so, i got my truck, Clyde put back together again, and, as long as the final piece shows up tomorrow, i'll be on the road tomorrow night. the truck is an 81 F150, 300 six, C6, 2.75 geared 9". i live in small town iowa, so to get ANYWHERE, i need to drive the truck for at least an hour. to that note, i was pondering looking into possible overdrive options, to help conserve gas.

im a manual transmission kind of guy, so i'll start there.

i have a SROD out of my old 87 F150, tha was behind a 300 as well. worked great. if i recall, it should slide in, the speedo gear will slip in, but i will have to locate a set of 80-86 pedals. also, my wifey still hasnt mastered driving a manual more than 20 feet at a time.

M5OD: i have one in my 95, and i love it, but i know that one wont swap. if i found one from a 87-91, would that swap in? yes, i know i would still have the pedal assembly and wifey cant shift issues....

then to automatic land....

AOD: i know some early models were considered weak, but upgrades are available. i think that no matter what, if i went with a different auto, it would get rebuilt before going in. what years do i need to look for, and what would be involved in that swap?

Are there other auto swaps available??

im open to any/all suggestions, as long as they arent cost prohibitive ( dont tell me to swap in a Tremec out of a new mustang) or ultra complex ( no, i dont want a stand alone computer and harness for an e4od)

Ideas?

 

An Automatic Overdrive would work wonderfully if you made sure the throttle valve cable/rod was adjusted correctly so it wouldn't burn up the transmission.

Other than that, make sure that if you're towing or hauling anything with any decent amount of weight to it that the transmission is in Drive and not Overdrive so it won't hunt and burn itself up as well. And make sure it has a transmission cooler.

 

I'm not sure you are going to see a huge difference in MPG by going to an OD since your rear diff is 2.75:1. Many (most?) OD's are rather high and will put your effective gear ratio into an area even the mighty 300 won't like. For instance, the AOD's OD is .67:1 so you are looking at an effective ratio of 1.84:1. For reference, my ZF5's OD is .76:1 and when coupled with my 3.50 diff's makes my overall ratio 2.66:1 and lets me turn right on 2000 RPM at 65 MPH. But your engine would be turning 1385 RPM at 65 with the AOD and I'm not sure that will enhance your MPG. However, some of the guys that know the 300 better than I may be able to tell you how well that engine would get along being asked to work at such a slow speed.

I don't know what the ratios are for others of the transmissions you listed, but I'm really not sure the view is worth the climb. Yes, the C6 is known to use power, but with your ultra-tall gears you may not see much benefit. I will say though that Ford's claim to fame on MPG with the 81's was with a 300 engine and one of the OD manual transmissions plus something like your axle ratio. I know that because I was given a magazine cover from '81 for Christmas that carries Ford's claim on MPG. It is in the shop and I'm not supposed to limp out there today, but maybe my wife will relent and I can get there tomorrow. If so I'll let you know just what that combo was in the advert 'cause it is probably the best possible MPG-getter Ford could come up with.

 

axle ratio would be changed if i went to an overdrive transmission, to something closer to a 3.55 ratio.

 

Oh yes, ive towed with many a overdrive auto tranny before. my dad and i have two big block auto trucks, a 93 and a 95. i have always had manual transmission trucks for my daily drivers, so this truck is a bit of a different step for me.

a little background. we have several trucks, many of which serve only one purpose. one is strictly a tow rig, one is our heavy hauler, one is an autocross truck, etc. Clyde, my 81 f150, is my daily transportation, or, rather, is going to be, when the bugs are worked out of him. he needs to be able to seamlessly drive down the highway, putt around town, be up for long distance road trips, he may haul a little, he may tow a little, but will not do a lot of either one, as the bed will be empty more often than not. Basically, the truck needs to be a well rounded combination of every truck in the fleet, but needs to master none of their jobs. However, with most of the other trucks sporting big blocks or windsors, i would like to expect a batter mileage out of Clyde, with his straight six. im not being unrealistic, but the truck Clyde replaced , the Farm Truck, was an 87, straight six, 4 speed od with 3.55 gears, and got 19-20 mpg all the time while doing the above mentioned duties. so something above 16 mpg out of Clyde would be nice.

 

I think a 16 MPG target is being a bit conservative. Here's the advert I was talking about (no, I didn't go to the shop, but couldn't sleep due to the many naps I had today so got up and found the pic I posted previously) and, as you'll see, Ford says that a 300 six w/a 4-speed OD tranny in a 4x4 got 18 MPG around town and 24 MPG on the highway. Yes, those EPA #'s were always a stretch back then, but 16 shouldn't be hard to achieve at all.

If it were me I'd do the research to find out the RPM where the 81 300 got its max torque and target my RPM at 65, or whatever highway speed you expect to cruise at, as my RPM target. I say that because the class on Internal Combustion Engines I took in engineering school said that max torque RPM is the same as peak efficiency RPM in most cases. So, given that RPM I'd back into the gear ratio for the rear end.

This site says the 1981 300 six puts out it max torque of 255 ft-lbs at 1400 RPM. Oddly enough, that's almost exactly the RPM you'll get with your current gears and the AOD in OD. So, I would seriously consider that combination - especially since what I've read says that an AOD can be built to take gobs of power and not detonate. And, because an AOD has fewer losses than the mighty C6 and has the lockup converter to boot.

My only concern is whether an AOD can be set up to hang in OD w/o shifting when on the road and driving in hilly situations. If not you might want to consider either of the SROD or the M5OD. However, I'm not reading good things about the SROD's longevity and I haven't found out what its OD ratio is. I've found lots of ratio listings, but they are all over the map and seem to indicate that there were a bunch of variations on that tranny. And, you asked about the M5OD bolting up to the 300, but from what I'm reading it came behind at least the 302 and 351W and since they share the same bell housing bolt pattern it should bolt right up to the 300.

So that would seem to say either of those manual transmissions would be a good option. However, I don't think either of them will have the steep OD ratio of the AOD so won't bring the R's down to the 1400 range with your current gears. And, btw, if what you want is MPG I don't think I'd swap your gears to 3.55's as my guess is that the current gears will give you the best MPG. Maybe not neck-snapping acceleration, but that's not what you said you want. And, if you decide after installing whichever tranny you go with that the 2.75's are just too tall then you can change that pumpkin out for one with lower gears w/o touching the tranny. IOW, give it a try, you might like it.

But, there are other considerations with transmission selection. One is the expense, and having an AOD won't be cheap and you'll probably never get back the money you spend there via the increased MPH over the C6. So one of the manual transmissions would be a cheaper option, although you will have to find the dual pedal mechanism. And, you will have to figure out which way you want to go with clutch linkage - mechanical or hydraulic. Both have their issues, with the mechanical linkages you will find well and truly worn. Trust me, I've been there done that, and you can find out how I fixed the linkage I have in Dad's Truck Build. It takes some work, but the parts are cheap and the results are satisfactory. On the other hand, the hydraulic linkage looks easy but has what I consider to be a huge problem - it is highly likely to crack your firewall unless you head it off at the pass. You can search the forum for posts about that, but trust me - it happens. So you will need a firewall stiffener or you are highly likely to have the problem.

And then you'll have to teach your sweetie to drive a manual. Hmmm, maybe the AOD is the cheaper alternative after all.

 

from what i gathered a few years ago, the SROD and the M5od had the same final gear ratios... but, for the life of me, i can't remember what it was.

the number in my head i would love to see is 20 mpg, highway. some on this forum claim that that's already possible with my current C6 and cosmic high gearing.

the hot rodder in me wants to put in a big block, a top loader, and 4.10's, but we all know that's just counterproductive to mileage. fun as hell, but counterproductive.

i'm going to start looking into AOD's. my C6 is working well, so i'm in no hurry, i can shop for a deal. what cars or trucks should i be looking for?

 

A truck just like yours. I don't remember when they started with them, but there was a post about that in the last month or two. Search for posts on it and you will find it quickly.

 

In my opinion, the 2.75 rear axle ratio is too tall for an overdrive trans, even behind the 300.
In Ford's opinion, it was fine, and they even went one step higher with a 2.47 ratio.
However, this was only with a manual shift OD trans. With an AOD the tallest axle ratio I have seen was 3.00.

I should also note that these ratio's combined with an OD trans was a 300 only thing. A 302 with OD trans was pretty much limited to 3.50/3.55 depending on year/axle model.

 

RW - Perhaps you should explain what axle ratio your truck has so so BL understands that you know whereof you speak.

 

Well, I do have a 300, C6 and 2.75 in the 80. Then I added 31" rear tires. Then I did this:



I want to drop the axle ratio down to 3.25 or 3.50 for better towing ability, but I think the 300HP/400TQ (conservative numbers) 390 in the 74 makes it a more viable tow rig.......

 

My son's 1984 F150 began life with the 2.47 rear differential and the manual 4-speed OD. It could and did return 24-26 mpg @ 60 mph with pre E-10 gasoline. It WAS a dog with regard to acceleration [could go to almost 40 mph in 1st gear!!! and idled @ ~8 mph ], but actually cruised nicely on the highway as long as steep inclines were not the normal. If a hill could be seen in the distance, it was wise to shift down even before approaching the hill. One could cruise @ 65 mph in 3rd [1:1 ratio] and the engine was still quiet.

The rear gears were changed to 3.55's and the truck was more fun to drive, but gave up 2-3 mpg. The 2.47 set pinion gear was so BIG it looked like a pumpkin!

Based on my experience, the most you might gain will be ~ 3-4 mpg only while cruising. Check your math as it might take a l-o-n-g time to recoup your expense to change the transmission.

 

There you have it - two guys that know whereof they speak. I'd heed what they said, esp the part about it taking a long time to get the cost back.

And, btw, I recently made a jetting change to Dad's truck in a quest to get that last little bit of MPG. It made the engine a bit grumpy until it got warm, and even then it didn't "feel good". I put the jetting back to stock and am wondering why I ever changed 'cause the truck is so much more responsive, easy to live with, and fun. IOW, I wouldn't spend a lot of money and then discover I don't like it. Yes, you can always change the axle ratio, but then you are back pretty much to where you are - after spending a lot of money.

 

My 83 F100 was that way. Empty on flat ground it was great. Up hill / with weight or both and it struggled. But you downshift and it was ok. I was never a heavy hauler but for bulky things it was great.

My 86 F150 302 EFI / AOD gets 19.5mph at 55mph. It's a 4x4 with 31's and 3.55 gears...

I'd say keep your eyes pealed for the parts you need. The AOD isn't the evil thing that people say, it's got the lockup TC and you can always manually hold it in drive for 1-1 when needed.

I agree that it will likely be a long time to recoup your investment.

 

I'm very pleasantly surprised at the mileage my 85 gets with the same 302fi/aod/3. 55 setup. Never did get the gear ratio but my 80 with the 300 auto was a bit underwhelming.