I'm working on a 7.3 swap into an older truck, with a ZF, and was wondering what other people with similar setups have seen for mileage.
Banks Powerpack filter housing, y pipe, and full 3" exhaust, and 3.73 rear gears.
The truck I pulled this engine from was a lifted 4x4 with 4.10 axles and 35s, and still got 13 towing a decently large trailer. The truck it's going into got 12 tops, and 7.5 towing with a nice new and decently built 360.

 

17-19 average mpg neighborhood id say.5 mpg(ish) less towing.
for 3.73's the idi era, you had to have the dually with 3.55 axle and the 215's rather than the 235's.those lucky to have been in this gearing range,must of loved their trucks.
what year pickup are you working with?

 

I hit 14mpg in my motorhome this summer in Florida

Cruise speed of 55mph, dually 215/85R16 tires all around inflated at 75psi rear, and 80psi front. N/A 6.9, C6, 3.54, stock exhaust.

 

This is what my truck generally gets empty: (see sig for specs)

24 MPG @ 55 MPH
22 MPG @ 60 MPH
20 MPG @ 70 MPH

All time best was 24.8 @ 55

and the best @ 70 was 22 MPG

I have crazy tall gearing but also an automatic so I don't think a 5 speed with lower gearing would result in significantly different results if everything is in good condition.

I've been meaning to try synthetic differential oil but never got around to it. For now, the truck is parked since I don't needed at the moment

 

The truck is a 69 F250.
Here's a shot of the engine in it's new home:

 

87 e350 6.9NA, c6, 3.55s, 235/85r16s, normal scale weight 7900
gets 16 on summer fuel, 14.5 on winter fuel, all driving is mixed highway/city with occasional towing
the turbo and the ZF should both have significant improvements over my numbers.

 

If your building an IDI in a 69 Ford, who the hell cares about fuel economy?


When your driving around in something as awesome as that, that be the least of my worries

 

My 93 gets right around 17 MPG @ 75 Loaded.

 

I've found that, relatively speaking, I can get a rough guess on mileage by crunching some ratio's.

For example, my dually with a C6 and 4.11's runs about 14-16mpg, but weighs (empty) about 6500-7000 lbs. (Looking at Josh's van above, they are comparative, his taller gearing is offset by heavier weight and wonderful van aerodynamics.) When I got my crewcab, I figured that going to the E4OD, and having 3.55's, that I would drop almost 1000rpm at cruising speed (55-60mph usually), and gain about 25-30% on fuel mileage. I haven't figured the exact numbers, but roughly I seem to be getting close to 20mpg with that truck.

Your project will weigh less than most trucks these motors are in, likely closer to the weight of David's truck, but without the tall gearing he has. My guesstimate at this point will be that you will be roughly around the 20mpg mark or thereabouts.

As John says, regardless, you'll have a different, fun truck, and 1-2 mpg really shouldn't be a factor IMO!

 

Sweeeeet!!!!

Always loved those older repower projects.
As mentioned that is a light truck and you will be above the regular guestimates.

I'd say 22-23 is easily within reach if you stay at, or below 65 MPH. At or below 55 could see 25 MPG. But again, it depends on the condition of the engine and fuel system. If everything is decent, then those numbers are attainable (don't let the cummins crowd tell you otherwise).

I should also have added earlier that my numbers assume a relatively hilly terrain up here in BC.

 

I hope you keep us posted on this swap, as far as the problems and how you overcome them.
We had a member start to install one in an older two wheel drive and scrapped the project because of some of the problems he ran into. His was a 66 if I recall right, but the frames are basically the same.
It depends on your goals for this rig, but if you are after mainly mileage, you can go with some 308 gears. That also depends on where and what type of driving you will be doing the most of. If mostly city, slow and a lot of stops and starts, what you have may be the best.

 

Typical short term use is going to be about 15miles on the highway at 65, with minimal in-town driving. Some light hauling, and rural highway driving at 50-55. I hate driving in this town, so I go an extra three miles on the highway to avoid it.
Long term use is going to be hauling a 20' gooseneck, again on the freeway, with occasional use as a commuter.
If I can nearly double my mileage, I'll be thrilled. At 12mpg, it just wasn't feasible to use this as a commuter, and I really need to retire my current commuter.
As far as challenges, the clutch master cylinder is a royal pain but I have that sorted out now. It'll be going in that little pocket between the body seem and inner fender well just to the left of the brake booster.

 

There is one potential pitfall I see here. These engines have a long warmup time especially if you are in a cold climate. Anything below operating temperature will reduce your mileage. All my numbers are favourable because of the long distances I was driving, which effectively overwhelms the wamup period at the start of the run.

For short distances of 5 miles or less in stop and go traffic, I am not alarmed to see my truck get as low as 13 MPG empty (thats usually with a heavy foot though).

 

Be sure to reinforce the firewall where you mount the clutch master cylinder, or it will give enough to cause problems and eventually crack and not work at all.

 

I have a nice big chunk of 14gauge steel to cut a reinforcing patch out of, so no worries there.
I also plan to use the block heater all winter so that should improve mileage numbers, at least somewhat. When I do the rear brakes in the spring it's getting synthetic 75w90, for an extra little boost too.
These don't seem like an overly common engine to swap into anything, I don't understand why. They are pretty durable, and have a very usable powerband, with much more automotive-style power delivery than some of the other more commonly swapped engines.