I have been thinking of adding an overdrive to my 77F250 4x4 for some time now and I am getting to that point that I need to decide. I have researched the Gearvendors Overdrive, ZF 5 speed, AOD, E4OD.

I am looking for the least amount of modification and cost possible like most people. I am building a 521 stroker (street performance), and keeping my divorced T-case. I really don't want to keep my old 4-spd. I prefer going with an Automatic, so the C-6 is really the cheapest and easiest way to do, but I don't have an overdrive. It seems all the overdrive options I am looking at, I would probably have 2500 to 3500 invested if I do that, as opposed to spending about 300 bucks on a C-6 rebuild. I have done some basic math on fuel savings and to see how long it would take for the setup to pay off. Here's what I have figured. Take it for what it's worth. My numbers are just guesses and probably will be much worse.

Currently 10 MPG, 19 gal tank = 190 miles @ $2.20/gal = $41.80. Filling up once a week = $167.20 per month.

If I save 5 MPG now @ 15 MPG, 19 gal tank = 285 miles per tank. Essentially I would save $83.60 per month on gas if I get an extra 5 MPG.

If I have invested $3000 on a setup and take my monthly savings, It would take me 35 months to pay off my transmission setup. Is it really worth it?
Something to think about. And again, my numbers are just for general discussion, and not actuals. What do You think?

 

In the end, the decision to do it or not will be yours. If you don't mind waiting 35 months to start saving money, then go for it. One thing you did not mention was the cost of gas going up in those 35 months. If that happens (and we all know gas does not go down in price) then your break even point will get closer. I too thought about ways to save money, but like other peeps pointed out in a different thread, the best cost per improvement is a good old fashioned tune up. Nothing else makes sense from a financial standpoint. It just takes too long to break even and then start getting your money back. JMHO

 

A 50% increase in mileage is unrealistic, on top of driving a stroker even with overdrive. I'd be happy to end up at 10 mpg with that engine when its done The speed at which overdrive would "help", wouldn't poor aerodynamics hamper any increased efficiency?

The only mod I can think of that might land a solid 50% increase in mileage would be a deisel swap

 

That's kind of what I thought also. To get 10MPG with my motor will most likely be unrealistic. I should point out that this truck won't be a daily driver so my concern for fuel savings isn't really that important, but I wanted to figure some generic numbers to see if an overdrive would really be feasible for me. I think It would be nice to have just to bring down the RPM's on a drive down the interstate to drive the speed limit, here in MN 70 mph.

And I also thought that a 50% increase in fuel mileage is unrealistic with an overdrive and I figured if I couldn't get that, than what's the point of investing the money. 3000 bucks opposed to 300 bucks is a lot when It really isn't necessary.

 

There's nothing stopping you from driving 60MPH right now.
That 70 is the Posted Maximum.

That doesn't mean you have to drive that fast

For Hwy use just put some lower ratio gears in it.

What ratio does it have now,3:54's ,3.73's.... I didn't see it mentioned

 

I have thought of changing the gear ratios also. Currently I have 4.10 Gears and thought about changing to 3.54. I have a Dana 44 front and Dana 60 Rear. Also, I'm gonna go with a 35 inch tire. I figured if I go with 3.54 and 35 inch tires, I could do between 70-75 mph at 2500 RPMs. I think this setup with a C-6 tranny is probably the way to go. I will have less than $1000 in this setup probably. A lot less than investing almost $3000 in the drivetrain. So, If you think about it, is overdrive really worth it?

 

My truck is used as a daily driver until I get my brother's Ranger in March. And with the stock 460-C6-4.56 gear combo it has, I get 10 mpg on the freeway, and that is doing 50-55 mph. I know the speed limit is 65 mph here, but with the 4.56 gears, the 460 is revving over 3000 rpm with my 33 inch rear tires :-( I thought of an overdrive, but like you have already mentioned, it is too expensive considering the little amount it would save/help the situation.

 

Bucks, 35's and 4.10's sounds like it should be the sweet ratio for your truck. How often do you drive on the highway vs. how often do you putter around town?

~3.55:1 rear with 35's. . .

70 mph = 2386 rpm
45 mph = 1534 rpm
35 mph = 1193 rpm
25 mph = 852 rpm

. . .If you spend anytime puttering around town with this setup, you'll be lugging your engine pretty badly, and destroy your main bearings. Not to mention you'll more than likely burn your C6 up having to gun your motor to get off the line.

~4.10:1 rear with 35's. . .

70 mph = 2755 rpm
45 mph = 1771 rpm
35 mph = 1378 rpm
25 mph = 984 rpm

. . .Not ideal, but better overall. A wide ratio C6 would help overcome some poor gearing during take off from stops as well, just something to keep in mind.

But is overdrive worth it? For me, it would be worth it in the right situation. Namely, the same situation Nighteyez is in. Low gears in the axle for excellent off the line and towing capabilities, and the overdrive to help me keep speed on the highway without throwing a rod through the block

In these old trucks I wouldn't think of it as increasing mileage, but more like "gas consumption management" Just making sure I'm not burning anymore gas than is necessary. . .ie, 10 mpg is pretty good considering you could be getting 5 mpg with a poor tune, defective motor components or mismatched drivetrain

 

Franken-truck, Thanks for the info. Maybe I will just keep my 4.10 gears, 35's for tires, C-6 and see what happens. I'm finding that when I search the overdrive topic, I can't find many people who have actually put in an overdrive system, especially for a big motor and truck. How well would an overdrive actually hold Up? If I had more information, I may consider the overdrive setup more. It's alot of money. I'd hate to invest alot in the setup and have nothing but trouble, when a standard C-6 would hold up pretty well and would have really nothing invested in that.

Maybe more and more people will experiment with different overdrive systems in their trucks and more information will be available on how it works overall. I can always change if that's what I really want.

 

Thats not something I have really though about, your punched out motor might be to much for some trannies. . .You might consider the E4OD swap? They can build them to last behind chipped out PowerStrokes, I'm sure you can have one built to live behind your stroker. Not to mention having programable shift points would be sweet depending on what your doing

There are a few people on here with overdrive swaps, you just need to digg for them lol. Like this well documented NV4500 swap behind an FE.

https://www.ford-trucks.com/article/...nsmission.html

Your 460 swap should be nearly identical since you need only a 2wd tranny with your divorced Tcase. And make special note of the $1800 price tag

 

Yeah, If I wanted the overdrive, I think the E40D or the newer 4R100 (I heard they are better) would be the way to go. Either way, a pretty expensive swap. The thing I don't want, like I mentioned, is knowing for sure if one of those trannys would hold up for quite a while. I don't want to have to be replacing or rebuilding trannys later on down the road. That could get expensive. That's why I'm so hesitant in the overdrive swap, when I know I could count on a C-6. One of those things I guess. What it is boiling down to, would I really benefit from an overdrive for my driving purposes.

 

Just something else to think about: the Gearvendors OD is 0.78:1, so it would help a lot during accelleration by creating a 6 speed automatic with very close gear ratios, but the tallest available gear ratio isn't going to be that much taller than stock meaning your highway RPM wouldn't go down that much.

For comparison the GM TH2004R transmission I'm using in my 84 Cutlass has a 0.67:1 OD, along with the lockup converter helping even more. With my 3.73 gears in that car depending on tire size my RPM's at 70mph should be less than 2500RPM even with small 235/60R14 tires.

Not saying whats best for you, but if it was my truck I would be wanting to stick with a manual trans and try and go with something that has a taller high gear if you keep the 4.10's, or if you go with 3.50's maybe a trans with lower gears to compensate.

 

To me you have to see what your driving habits are. Mostly hwy or city. As you know and already stated a overdrive is of no use in the city.
Do you drive hard and fast or recreation?

I have a 77 F250 351M with 351 rear end and FlowMaster exhaust that I pull my 22' 5th wheel with. I though I was working it too hard,So I had a GeerVender Over/underdrive installed. LOVE IT. I grant you, It is pricey
I don't go any faster,But by being able to split the geers I go a LOT easier.
So for my needs it was money well spent.
I have had this rig over 9000 ft and pulled the step grade with easy.This is going to make my truck last a lot longer.
So like I said it is what every YOUR needs are.Forgot to minition that the big increase in milage. Can't say what it is,But I get a lot more miles out of a tank.
Don

 

I've got a mildly modified 400 and C6 in my '77 F150. I normally drive conservatively and cruise at about 4mph over the speed limit. That nets me about 10.7mpg. I spent a couple months driving no faster than 60mph, accelerating slower than any other car on the road including semis, trying tonnue covers, inflating tires, and so on. My best mileage was 11.79mpg. I gained a whole 10%. You will not make 50% (10mpg to 15mpg) unless you drive like a maniac now. The driving style will make more difference than the mechanics.

Also all of your choices seem to be decidedly towards worse mileage. 520cid? Bad economy, great power. C6 Automatic? Tough as nails, but sucks fuel. I don't know what to tell you. It's a big truck with a big engine. Gas mileage will suck no matter what you do, with the possible exception of a turbo diesel conversion.

 

521 stroker and fuel economy does not compute. why not efi 460