I'm looking to get my first IDI here soon, the truck's an 86 F350 Crew Cab 4x4 with a ZF-5 swap, haven't heard back about miles or rear end gear yet.

I eventually plan on throwing a turbo on it and an aux overdrive unit but that's gonna be out of my budget for a few years.

I know not to expect Civic MPG out of it, most of the driving it'll get will be highway (70 mile roundtrip commute to work until my other truck's fixed) so if I could get llike mid to upper teens I'd be happy.

Any other mods besides the overdrive unit and turbo to help make these things more driveable?

Thanks,
Dylan

 

Mine gets about 15 MPG, not a dually, 4wd with a c5 I think. I drive slow. It is my understanding, the turbo isn't cost effective but someone more qualified should comment.

 

If you have the old 3 speed automatic, it's a c6. The c6 isn't known for fuel mileage. My 7.3 idi got 12mpg with the c6, 35 inch tires, 4 inch lift 4x4 with 4.10 gears. I put the zf 5 speed in it and with no other changes I now get 15 mpg on the highway. If I didn't have the big tires and lift on it, I probably could get 17 mpg.

 

If you don't mind sharing, about how much does it cost to make that swap?

 

The junkyard wants $1000-1500 for a zf 5 speed. I got a wrecked truck for $1000 that had the zf in it, and then sold parts off it and the old c6 and ended up having about $500 in the tranny.

I then found out about the flywheel problems the original setup has, they came with dual mass flywheels and they wear out. They sell single mass flywheel conversion kits, back when I did it they were about $700. You can now get them for about $300 on rockauto. It comes with a new regular flywheel, but they can be a little noisy. That is why Ford went to the trouble to use a dmf, the smf makes your tranny sound like it has gravel in it sometimes.

I didn't go with the $700 for the kit I am cheap. I found out on another board there is what they call the Lucky Mod. It was a guy on there whose nickname was lucky, and he took the dmf apart and bolted it solid, and then used a clutch disc with springs in it. That is what I did, it's still going after 8 years.

 

Why do you say that on the turbo?
I guess it depends on how much you will drive it after the turbo install?


Not a Ford 6.9 but a 86 K5 Blazer 4x4 with a 6.2 diesel 4.10 gears and 700R4 OD auto trany.
At 100K I added a Gail Banks turbo kit best thing I ever did.
It has be 14+ years since I had that truck and I am sure I was getting in the low to mid 20's mpg and that was pulling a open car trailer of about 5K lb @ 70 mph.
I sold the Blazer with 230K in 02 for $500 only because I had no place to keep it.


I can tell you I don't get close to that with a 02 Dodge 360 with auto OD 4x4 with 3.92 gears doing 60-65 mph more like 5-8 MPG pulling that same trailer.
Even not pulling the best I have gotten is 17 mpg @ 60 mph with this truck.


I think it will do pretty good with a turbo and OD unit for MPG.
Dave ----

 

Badly tuned C6 automatic trucks with 4.10 gears can drink 10-12 mpg. More when hauling...

My F350 crew cab 4x4 T19 four speed with 4.10 gears got 15 mpg. With the gear vendor overdrive engaged (3.20 effective ratio) this went up to 17 mpg.

My friend bought a Supercab 4x4 C6 automatic truck with 3.55 gears. It got 14-16 mpg. We ZF swapped it (2.77 final drive ratio in 5th gear) and even loaded with 2,000+ lbs. of cargo the mileage increased to 21 mpg at 60-65 mph, 18-19 mpg at 70-75 on the interstate.

These diesels need overdrive gearing to reach good mpg. There is a guy who was able to *almost 25 mpg with his 6.9, and most of that was with gearing. Here is his thread that you might find interesting.

https://www.ford-trucks.com/forums/7...rs-thread.html

As far as the turbo, it is nonsense to say that it is not cost effective. Of course it is not if you are talking about mileage gains. The only time it really helps is when you are towing or hauling really heavy, it may buy you a mpg or two. The rest of the time it usually results in less mpg because it produces so much more power with ease that your driving style naturally adjusts to take advantage of the extra get-up. So if you are trying to justify a turbo because of fuel savings, just stop. However in terms of performance and capability then absolutely yes. It is totally worth it and the very best thing you can do to the truck. Used turbo kits are just not that expensive. But if your goal is mpg, invest in gearing before thinking about a turbo.

 

Well, that truck sold today unfortunately. So the hunt for an IDI continues, there isn't too many for sale in my area now it seems

 

I wasn't planning on installing a turbo for mileage gains only, just seemed like an added bonus. I was wanting to throw a turbo on it to just give the truck a bit more go

 

All my research was regarding MPG. If you want more power and don't care about MPG, then cost is less of a factor, if it is cheaper than buying a new truck, then go for it. If MPG is important, you have to drive a lot of miles to make the turbo profitable. I haven't read about the gearing but will do so immediately, thanks for the link.

 

I've had my IDI for a few months and I love it. It gets lousy mpg though. Better on the highway but usually I just cruise around town on the weekends with it. Sorry, I can't give you a number. The fuel gauges are broken.


Hopefully another one pops up soon you can look at. They are cool trucks.

 

I have a 7.3 IDI, no turbo with a ZF 5 speed and 3.55 gears. I normally average 16-18 mpg with it.

 

You hear old stories of these diesels getting close to 20mpg. Not so anymore. When they changed the diesel fuel and went to the low sulfur stuff, it zapped it and you can't get those mileage numbers like you used to in the old days. You may be able to get close with a double overdrive or the 3.55 gears and flat ground, but it's way more difficult now.

 

That's a bummer.

 

Um, you don't need the fuel gauge to calculate fuel economy; it's impossible to calculate fuel economy with a fuel gauge anyway.

As for ULSD/mileage, I've read that it's an urban legend. The cetane level was boosted enough to provide pretty-much the same energy density. I've never run a vehicle on both types of fuel (who has?), so can't provide proof either way.

I see no reason you can't get 18 MPG or more with a ZF5. My '85 NA with a T19 and 3.55s gets 15-17 all day. I think double overdrive would be overkill, and financially would never pay off. I know it's apples/oranges, but our '95 PSDs with ZF5s and 3.55s get 20-22 MPG, provided we don't go over 2000 RPM.