Hi all,
I'm new here. I've been driving and modifying Ford trucks for over half of my life and currently own and maintain four of them. My pride and joy is a '84 Ford F-250 4x4 I did a frame off restoration on about ten years ago. The truck originally had a 351W with a C6 automatic, but I switched it over to a 460 with a four speed manual from a donor truck. I rebuilt the engine myself, it has:
Flat top pistons with large chamber (96cc?) heads. The heads have been mildly ported on the exhaust side with a three angle valve job, also shaved bringing compression ratio to around 9.4:1. Camshaft is a mild Crane Powermax, I can't remember the specs but its around 212-218 duration at .050". The timing set is a '68-'72 non retarded style, cam timing set straight up. I'm running an Edelbrock Performer intake with a Holley 4 bbl, MSD 6-AL ignition box with Blaster 2 coil and Super Conducter wires.

I love the truck and it has tons of power, but the gas mileage is horrible. The truck has 3.55 gears and if I try to keep my foot out of it I still only average about 9 miles per gallon. The engine runs about 2300 RPM at 60 MPH. I've located a complete '88 F-250 with a ZF 5 speed I would like to swap in. I've gone through everything and know what to expect if I swap it in. I'm sure someone here has done this conversion before, just wondering how much of an improvement in fuel mileage I can expect to see? I would be thrilled if I could 12 miles per gallon out of the beast, but is this expecting too much? I'm just trying to justify the expense of converting this truck and whether its going to make a significant improvement.

Also, I was wondering if a good set of headers would make any noticeable improvement in fuel mileage?

 

I did the conversion. I run 4:10 gears instead of the 3:55's. I got a 4.5mpg increase going with the ZF instead of the C6. I was able to get a bit more by installing a vacumn gauge and and adjusting my driving habits to keep it happy instead of watching the tach.

 

Wow! I'm definitely going ahead with this. I did have a vacuum gauge in this truck at one time, I'll have to put it back in. Just curious, what is your engine RPM at 60 mph with those gears, and what kind of mpg do you average? I was expecting to see around 1800 rpm with the ZF and 3.55 gears.

I checked everything over on my truck yesterday and it looks like the only snag will be the transfer case shifter. I just installed new carpet a few years ago so I want to keep the shifter in the same location. I'll have to cut a hole in the ZF floor pan and I think I have figured out a way to redesign the shifter linkage using Heim joints so I can keep the transfer case shifter in the stock location.

Anyone else who has done this conversion, I would love to hear your results.

 

That rpm at 60 is about right. I believe the 5 sp has the same overdrive ratio as the 4OD (.70), which I have and run at 1600 rpm with a 3.08.

 

I cant remember what my rpm at 60mph is, I dont drive the truck too often anymore. I am taking a road trip this weekend so I will try and remember to check. I do remember that when I had the C6 with the 4:10's I wouldn't go any faster than 70mph. With the 5spd I can comfortably bury the needle on the speedometer.

My mileage is always changing. I was getting 8mpg with the C6, it increased to 12.5mpg with the 5spd, and then dropped back down to 10-11mpg when I replaced the factory carb with an edelbrock. The extra power was worth it though. I do have a lot of extra weight on my truck though which helps kill my mpg as well. My bumper is heavy enough I had to get extra springs to keep the front end up.

 

I have a 86 F-250 with a 6.9 and 4 speed manual transmission. I would like to put a a overdrive in but don't know what transmission to look for. I know some of the newer trucks even came with a 6 speed. The only problem I have run into before is that when they changed to electric speedometer's I can't hook up my manual speedometer.

 

A ZF 5spd from behind a 88-91 7.3 diesel motor should work. They share the same bolt pattern.

 

But will I be able to hook up the speedometer?

 

Is your truck a 4x4? The speedo cable is mounted to the transfer case if it is. If not, the speedo cable should attach to the tail housing of the tranny. As long as the donor truck is pre '92 you shouldn't have any problems with a 2 wheel drive; 4x4's should be able to use a ZF up through '94.

I'm doing the conversion right now and its going very smoothly so far. I even discovered my original NP 208 transfer case will bolt right up to the ZF, although its about 2" shorter in length so I'm not sure if I'll have driveshaft length issues. I'm hoping by tonight I'll have it together enough to move it under its own power again, but the transfer case shifter linkage is going to take some time to figure out.

 

Just a quick update...

Its moving under its own power again but I haven't road tested it yet. I don't think this swap could have gone any more smoothly. My original transfer case bolted right up, the drive shaft lengths were unaffected, slave cylinder fits, and the original floor pan even fit with a little minor hammer work in one small area. The best part is my original transfer case shifter lever bolts right to the ZF and ends up being in the exact same location on the floor, absolutley no modifiction needed! I had expected this to be an issue given the location of the shifter on ZF equipped trucks, but it fits perfectly.

I only ran into two unexpected surprises. First, I knew the clutch was different but didn't realize the ZF pressure plate uses a wider bolt pattern, so it wouldn't fit my flywheel. Fortunately I had the flywheel off of the donor truck, so I just switched. Second, the cross member is moved back about 2" from the original location so the two holes in the lower frame rail (one on each side) won't line up. I'll have to drill new holes in the frame this weekend. Other than that I'm very pleased with how easy this conversion was and I only wish I had done it sooner. I'll try to post some pics as soon as its all finished.

The worst part is with the forecast for gas prices this summer I don't know how much I'll be able to drive it now.

 

did ford use a 6 speed that would fit on a 6.9?

 

I know ford used a 6spd, but I'm not sure what it was put behind.

Hey Rowdy, did you remove the old AT lever or leave it on the column? I left mine on just to mess with people. I did accidentally try and use it once after my conversion though.

On the crossmember deal, I used the crossmember from the donor truck with my original drop brackets. I still had to use the tranny mount backwards and elongate the holes to get it to fit. Glad to hear the t-case worked out, it looked like it would but I used the BW1356 from my donor and cut a new hole in my floor.

 

The six speed was used in '99 and up Super Dutys. I'm not sure how much work it is to install one, but I think the final overdrive ratio is about the same as the ZF. They are quite large so I'm sure the swap wouldn't be anywhere as simple. If you still want to do it, I know a few guys over at thedieselstop.com who have installed one behind an older diesel.

I took the automatic shifter handle off. I wanted to keep my tilt column but didn't have a manual column with tilt, so otherwise I would have switched it. But...I'm a real nut when it comes to details and I love the sleeper look. That five speed shifter just didn't look right for my truck so I stopped by the local welding shop and bought a piece of 5/8" stress proof rod and bent it to match the contours of my original four speed shifter, welded a square stub on the end, then put the four speed **** on the end. Total cost was about $3 and two hours worth of time. It looks totally stock, no way anyone can tell its a five speed.

I pulled the drop brackets off the donor truck. They curve back and move the cross member back about 1 1/2" so the mount fit perfectly. I only had to drill two holes in the frame but otherwise the entire conversion was a rather simple bolt in.