I read a few threads where IDI's get 20+ hwy mpg.

Well I need a truck for my life projects, and now my ex moved far away. So I need to do bi-weekly 300 mile trips to see my boy. So I thought a IDI tweaked for mpg could suit the bill.

So is 25+ achievable if the truck gets a bit closer to glorified car status? Like take a 250/350 have the suspension tuned down to light duty, and ditch the D60 for a SF 8 lug?

I can not get a car as I am on a 1 in 1 out policy for quite awhile, and rather have a truck at this point. Not that I am opposed to cars, but now I am a truck guy.

 

I think 25 is a pipe dream. Even 20 average is hopeful. both my trucks are well tuned and get around 16 to 17. I had a 2001 GMC 5.3 liter auto gasser that saw 20+ even in city driving constantly, 17 pulling my boat! For what you spent trying to squeeze even 20 out of an IDI, you could pick one of those up.

 

25+ isn't happening. Not consistently anyway. High teens overall average and lower 20's on favorable highway runs is more realistic for an mpg tuned IDI. These things are just plain heavy. If your preference is just "not a car" but you still need good fuel efficiency I would second the suggestion for a lighter gasoline pick up. The difference in fuel cost and maintenance on these is against you if you don't need the heavy duty capabilities and towing power of the diesel. I love mine, but it is not practical to commute in it. Fuel, oil changes, tires etc., will eat your bank account alive.

 

I've had 3 IDIs: one factory turbo E4OD F250 4X4, a non-turbo F250 supercab 4X4 ZF5, and a F250 regular cab 2WD ZF5 with the Banks Sidewinder kit. All 7.3s, all with 3.73 rear ends. 15-16mpg highway with the E4OD truck, 17-19mpg highway with the ZF5 trucks. I haven't towed enough to notice a difference between the turbo and NA truck.

You could probably get low 20s on the highway with the right rear end and a ZF5, especially if you didn't mind giving up some towing ability. Really your only expense will come from the "meh" mileage; oil changes are cheap if you buy bulk oil from WalMart. Sure the components are heavy duty and expensive to replace, but they are designed to withstand very high loads and if you're only using it for DD use they will last forever (i.e. brakes designed to stop 15k of truck and load only being used to stop 7k of truck). Buy a good set of Michelin highway tires and drive the sucker! I can attest to the extreme simplicity of repairs on these vehicles as well, so you have that going for you.

 

Cummins do pretty well with the right gears and a conservative power level, probably better than an IDI (given the smaller displacement), but given the initial cost of a diesel truck plus the cost of diesel now, you're better off getting a newer gasser like akamacgyver said. Most of them can nail down at least 20 on the highway, plus they're newer and more comfortable. Not sure about your area, diesel is almost a dollar more than 87 octane here. If I wasn't towing a very large trailer, I wouldn't want to be putting a lot of miles on a diesel right now.


I hated taking mine on the highway- it had 4.10 gears so I had to keep it at 65 to not ruin the mileage completely, and the cruise didn't work because it had the typical jumping speedo.

 

back when diesel fuel had a cetane rating in the 55-60 range, 20+ mpg on the highway was no problem even with 4.10 gears and 4 wheel drive..
now that diesel only has a 40-45 cetane rating you will be lucky to see 16 mpg doing the same driving

 

So this analysis would be much easier if you listed some constraints: i.e. how good are you with a wrench, how much are you looking to spend on a vehicle, do you have other requirements like carrying people in comfort in all of the seasons, etc.

I use my IDI when it's just me and I'm just hauling relatively light or running errands. I have a F250 Superduty with the 6.0 for heavier work or when people are going with me and I want comfortable cruising. The wife has a diesel VW Passat that will get 50+ mpg on the highway cruising at 76 mph...different vehicles for different needs.

 

Banks Powerpack exhaust on a 7.3 with a ZF and 3.73 rear gear in a 2wd 69 F250- It gets high teens not being nice to it, and over 20 on the freeway. As a six cylinder, the truck got similar mileage but it has a lot more power now.

 

If mileage is the #1 concern, then the IDI f250/350 wouldn't be my choice of truck. They do decently for what they are....a heavy duty truck that can tow reasonably well for their price tag. Just have to decide what you are willing to trade off.

A couple of other options with the IDI are:
1. They can run on many different fuels. Waste motor oil, waste vege oil, bio etc. This can make them cheap to operate in the correct situations. Just realize you will probably have to replace your IP/Injectors more often with alternate fuels, so do the math to see if it benefits you.

2. You are talking about making the 250 lighter. This could help, or you could swap the IDI into an f150. Partner it with a manual trans and hwy gears. If I remember correctly there is a person over on oilburners who did this. I think his mpg was pretty impressive.....maybe 30+ IIRC. Don't quote me on the exact mpg figure since I am going from memory on something I read a couple of years ago.

Remember to figure other costs in to your operating cost too. Insurance, registration, maintenance etc.

My '88 e250 gets in the 17-19mpg hwy range under the right conditions. 3.54 gears, c6 w/aux OD, flat ground and CC set at 60-65mph. Winter time I get in the 14-16mpg range, but I don't try to drive conservatively since it doesn't seem to matter. Trucks usually do better than vans due to better areo.

 

If you had a 2wd single cab with 3.55 and OD, especially the E40D, with a turbo and very conservative pump calibration, were willing to drive 55 i would say 25 is definately attainable.... practical.. no. My 79 - 7.3 ZF, 3.55 and 35" tires pulled down 24.6mpg. However my average speed was 45-55mph. That said, it has 6" of lift, 35x13.50 R18 rubber, draggy brakes, and lets not forget, 70s pickups were not known for thier aearodynamics. Also a supercab flareside. Under normal circumstances, IE not being impeded by traffic during road construction, and driving like a person that really enjoys my stacks, i would average 18.8 over 7 tanks 50/50 driving. I think my dads 88 7.3 / C6/3.55s gets somewhere around 17/18 hwy, 2wd single cab, but no lockup and no OD. E40D or ZF, 3.55s, nice big air dam in the front and a tonnea cover would net you low 20s consistantly i would wager.

 

Low to mid 20's are attainable with the right setup as others have noted. I have a buddy who was getting 25 pretty regularly on long trips. But that is with a zf5 swapped into his standard cab 6.9 2wd with 3.55 gears. Basically THE most efficient setup.

Cummins is a different story, and "mild" tunes are not needed to achieve stellar fuel efficiency. I regularly get 23 in combined driving out of my extended cab 4x4 duallie and I'm somewhere around 400-450hp at the crank. Direct injection with a pump that does timing (P7100 does not) have pretty durned good efficiency potential. I suspect my engine setup in a standard cab 2wd would be capable of high 20's. But for context, when I got the truck with a preturbo exhaust leak and a sticky timing actuator in the IP, I was getting 18 topps and usually more like 16.

Cummins trucks are pri$ey though. And the front end on the coil sprung 4x4's is pretty junky.

But with my Ford, I expect about the best I'm getting with 3.55's and a 4 speed is about 18 on a good day with a tail wind. I hope to get into the low 20's with a ZF5 once I find the time. I would be *really* surprised if I ever attained 25.

Also, I wouldn't expect much from tinkering with the suspension, but if you can live with a 2wd, I bet you'd pick up measurable economy improvement, and the purchase price will be lower too. I'd look for a truck with a ZF5 and 3.55's. Driving style with diesels is more pronounced in it's effect on fuel efficiency than gassers IME. Obviously don't lug it but keep the rpm lowish and load it up rather than wind it out.

 

Shorty F150 2wd with a Perkins 4cyl! Theres a fuel efficient truck!


But as others said, upper teens is pretty much realistic. I hit low 20's regularly in my Club Wagon, 3.55 with E4OD, but that's cruising around 55mph.

 

totalloser does bring up a good point about diesel engine operating parameters that I recently have noticed while driving my 6.0 and watching my Scanguage II. The engine consumes FAR less fuel under lower RPM/higher throttle (so there is no throttle, but for the sake of ease of discussion I'll call it that) positions than it does at higher RPM/lower throttle positions. They definitely are more efficient loaded up than wrapped out (as long as you aren't lugging them to their detriment).

 

I've worked extensively to mod my van to make it as efficient as possible. 25mpg is nearly impossible.

I installed a GVOD, synthetic lubes, aluminum driveshaft, a turbo, bigger tires, PV array, etc. With my mods, I was getting about 20mpg.
Also, i run WVO, which saves money, but I still have to pump grease in the garage which eats up some of my time. The more efficient my van is, the less grease it uses, the less I have to pump.

Things to do on my list are: more efficient torque converter, e-fan, e-fuel pump, touring package, 3.55 gears, etc.

Regardless, the amount of money you spend eco-modding your truck would get really expensive. I just like tweaking my van, so it doesn't bother me to spend the money.

(Note, don't install a turbo to get better mileage. It's just too much fun to drive. I used to drive 55mph everywhere, but now with the turbo, i drive 70 and pass everything in sight. Some times in the city, with my turbo and my new bad driving habits, i get as low as 12mpg)

 

If tinkering is on the table, and fuel efficiency is king...

short bed standard cab 2wd f150, small block ZF5, 4bt with turned up VE pump, (many 4bt's came with the small block ford adapter) and pick a choice gearing. Keep in mind once you are talking about a 9", gearing *taller* than 2.5:1 is available, and the 5.72 first/reverse of the wide ratio ZF will even it out decently. I bet it would break 30mpg. Heavier coils would be a must though.