Best MPG engine Size and setup

[csdineley]

So, I have an 87 C20, 350 TBI, TH400, 4.10 gears. And on my best day I have only gotten 14 mpg at 60 mph. But I normally get 12mpg. So I don't think efi is going to be a big help. Now for the Ls engines both my brothers drive newer Chevy's. Both are 1/2 ton 2wd, one is a 4.8 with a 5 speed (nv3500) and he avg. 20 mpg. The other brother's has a 5.3 with a 4 speed (4l60e) and he gets 16-18 mpg.

 My plan is to swap in a 5 speed or overdrive auto to hopefully get me up to 16 on a good day.

[Edahall]

Just remember, any engine you put in this truck will lose a significant amount of fuel economy at 75 mph vs 60 mph.  It just takes so much more power at 75 mph to overcome wind resistance.  Even with the most fuel efficient engine (the 6.2 diesel), I'm barely able to squeak 20 mpg at that speed.  That extra 15 mph uses a third more fuel.

[Stewart G Griffin]

1) Is 7000 really the weight you are at?

2) i don't think there is anything you can do to improve mileage significantly (see my mpg thread) with the exception of going diesel, going down on cubes or LS.


If you are grossing $300-400 per day, then it may be cheapest to do nothing.

[78 Chevyrado]

1) Is 7000 really the weight you are at?

2) i don't think there is anything you can do to improve mileage significantly (see my mpg thread) with the exception of going diesel, going down on cubes or LS.


If you are grossing $300-400 per day, then it may be cheapest to do nothing.

1 - yes right around there

2 - that sucks...lol  but i know I'm moving a lot of weight and low aero-anything....

3 - the only problem I have with going diesel, or even LS, is there may be signifigant downtime, as I'd be treading new ground. 

I have a special interest in driving this particular truck though.  And I want to keep it working.  I know whatever I do I'll pay a penalty for that.  And whatever MPG the newer trucks get, the older trucks are lighter, so I'd get a few more MPG. 

I checked on it one day, my 78 C20 Weighs 5200lbs empty, and the corresponding 2012 model 3/4 ton HD truck weighs 6800lbs empty.  So I have a 1600lb weight advantage to start with.  The newer trucks are better, but are heavier.  So basically my loaded truck weighs what a newer 2500HD does empty.  I'd like to go 75MPH just to get mnore work done faster.  Faster is better, but no matter what I'll have to find the balance.

[Edahall]

There is not much mpg penalty for weight on the freeway at a steady speed.  The main reason the newer trucks get better fuel economy is better aerodynamics and a slightly more efficient drive train (the big one being a lock up torque converter).

Have you thought about installing a 700R4 transmission?  You might also have a company grind you a cam that optimizes fuel efficiency at 75 mph.  I know this is done with boat engines.

[Stewart G Griffin]

In that case, the easy/cheap things would be to change the gear to 3.08 or higher.  Switch to smaller tires---235/75 or smaller.


2) i think aerodynamics can help greatly;  i have not had time to experiment further and probably will not.

3) If you want to get more drastic, i would suggest in order of level of seriousness:

a) get a 4.3 v-6 from a junkyard, get it running and tuned on a stand, or a tire or cinderblocks etc.   This, i feel, is the most plug-and-play out of all the engine swap options that will bring large mpg gains.   i'm not sure about the mounts/exhaust, but everything else is plug and play except for the fan shroud.    This route, i feel, results in the least amount of down time.

b) If you want to go LS, then i would also get it running on a stand first.   Then work on each department separately;  For example everyone says you need to install an electric pump, but it's not that simple.  You need to mount it correctly, install a relay properly, wire it properly etc.  Then move on to the next department.  Things like this can be done while still driving the truck daily.

c) Same with diesel.  Research what is need for each step.  For example Hydroboost brakes---what parts are needed and what exactly needs to be done?  What type of torque converter and how to get one.  All things like this can be done and parts and tools purchased while still driving the truck.

[square81]

I'd have to agree with the OD option. Keep the 350 and go with a 700r4. Its going to be a big help forsure plus less down time doing the tranny swap, save the LS swap for down the road when you have more time. Going with the 700r4 will be easier, cost less and you dont need much to make the swap happen. Whats the gear ratio options for your rear end. Changing the ratio might help out as well.
 My buddy has a 3/4 ton 2wd with a good running 305 and a 700r4. He carries all his tools too(mobile welder fabricator jack of all trades guy). He uses a contractor cap on his and gets about 15 to 17 mpg. The cap dont look real eye pleasing but it helps.
I think Ls swaps are cool too, more efficient, better drivability and make more power no doubt. But are you going to gain enough mpg to make it worth while doing that swap.
 Refridgerators are only so areodynamic that how I look at it. 

[79gmc15]

What is a contractor cap? I agree, best thing is to do is in steps. Especially if you get more money down the road. Get the 700r4/modern equivalent built up first, then install. Next would be gears, then motor. Get a plan down and stick to it.  You want this done right and last because it is your livelihood. You want strong reliable parts used. Break this up in chunks and it will cause less downtime.

[78 Chevyrado]

 I have looked into 700r4s and it would workwell, even has a mechanical lockup, so no electronics needed.  Lowestgears i can find for my 14bolt is 3.23 or something close to that.  Not enough to warrant a swap from a 3.73.  Im thinking build the torquey engine i want, back it with a hevilybuilt 700r4 and add FAST EFI to itfor driveability.

The 75mph mark is just a goal.  Anything will be better than the worn 350 and th400 i hav3 now, even if i replaced everything to stock. Id love a gear vendors unit but not for $3500.  Theyre insane....

I wondered if adding the airdam off an 87 wouldhelp?  I know i lost mileage on my 87 when i took it off for looks...

[Edahall]

I wondered if adding the airdam off an 87 wouldhelp?  I know i lost mileage on my 87 when i took it off for looks...

Yes it would especially at 75 mph.  Also, lowering helps even more.

[brownturd]

One thing to think about is that driving a 100 miles at 75 mph vs 65 mph you will only get there about 15 to 20 minutes faster by driving 75. Is that time saved worth spending about twice as much on fuel?  Probably not

[79gmc15]

Yeah I don't understand the need of 75 MPH. At that speed, that's death in these trucks. I would hate to drive a 7k+lb 25 y/o truck at that speed, for what?  A few more minutes and more money spent on gas, endangering others around you. The new trucks can survive a crash then older trucks. Not to mention they stop faster.

[78 Chevyrado]

This is true about the time savings...  hadntthought about it that way. I have no problem getting up a halfbhour earlier, i just like to go with the flow, which aroundhere can be 55mphto 85mph depending on the area.  And for safety, the faster i go the farther i am from the car in front of me.  Im not worried there, as my brakes are good enough, if i ever panic stop, i feel extremely nauseated for the next 4 hours...  i almost puked one day, it stopped so fast.  Itll stopabout the same distance as my 2008 impala.  Love the big brakes and the bfgs.

Like everyone else, i just wanna see how to get the most out of what i got.

[Stewart G Griffin]

i wanted to add another suggestion that would be fast and easy and raise mpg---pretty much a bolt in affair except you would need to add a fuel pump for a carb which is easier than adding one for fuel injection.


i think the 91-96 impalas had a 262inch v-8.  i think it was code L99 or something.   Run this with carb instead of fuel injection and this could be bolted in in less than 8 hours.   i think the engine will have enough pep to move everything.   

[Jason S]

My thoughts, based on a lot of research into the matter and trying to maximize the dollar while keeping an old truck...

Just as a point of reference:  My 2005 Chevrolet 2500 HD, 6.0L, 4x4, 4.10's, 5-speed NV4500 gets about 14 mpg on the highway. I can get 15 mpg if I watch my speed or as little as 13 mpg if I don't care. 

Prior to being transferred, my daily commute was 36 miles roundtrip.  I tried to maximize my fuel economy in the 2005 and could only get 14-15 mpg.  The reason I initially swapped the '73 GMC K2500 to to run on propane was the price of fuel, $2.00/gal vs. $3.85+ (at the time).  The '73 GMC with a 4" lift, no overdrive, horrible aerodynamics, iron exhaust manifolds, worn valves and 4.10's cost less to drive to work than the 2005. The price to swap to propane was significantly less than an aftermarket EFI system, with rebuilding used items it was closer to what it would cost to swap a factory EFI (but a lot simpler to do). After the swap, I could start the truck up at 10 degree outside temperature and pretty much drive off.

I liked the propane enough that I installed a propane system on my '74 Chevy K10. I haven't measured the fuel mileage on it yet, but I can start it and drive it like it was fuel injected.

I'm currently planning out a rebuild for the '73 GMC.  At this time I plan for a 700R4, keeping the 4.10's and rebuilding the 350 (355 or 383).  The key part of my rebuild, besides the overdrive, is to up the compression ratio from 9.1:1 to 9.5:1 or more, using a cam that maximizes low end torque along with better flowing heads and exhaust.  With that, I should be able to take advantage of overdrive at highway speeds be able to use the 110 octane available from propane, yet still run on 92 octane gasoline if I need to.  With the low price of propane (compared to gasoline) combined with better efficiency, operating cost should be closer to the '01 Tahoe with a 5.3L and overdrive (averaging 20 mpg city/highway) than the '05 2500. From research, if I were to go with a dedicated propane system and lose the gasoline carburetor I could get even better mileage and performance.

Overall, my idea is maximizing the torque at realistic engine speeds (i.e., <5,000 rpm), have overdrive to keep rpm's down at highway speed, have a C/R that can better squeeze the propane but still be able to run pump gas and have the lower gears. Really, I'm trying to have my cake and eat it too... 

If you were to consider propane there is an added bonus: there are tax credits that are offered by the federal government as well as many states to swap to an alternative fuel (e.g., propane).  So a good portion of your initial investment can be returned by tax deductions while saving money on fuel.