit leary of "advertising".

 

Good question/point. Hopefully Ken will chime in here as he has the results of swapping a C6 out and installing a manual tranny with a 1:1 top gear. (I forget which one he has in this week. ) But, IIRC, he believes he got 1 more MPG with the swap, which effectively eliminated the torque converter w/o changing the gear ratio.

As for the Hughes TC, I was doing a 351W build a few months ago chasing MPG, and talked to a Hughes technician. He said I would get 1 to maybe 1.5 MPG increase by going with their Fuel Miser TC. That's interesting because the best a TC could do would be to eliminate all slip, and yet a TC w/o a clutch can't ever get to zero slip. So, given Ken's results of +1 MPG with the manual tranny I have my doubts that the Fuel Miser TC would give more than +1 MPG.

Having said that, I have a brand new Hughes Fuel Miser TC that's never been used nor even installed. I didn't install it because my plans changed with that engine and I sold the truck it was going in. So, I'll make someone a very good deal on it. Just PM me or send me an email at janeyandgary@gmail.com if you are interested.

 

Well now, if you never used the overdrive gear, that makes a difference. That means you pretty much used it as a 3-speed, and no wonder you didn't get better gas mileage.

I agree with the fact that the C6 is a stronger transmission in stock form than the AOD and is much better suited for towing.

But I don't see how anyone can say that an AOD with 3 gears plus overdrive (.67:1 ratio) doesn't get better fuel economy than a C6 transmission with only 3 gears (1:1 ratio).

 

Good point, Lariat! Driven wisely the AOD would have to return equal if not better MPG than a C6 since you could just lock out the OD and you would have the same top gear as the C6. Further, in the right circumstance you could do even better by using the OD. In addition, some have said the C6 has high internal loss while the C4 has less, and IIRC, the AOD is based on the C4.

So, let me restate things: IMO swapping out the C6 for an AOD isn't worth it for what honocor said he will be doing as the gain, and there should be some, would not offset the expense and hassle. And, that's side-stepping the reliability issue for towing.

 

doesn't the AOD TC still lock up in 3rd above a certain speed, which would give you lower highway RPM then a C6. I seem to recall a C6 needing more power / energy to operate then an AOD / C4 which should also mean all things equal the AOD would be more efficient use of power thus better fuel econ.

 

AOD is based off the FMX which is based off the Cruise-O-Matic.

As for the Fuel Miser TC getting better MPG than the manual, the only thing I can think of is the torque multiplication you get when starting out. I have no idea how that factors in but that's all I could think of to get the same or better MPG than a manual.

I'm in a similar boat. Long story short I'm looking at an 86 F250 Supercab that's going to be THE truck. It's a 351/C6 combo and I'm thinking of how I can get the best mileage and none of Fords OD autos really impress me.

The main concept of the build is stone cold reliability through simplicity. That's #1 priority. Even built I question the ability of the AOD to survive in an 8600lb gvw truck. Especially one that will tow 6-7k... The E4OD might but I then need a computer to run the trans and I'm getting away from my "simplicity" goal. I have an E4OD but it needs to be rebuilt so that plus the computer I'm at least at $2000.

I think for me and the amount of driving I do with the truck I'm going to stick with the C6 and just suck up the couple MPG loss. If I can average 10 or so that would be great. My 2004 Dodge 1500 with the 4.7v8 and 5spd Auto gets 11.5 or so average. That's a double OD trans...

In the end I think I'm going to go Manual. I plan on doing some limited plowing with the truck and I don't know how I'm going to like a manual but for the money I think it's the best bet to get me the most MPG while remaining simple and reliable.

Other than that, go over the basics. Tire pressure, tune, eliminate unneeded junk to keep the weight down and drive easy.

Lastly, for grins and giggles I figured out what a 2mpg increase (10->12mpg) would save you if you drove 12k miles/yr and gas was $3.50/gal. Savings are about $700/yr.

Here are some other numbers:
All assume starting at 10mpg, driving 12k miles/yr and gas costs $3.50/gal. Savings are per year.

New MPG - Savings
11mpg - $382
12mpg - $700
13mpg - $970
14mpg - $1200
15mpg - $1400
16mpg - $1575

So if you gain 1mpg by tuning up your truck you'll save almost $400 in a year... Kinda puts the $100 or so in parts into prospective...

 

Thanks. That's what I was trying to say with "In addition, some have said the C6 has high internal loss while the C4 has less, and IIRC, the AOD is based on the C4." And, if the TC does lock up in 3rd then you would drop 200 - 300 RPM of converter slip, which would make the AOD neck and neck with a manual tranny with 1:1 top gear.

Still, the thing that concerns me is the .67 OD as it is a huge jump. Were it more like the .76 of the ZF I would have been thinking about having an AOD beefed instead.

 

a manual trans will always be more efficient, mechanically. There's no fluid mass and pressure to build within a TC, no internal clutches to slip, etc...

I think the OP and I both are trying to maximize the strengths of the C6 while trying to improve fuel efficiency.

The great thing about the deep OD with the AOD is that it lets you run a steeper rear end gear, which will will improve take off. AOD w/3.73 = C6 w/3.00

If the TC nets 1.5mpg, shift kit nets 1.5mpg, the "small improvements" start adding up quickly! Right now with the truck below my best tank EVER was 14.2MPG, 60* air temp, flat stretch (~165mile stretch), 55-60mph, only 1 stop sign! I currently avg 9-10mpg mixed driving.

 

Yes, the C6 uses much more power to operate than the AOD does, so the AOD is more efficient and produces better fuel economy for normal street use. Otherwise, Ford would have never used it.

Ford plainly states NOT to tow in overdrive. And auxiliary transmission coolers were installed with the AOD that had the towing package. So I think the idea that the AOD is "weak" is somewhat exaggerated for normal street use and light towing purposes.

That being said, if your #1 priority is "simplicity," then the C6 is the transmission for you. The E4OD is much more complex than the AOD, and the AOD is more complex than the C6. If you are going to tow 6-7K, I would definitely recommend the C6 over the AOD. There is a reason why Ford never put the AOD behind anything bigger than a 302 engine in the F-Series trucks. And its rare to find a 302/AOD combination in an F-250, because an F-250 is considered a heavy duty model.

True, and that's always FUN! At 55 MPH, my 302/AOD with a 3.55 gear is cruising at a slumber of only 1500 RPMs. With the AOD, you will be able to run a lower 4.10 gear, and even then it wouldn't scream at 55 MPH like a C6 would with a much higher 3.55 gear.

 

I occasionally used OD on the motorways (freeways), where the truck could be driven at reasonable rpm (around 75mph in OD due to 3.54 axle & 30" tires), but there was so much wind resistance that there was no fuel saving - in fact, the opposite.
In an aerodynamic car it would be a different situation.

My intention was to swap to a close-ratio T19, but the AOD died too quickly, so a C6 had to go in temporarily, followed by the NP435 swap later - T19 next week, Gary! Seriously!

In the process of all this though, I learned a couple of things (just 'seat-of-the-pants' observations, in my application)

- the power loss difference between a C6 & an AOD seems pretty much non-existent
- the C6 is actually slightly lighter than the AOD......10-15lb IIRC
- NOT using OD, the C6 & AOD gave me the same mpg, despite the AOD lockup in 3rd (& 4th)
-swapping from C6 to 4spd manual has so far gained 1mpg, running light (not towing or loaded) but I think the C6 mileage would fall behind, loaded.
-while the mpg difference between C6/Np435 is only slight, the power difference is noticeable, especially on initial acceleration & climbing a hill
- with the manual trans, my vacuum gauge reading was often 3 inches Hg higher on hills than with either automatic; but not much difference on flat roads.

Hope this might help someone