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AOD and fuel economy considerations

3T

Full Member
Joined
May 24, 2006
Messages
167
I just finished with the EFI swap. I got the AOD tranny (recent rebuild with 7,000 miles on it) from the 93 Mustang donor along with the engine. My C-4 is leaking so bad right now - I either need to fix those leaks or start on the tranny swap. After reading various threads on AODs, I am a bit hesitant. The only reason I would want to do the swap is for increased fuel economy. From what I read, there are some hints of little changes in fuel economy but also seemed to depend on rear end, size of tires, etc. I could not find any information that answered my questions. So if I run 31-33" tires with 3.50 rear end will I gain greater fuel economy? In other words, is it worth the money and work it will take in an effort to increase fuel economy?

Thanks!
 

COBlu77

Sr. Member
Joined
Feb 10, 2004
Messages
867
Loc.
Arvada, CO
I doubt you'd see a significant MPG increase, unless the majority of your driving is highway. Even then with 3.50 gears and 33" tires, your RPM's at highway speed might be below your engine's efficient power band and you'd be constantly shifting in and out of OD.

On the other hand, with proper gearing, an OD tranny is great for highway cruising. I love my 4R70W, but I have 4.56's
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
...So if I run 31-33" tires with 3.50 rear end will I gain greater fuel economy? In other words, is it worth the money and work it will take in an effort to increase fuel economy?

Thanks!
I will say no. I have a 4R70W and 4.88 gears. That comes out to about a 3.42 gear when in overdrive. With the 33" tires that works out just about right.

A 3.50 gear with an AOD .667 overdrive comes out to a 2.33 effective gear ratio. That would work in an aerodynamic mustang with short 26" tires, but not in a tall tire truck.
 

broncnaz

Bronco Guru
Joined
May 22, 2003
Messages
24,341
Ditto
you would need at least 4.11 gears to run the AOD and see any gains. Right now with the C-4, current tire size and gearing your already in overdrive. even now the EFI engines power band may be higher than your hiway rpms if so you wont see any increase in mileage. in fact many people swapping to EFI see little to no mileage increase. Its all about the camshaft it has to be right for your rig and driving needs.
Good time to put new seals in the C-4 if its out. wont be hard to stop the leaks.
 

Thwak

Jr. Member
Joined
Oct 24, 2007
Messages
129
Loc.
Clear Lake, TEXAS
just curious. i have 33's and 4.11's. will aod help my hiway mileage. thinking of going to 35's after these wear out. will my motor be out of its powerband in od.
 

PaulS

Jr. Member
Joined
Dec 6, 2007
Messages
96
Loc.
Greater Seattle area
You don't want to put a heavy load on an AOD when it is in OD. If you do so without replacing the thin OD band with the 2" band and drum you will burn it up. Od is for light load cruising and down hill use. I think you are better off with a C4 with 3.0 to 3.7 gears. If you have more gear than that then the AOD is a viable (although expensive) alternative. It costs about three times as much to make an AOD take the same torque and HP as a C4.
 

Hazegray

Sr. Member
Joined
Aug 10, 2004
Messages
795
If you are driving on the highway alot, then go with the AOD. If it's just an occasional jount, then stick with the C-4.

Also, I found out with AOD (w/33's and 4.11 gears) and a 302, the motor was having difficulty "pulling" on the highway. It was constantly hunting from OD to Drive. After I installed a 351W, it quit jumping out of OD...pulling mild inclines with ease.;D
 
OP
OP
3

3T

Full Member
Joined
May 24, 2006
Messages
167
So what would I need to do to get greater fuel economy? And if that is possible how much MPG can I expect?
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
So what would I need to do to get greater fuel economy? And if that is possible how much MPG can I expect?

Providing that you were overgeared and turning too many RPM on the highway before and with OD you are not below your engines powerband you could see 1-2 MPG increase typically. More or less is possible.

No your 8MPG pig isn't going to magicly be getting 20 MPG.

As for the 33s and 4.10s and thinking of going to 35s, an overdrive would be questionable with the 33s and would be pointless with 35s. The 35s are already an overdrive.
 

76MaBronco

Bronco Guru
Joined
May 21, 2007
Messages
4,219
Loc.
Massachusetts
Does anyone have a listing of the powerband of certain engines?

I'd like to include it in my gear calc so it makes it more complete.

I have an EFI 302 out of a '90 Lincoln, ZF 5 speed and running 31's (it's what I have right now)...might go to 32's or 33's, not much bigger than that. I bought lockers that take 4:11's and higher.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Any published powerbands will be for full throttle. But generally you don't drive around all the time at full throttle. A lightly throttled (loaded) engine runs more efficently at lower engine speeds. Heavily throttled (loaded) engine runs better at higher RPMs. A lightly loaded engine turning high rpms is wasting a lot of fuel just to spin the engine. A heavily loaded engine at low speeds (lugging the engine) creates high cylinder pressures and requires a rich air/fule mixture to keep the engine alive.

So you have this double edged sword. by running without an overdrive you turn too many RPMs and are wasting fuel just by spinning the engine. Go to overdrive and lug the engine. The balance between the 2 points is what gets tricky.

As for the powerband I spoke of earlier, you just need to drive the vehicle and feel the engine. How well does it run at low speeds with some load on it? Too much load at too low of a speed and it isn't happy. Then you need to guess how much load you have at cruising speed and figure if the engine will be happy pulling that amount of load at the RPM you want it to run at.
I am sure there is a science to it, but it really comes down to how it feels to us.

My torque built 351 is happy running 1000 RPM under light load and at 2000 RPM is completely happy pushing my brick down the highway.
Another project with a stroked 302 hates any load under 1500 RPM but is ina a body that goes down the highway much better. It still cruises nices at 2000 RPM. But that cammed and stroked 302 would not be happy pushing the bronco down the highway even though it is the same RPM. That is because the load is higher in the Bronco. Again, a balance...

Throw in a bunch of variables, how flat are your roads? how fast do you want to drive? engine size, cam, etc...

This will be debated for a long time. Why? There is no clear answer. If there was then this wouldn't go on forever. The answer would be given and the thread would end. But no, the debate about gearing will go on forever. Often with what some people think is "perfect" while others with the same gearing hate it, often for completely different reasons.
 

surfer-b

Contributor
Bronco Guru
Joined
Sep 7, 2006
Messages
2,972
I have 92 mustang 5.0-all stock, AOD, 3:50 gears, and 31" tires, I very seldom use OD. I need to drop to at least 3:90 an probably wouldn't hurt if I went with 4:10's, if I had it to do all over I would stay with the C4, as others stated its much cheaper and easier to change gears than it is to change trannys. You also will have to modify the driveshafts, also the tranny pan to clear the front driveshaft, then there are the shifter mods you will have to consider.
 

surfer-b

Contributor
Bronco Guru
Joined
Sep 7, 2006
Messages
2,972
IF you want milage swap in a Diesel, this seems to work for others, I dont know if its worth the trouble unless you just want a project or you drive it everyday for long distances.
 

76MaBronco

Bronco Guru
Joined
May 21, 2007
Messages
4,219
Loc.
Massachusetts
I'm thinking 2000 is a good highway speed (65)...and with 31's, 4:11's gives me that exactly. If I go to 33's, I'm better off with 4:33's (the little difference is really trivial...)
 
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