Best MPG engine Size and setup

[78 Chevyrado]

I Think this summer (or at least when it warms up)  I'll be replacing the stock 350 and th400 in my truck.  I would like to go the most fual efficent route...  I know, thats a stretch...

I'm now a general contractor and as such, I sometimes drive 10 miles one way or 110 miles one way to the jobsite.  Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...

I'd like to keep it with a SB chevy.  I'd go with the new LS engines, but if something doesnt work out, The truck absolutely can't sit around waiting for parts and such.  If the trucks sitting, I'm losing about $300 per day..  I'm thinking so far to keep the 350 but put in a 700R4 Built heavily or a 4L80E.  It will have OD.  I need to be able to go at least 75MPH without the twice the fuel consumption penalty.  I know it will use more fuel, but not double I hope...

78, C20 with 1 ton suspension, 33.6" tires, 3.73 rear, and it weighs about 7000lbs with all my junk in it which I have to carry all the time.

I'm thinking a 350 with an OD trans and something like that FAST EFI Setup.

I know the LS engine like a 5.3L would do best, but if I do that, it has to be up and running in a week.  I could do that with a SB and EFI.  I'd want a roller cam block  or a retro cam. 

What do you guys think? 
350?
400?

5.3L
6.0L

4L60 or 4L80E?

I can do the work, but after I get my parts together, I want it finished and running and ready for work in 7 days at the most.  I'll lose a lot of money otherwise.  It could go a couple of days more, but if it goes much past this, I'll have to rent a truck...

I can go a couple of days without the truck, as I can drive the car if the job doesnt require too many tools, but every time I dont have the truck, the tool i need is 100 miles away in the truck... ya know...

So with these needs, SB or LS?  700r4 or 4L80?

Thanks Fellas!

[jaredts]

What mpg's do you get today?  What kind of shape is the 350 in that's in your truck now?  How about just dropping a 700R4 in and see what that does for you?  A th400 is about the toughest automatic ever made, but one of the worst for fuel economy.  That alone would make a big difference.

[1HI4x4]

A 4bt Cummins with 47RH would probably get 20 mpg in your truck. I'm a Chevy guy to the core, but I'm also a diesel guy.

It would be a easier swap than an EFI swap would be.


Here's a link to some fuel mileage numbers, the bottom truck is a two wheel drive Durango, all the other are 4x4 jeeps with 33" tires.

http://www.4btswaps.com/forum/showthread.php?10409-4BT-MPG-Challenge

[Remon.]

Very curious on the outcomes of this topic!

I'm in a comparing situation as you're in. I want to use my truck for work (starting general contractor) after it's ready for the road and the Dutch climate.

I've almost the same setup as you have: '77 C20 Camper special, so 3/4 ton and 1 ton suspension, sbc350, th400 and probably 3.73 or 4.10 gears. I only drove the truck 40 miles home so I don't know the MPG's, but probably not so good

My formal truck with the same setup, except for a LPG installation and 4.10 for sure did on a good highway day: 7 MPG but more often under 5.5! And especially in Holland fuel isn't cheap.

I was thinking of swapping my th400 with a '87+ th700r and just see what happens. Friend of mine had pretty good results with this swap and is also very pleased with the low RPM's.

[audrima]

I to have a 78 C28 350 sm454 no clue the gears in the rear.

I plan on replacing the sm454 with a sm456 Going to freshen up my 350 rebuild the carb, new exhaust manifolds, replace the hoses and belts, full tune up, make sure it's in the time, tighten the valves. new oil pump new water pump.  etc then rebuild the rearend with better gas friendly (if I don't have it already) gears. then just the general stuff. make sure the tires are good and filled right make sure it's aliment is good Etc.

That will get me about 15-17mpg if I do it right,  but in the long run when I fully restore her I am going diesel when I find one that I like.


Mariah =^_^=

[DustyRusty]

I agree with jaredts on swapping in a 700R4 for cost reasons alone.  Get a later transmission because the earlier ones were prone to cracking around the bellhousing, IIRC.  I don't know how to ID the difference, however.  I'm sure others will know.  The disadvantages of a 4L80E are that it is usually more costly and you will need a stand-alone computer to control it.  If you decide on an electronic transmission, consider a 6L80E or 6L85E if cost isn't critical to you. 

Rusty

[Edahall]

  Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...

No engine is going to get good mileage at 75 mph in our non aerodynamic trucks.  That said, OD is a good start but there is a lot more to gain through aerodynamic mods.

[78 Chevyrado]

  Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...

No engine is going to get good mileage at 75 mph in our non aerodynamic trucks.  That said, OD is a good start but there is a lot more to gain through aerodynamic mods.

This is true, so I'm looking for the best possible while driving a brick $%^& house down the road...lol


Right now, I have the OE 350 and TH400 and get about 12-14MPG...   as long as I stay under 65MPH.  The Truck has 143K on it so does the 350 and TH400.  They still run good only smoke at startup... bad valve seals....  all else is good.  don't know how long it will last though, as some days I'm so ready to get home, I'm running 85MPH with traffic just to get to my shower n bed faster....   It's gonna blow one day, I knoiw it and expect it, I just hope to be as prepared as posible when it does go.....lol

[Da67goatman]

If you wanted the BEST mpg while still having the power to pull a house you'll need turbos.  No other way, small displacement gets you mileage while turbos give you the power when you need it.  I would look into the new Camaro v6 and turbo it.  300hp and 24mpg factory, throw in the turbo and upgrades associated, 500hp and 26-28mpg.
 Pretty sure it comes with a nice 6 speed auto so you'll always have the right gear too boot.

[Edahall]

  Mainly my current setup is killing me because I don't go over 65MPH, ever, because if I go 75mph vs 65mph I use almost twice the gas...

No engine is going to get good mileage at 75 mph in our non aerodynamic trucks.  That said, OD is a good start but there is a lot more to gain through aerodynamic mods.

This is true, so I'm looking for the best possible while driving a brick $%^& house down the road...lol

Yeah, I understand what you mean.  Keep in mind that at 75 mph, a tremendous amount of energy is being used to overcome wind resistance.  Our trucks are not aerodynamic so a newer 5.3L in a newer Chevy truck is not going to get same mileage at 75 mph in our trucks.  Also, gas engines tend not to be very efficient under heavy throttle.  A lot of the fuel is being used to prevent pinging.

The best possible mileage in your truck at that speed is going to be either the GM 6.2L diesel or Cummins 6BT.  I can comment on both of these since I own each.  My 1982 Suburban (2 wheel drive, 3.08 axles, 700r4) with the 6.2L gets close to 30 mpg at 55 mph and at 75 mph, it'll get around 20 mpg.  The Cummins 6BT in my 1990 Suburban (4 wheel drive, lifted) gets 26 mpg max and at 75 mph, 18 mpg is what she'll do.  However, I have a feeling that the Cummins might even do a little better than the 6.2L if it were in the 2 wheel drive Suburban.

If you want to stick with gas, I would probably put in a 5.3L or 6.0L engine.  However, at 75 mph, I doubt you will get very much more than 10-11 mpg.  I have a friend with a newer Chevy 3/4 ton that has the 6.0L and he complains of only getting 12 mpg on the freeway but he drives fast.

Also many times, a bigger engine will get better fuel economy when worked hard.  Several weeks ago, my friend and I were following each and he's got a 2012 Chevy Impala and I've got a 1997 Lincoln Mark VIII (295 hp engine).  With drove 80 mph the whole way.  When it came time to fill up, he got 23.5 in the Impala and I got 24.1 in the Lincoln.  That said, his Impala will get over 30 mpg driven easy but the most I've ever been able to squeak out of that Lincoln is 26 mpg. 

[Stewart G Griffin]

Is 7000lbs. the max you will ever need to move?

[Captkaos]

As for aerodynamics and fuel efficiency our trucks have about a .41 drag coefficient vs say a Corvette with .30 vs say a mid 80's Caprice at again, .40...  Needless to say, our biggest issue is frontal area.  And there is a formula that will calculate the drag limited top speed of a car and it is based on the horsepower vs Cd..  That being said most trucks are going to max out at about 22-25mpg if driven at around 65-70 mgp.  My 1987 got 22mpg on all the trips we took, it was an old underpowered 305, but it got 22mpg all the time on the highway,  In the city, it typically got about 15-17 with 55mph speeds.  That being said, If you are after efficiency with gas you can go with a big motor making alot of torque that can be lugged around all day turning no more that 2000rpms or a small motor that can make good torque and be overdriven under low speeds.

If you are wanting to move away from pre GEN III motors, I would recommend a 4.8L or a 5.3L with an overdrive if mpgs are your goal.  You should be able to achieve 22mpg, and if you are after cost, find a low mileage pullout 4.8L/4L60E combo as they are skipped by all.  A friend got one to put in his 70C10 for less than a grand and it had 15K miles on it.

If you are wanting to stay with a GEN I, I would build it for efficiency and not power. I would probably start with an overdrive unless you are geared high already, but if your numbers are 12-14 in the city, I don't think it will improve much with either option.

If your choices are a GEN I with an OD and an aftermarket system OR a LS motor and OD, HANDS down I would go the LS route as the EFI systems on them were designed to be driven under varying conditions and have a 100K warranty.  aftermarket injection systems were built to achieve Power in Hobby categories.  If the GM ECM dies you can get one for nothing.  Not going to be the case with an aftermarket setup...

[Edahall]

The best fuel economy at 75 mph I know of is a guy on a different board who has a early 80's Chevy 6.2L diesel that has been lowered.  It's got a 700R4 and 2.73 gears.  He's claiming 27 mpg with the setup and I believe it.  I've thought about lowering my Suburban to gain fuel economy at fast speeds but I'm usually not in a hurry.

[Captkaos]

No doubt if you are after mileage and your options are not limited to gas, the 6.2L is going to be hands down the best option as that is why it was originally created.  25mpg is the norm.   Even 4x4's get up to 20mpg with a 6.2L.

[78 Chevyrado]

No engine swap it discounted here... 

I want the 75MPH mark, because at 65MPH I get 15 or so MPG...  at 75mpg I get 8 or 9MPG...  I'd like to go 75MPH just to save some time, and time is money.  I realize these aren't effiicent trucks, but I guess I just want to get the best mileage possible...  BEcause gas saved is money in my pocket...  Even if I got way better mileage at 65MPH.  I get about 14-15MPG now driving 60-65MPH everywhere.   I just figure (HOPE) an LS 5.3l or 6.0 with OD would do better than my current worn out 350 and TH400.  Grandad already teared up on me about using the truck he retired from working with to start my career, so this C20 is a work truck from here on out.... 

I heard 6.0l suck gass bad...  I picked a 5.3L with OD because I havent heard anything about them being badly inefficient..  I don't haul a triler often enough to consider that on my new engine... 

For instance, today, I drove 65m to marietta, then 37m to snellville, then 42m to flowery branch, all to some odd small jobs.,..  So with the possible 144m+ per day, I want to save $$ where I can.  But I have to have my truck and all my tools pretty much wherever I'm sent...  never know what your gonna need....  So thats my situation.  I had to add 15psi to the back tires to make up for the load imposed on the truck by my tools, etc...  Thats how much I haul daily....
 
  I'm in my truck 3 or 4 hours daily...  couldnt be happier, but i need to save gas all I can...  Right now I find myself driving 55mph often...  well thats too slow to make a living daily...  If I take a day off I lose around $300-400  So whatever the cost in $$$, but I'm limited by time...  So I wanna go the 350 with efi route because it bolts in with a minimum of problems....  the ls could mess me up time wise...  Though the ls is the ideal.