I have 2 Chevy trucks, a 1991 with a 305 and 135K and a 1990 with a 350 and 129K. The 305 gets about 13-14 MPG after an extensive tune-up and header install. The 350 gets 10 mpg, but needs a tune-up, I just bought it. The injectors on the 305 don't mist as good as they used to, but should be ok for awhile longer. I don't want to replace them cause I plan to sell this truck. Haven't checked the injectors on the 350 yet.
A friend told me I could put a carb on the 350 and get lots better mpg. I'm not so sure. Any thoughts on this?  

your friend is bluffing, even though it might be much less noticeable than a carb vs port injection setup, TBI still atomizes the fuel better to begin with, so it has at least some effect on the fuel atomization once its in the cylinder

Thanks Gibs, that confirms my gut feeling...

Guess the first action is an extensive tune-up, then check the mileage. Injectors may be needed after that. I'm hesitating on the injectors because I think they are more than $100 each, and I want to see what I can get with a tune-up first. I'll take a look at 'em anyway...

Thanks again.

Bob

A carb, unless you jet it dryer than a popcorn fart, will never deliver the mileage of TBI and isn't even in the same ball park as port injection. Mileage and emissions control is why the carb was abondoned by all auto manufacturers.

You didn't sight what is the configuration of your pickups, how they're driven nor their state of tune. Also, they both qualify as high milage engines and are subject to wear.

Generally bigger like 3/4 ton versus 1/2 ton get lesser mileage. Stiffer gears, 4 wheel drive, automatic transmissions and using air-conditioning tend toward burning more fuel. Short trips with stop and go whether on or off road get poor milage compared to cruising at legal speeds on the highway for lengthy distances. Towing or hauling heavy loads eat into mileage as does a heavy foot. Certainly aging systems that support the engine reduce mileage as they drift out of tolerance or become disfunctional.

To demonstrate these effects I'll use my S-15 pickup. It's a 2 wheel drive standard cab that weighs 3210 pounds with me (200 lbs) in it. It's running a 95 LT-1 350, 4L60E automatic, 3.08 posi rear, and 95 Z-28 rims and tires. In this configuration it gets 18-20 mpg commuting on an in-city freeway, it gets 15 on surface streets with lots of stop and go, cruising long distances at 70-75 mph it gets 24-26 mpg. When originally installed the engine and driveline had 48,000 miles on them and passed the annual emissions test with great ease, my pizza and beer breath was more of the threat. I stay on top of this this thing with proper and timely maintenance. It now has 108,000 miles on the engine and driveline and the emissions are steadily increasing at every test to where I can see some major work on the systems and perhaps the engine are coming in 2 or 3 more years. I can also feel the power diminishing as it ages and fuel mileage has crept down a consistant 2 mpg over the last several years since it was installed. So all the indicators are pointing at an aging motor that with it's systems will either have to be rebuilt or replaced in the next few years.

The money a decent carb costs and then more money to get it into tune with your engine would be better spent fixing the factory systems that are aging out.

Bogie

    USE HI-PERFORMANCE SYNTHETIC OILS LIKE ROYAL PURPLE, AMSOIL, OR REDLINE AND YOU MAY SAVE YOURSELF THE TROUBLE OF AN UNESSARY ENGINE REBUILD! BUT IF YOU LIKE SPENDING TONS OF *** AND DOWN TIME REBUILDING AN ENGINE BEFORE IT IS EVEN NECESSARY THEN GO RIGHT AHEAD! I CAN'T STOP YOU, BUT REMEMBER! THE REASON SYNTHETICS WERE DEVELOPED IN THE FIRST PLACE WAS TO REPLACE DINO OIL BY PERFORMING THE VERY SIMPLE TASK OF KEEPING THE INTERNAL PART OF AN ENGINE AT LIKE NEW CONDITIONS! IF AN ENGINE IS KEPT INTERNALY CLEAN THEN WEAR IS VIRTUALLY ALMOST NON-EXISTANT, AND IT WILL PERFORM LIKE NEW, MILEAGE WILL GO UP, HORESEPOWER AND TORQUE WILL GO UP AND TEMPS. WILL GO DOWN! BUT GUESS WHAT? CARBON BILDUP BEGINS TO SHOW ITS UGLY FACE IN AN ENGINE WITH HIGH MILEAGE! THIS CAUSES MILEAGE TO GO DOWN AND LEADS TO FAILED EMISSIONS! SO TO AVOID FAILED EMISSIONS, THIS BUILDUP MUST BE CLAENED OUT! THERE ARE TWO WAYS TO DO THIS.

1. MANY OF US THINK THAT WHEN AN ENGINE GETS TO SAY 150,000-200,000 MILES A REBUILD IS IN ORDER. WRONG! WHY WOULD YOU WANT TO TEAR DOWN AN ENGINE JUST TO CLEAN OUT THE CARBON BUILDUP YOURSELF WHEN YOU COULD USE THE COST EFFECTIVE METHOD #2?

2. IF AN ENGINE WITH THAT HIGH OF MILAGE HAD BEEN SINCE DAY ONE USING ANY OF THE THREE OILS I'D MENTIONED ABOVE INSTEAD OF DINO OILS, THEN IT WOULD STILL BE AT LIKE NEW CONDITIONS IF IT WERE TORN DOWN! 

    I'VE SAID THIS MANY TIMES BEFORE AND ENJOY DOING SO BECAUSE I BELIEVE THAT FUEL-EFICIENT MONSTERS ARE POSSIBLE AT WHATEVER THE AGE! NOT ONLY IS THAT POSSIBLE, BUT I AM OUT TO PROVE THIS BY OBTAINING 40 M.P.G. ON MY '96 S-10 SS WITH THE VORTEC V-6 WITHOUT THE USE OF ALTERNATIVE FUELS, HYBRID TECHNOLOGY, OR A 4 CYLINDER ENGINE, AND WITHOUT DESTROYING THE LEGENDARY SUPER SPORT MUSCLE CAR PACKAGE. I JUST RECORDED A 29.0-29.9 M.P.G. AVERAGE ON THE HIGHWAY AT 65-70, AND A 28.0-29.0 M.P.G. AVERAGE IN CITY DRIVING AT 35-40 YESTERDAY! I AM USING A SCAN GUAGE TO TRACK MY MILEAGE! VISIT (scanguage.com) FOR ALL THE INFO ON THIS COOL GADGET FOR LATE MODEL CARS AND TRUCKS! BY THE WAY, MY TRUCK HAS A LITTLE OVER 113,000 MILES ON IT, AND IT USES THE R.P. 5-30, AND THE PURPLE ICE COOLING ADDITIVE, AND WILL BE USING THE R.P. AUTO. TRANNY FLUID REAL SOON!     

    I REALIZE THAT AN ENGINE REBUILD WILL HAVE TO BE PERFORMED AT SOME POINT, BUT 200,000 MILES OR LESS IS UNECESSARY! I BELIEVE THAT 300,000 MILES OR MORE IS A HECK OF ALOT MORE REALISTIC IF METHOD #2 IS USED! THANKS FOR YOUR TIME AND HAVE A GREAT ONE!

      MAY DINO OILS REST IN PEACE, AND LONG LIVE THE BOWTIES!     

this is a topic about TBI and mileage bring your spam some place else...

Right on Gib, that WAS a commercial.

Bogie, you have very good points also...

Truck 1- 91 4WD heavy half-ton (3/4 ton susp., 1/2 powertrain), 305, auto, reg cab, long bed, no AC, 135.5K miles. 265/75R16 Cooper Discover load range E. I run these at 40 psi unless loaded or towing. I bought this one with 12K miles in 93. Historically, I've never gotten better than 13 mpg, in any use. Always felt underpowered in OD, so used 3rd almost exclusively. I've increased the size of the tires gradually over the years, i think the orig size was 225's.
Since the header install, power is up, mpg is up, I can use OD more often. Most recent mpg was 14.5, uncorrected for larger tires. Part of that tank was a 200 mile round trip with sections of hills & curves, mostly at 75 mph with passing speeds up to 85, hi-caution areas at 55 (brakes are excellent, have ABS in 2WD, very good tires, recent pads). The rest was city driving.
Very pleased with this difference in mpg. Steps taken:
1 headers     2 rotor/cap/plugs/wires
3 timing chain/gears    4 muffler/tailpipe
5 fuel filter

Truck 2- 90 4WD Z-71, extended cab, short bed, 350, auto, AC, 128K miles, same tires, not sure of load range. I bought this truck 6 weeks ago, currently not insured so not driving it. I did use it for a 700 mile round trip immediately after buying. Was surprised to only get about 10-11 mpg. Used AC about 30 minutes to check it out. Usually don't need it here (AK). Has modified exhaust, but no headers.
These are steps I plan to take to improve mpg, in no particular order.
plugs/cap/rotor/wires/fuel filter
Check tire pressure
check fluid levels (diffs, xfer case)

Comments/suggestions gents?

   I'm sorry if I sound like a commercial, but I DEFFINATELY AM NOT GIVING OUT SPAM!!! I'm mearly giving you guys a wealth of personal experience information on how to improve the mileage on any vehicle and not sacrifice any muscle! I do not state but form personal experience! Every Hi-Performance part I install on my truck is carefully tested by me for fuel economy! I do not install any part on my truck without doing extensive research first! I want to make sure that each part I buy can deliver on both muscle and economy! 

   Trust me! I may only be 25, but I know alot about fuel economy and am always trying my very best to help those who want to obtain better mileage out of their vehicle and not sacrifice muscle. So if I always sound like i'm a commercial it is simply beacuse I sometimes feel no-one is listening to me or believes what I say! I even think that some poeple believe what I write is bogus. But I keep trying hoping that maybe someday soon lots of people will catch on to the things i'm stressing and realize that it is not spam or snake oil, but is just a wealth of information on improving mileage without hurting the Muscle we all love. 

   It is a proven fact that most vehicles are only built to meet the minimum emission requirement, and not exceed it, yet we've been told that the automakers of the world have been building the vehicles we drive to their best possible efficiency, and any improvement is impossible. It has been found that by simply improving the engines eating and breathing abilities, greater efficiency in the form of more M.P.G., Lower Emissions, and Increased Muscle are all a byproduct of that. This is something you DO NOT see from the factory unless you consider Displacement on demand as a qualification

   You see! We the people have been ingrained with (DECEPTIVE THINKING) from previous generations. This type of (DEEPTIVE THINKING) is nothing more than a dealdy disease. So my point since day one has just been trying to wake us up to the fact that this (D.T.) has been the main contributor in the push for hybrid, hydrogen, and fuel cell cars, and If we stand on the sidelines and don't do anything to stop it we may as well kiss the internal-combustion engine goodbye!

   .      

       MAY DINO OILS REST IN PEACE, AND LONG LIVE THE BOWTIES! 

Geeze, even my LT-1 powered S-15 feels undepowered in overdrive.

Actually you've got some pretty big pickups here and I can understand that a 305 is working pretty hard. Larger diameter tires don't help that feeling either since they effectivly reduce gear ratio, tuning the engine even slower which moves it further down the torque curve, so you compensate with more throttle.

Somewhere there is an ideal combination of engine size and RPM for the load  (weight, aero drag, road speed) that the engine sees. The factory does a poor job in advising customers about the real world power requirements of trucks and how often if an engine is too small for the job, its fuel efficiency and life expectancy are not as good as that provided by a larger engine.

Truck number 1 could really use a 350,377 or 383.  A big block 454 is probably too much on the other side of the bigness curve and will be a bit of a gas hog simply because you have to feed them inches whether you use their full potential or not. The mods you've done or plan to do to both trucks are in the right direction. you also want to check valve motion to insure that the cam lobes are OK. Chevy engines do have an appitite for flat tappet lobes, so when they get some miles on them it's a good idea to pop the rocker covers and take some measurements. Truck number would also be happier with a 350. A 305 in these 4000 pound plus vehicles is just stretched too far to be really happy.

Mountains, city, towing, high cruise speed and cold wet weather (I did see "AK" like in Alasaka didn't I) all conspire to reduce mileage.

A synthetic oil might provide some mileage benefit, but anymore these days the chemical and structural difference between synthetic and mineral oils becomes less distinct every year as the refiners contine to rip mineral oils apart and reorganize the molecules for improved performance. In a few more years and all oils will be what we today think of as synthetic. You're already seeing 5 and 7 thousand mile oils that last because of molecular structure rather than a mixture of exotic chemical cocktails. Bit by bit we're catching up to Europe, where  pollution law does not permit oil changes with the frequency that we are used to. But still today here in the states synthetics do show a small lubricity improvement over classic mineral oils, especially in shear which really reduces piston to cylinder wall drag.

Bogie

  Hey guys, I was talking to a guy who deals in Amsoil and he told me that their oil is 100% synthetic. What he meant by that is Amsoil takes corn oil or other natural oils and restructures them for use in an engine. He says that under no curcumstanses does Amsoil ever use any petroleum basestocks. So if this is true then they absolutely do not pump oil from the ground as we are used to seeing other companies do. Redline does the same thing Amsoil does. If this is the case then Amsoil and Redline are the only true synthetics. I am currently a Royal Purple user and not sure if it's 100% Synthetic. I have had good success with it, and don't plan to change unless I find it to not be 100% Synthetic. Will be calling R.P to find out. I've looked up this info. on many different sites, but they all have many different oppinions. Just thought I'd pass this info along incase anyone was wonderin'. 

Why does synthetic gear oil smell like petroleum gear oil? Do they all put the posi additives in them?

Why does synthetic ATF smell like regular ATF?

Why is Amsoil Racing oil red, but their "regular" synthetic oil is oil colored? They both are 100% synthetic... so what's the difference?  On a similar note, RP's oils are purple in color... why? What do they add to it to make it that color, and does it effect the performance of the oil?

and what about Mobile 1... it's pure synthetic too, right? Why doesn't it get mentioned as much as the other more expensive synthetics?

and what does any of this have to do with TBI vs carburetors fuel efficiency? 

EXTREME DRIVER - drop off the thread dude, you are totally OFF TOPIC. Start a new thread on your subject and folks who are interested will respond. And don't post another of those "I'm sorry, but blah, blah, blah (trying to help you), blah, blah, (because you're idiots) blah, blah, (can't see the light), blah, blah messages.

Bogie, more good points, and I hadn't considered that the larger tires would affect the power availability. I was looking for better load/towing capacity, mileage too. With everything considered, I guess this 91 truck is not doing badly at all for mpg.
I'm thinking of selling the 1991 with the 305 however, I like the extended cab on the 90 so much. But I want to have more time with the 90 to evaluate it and upgrade the mpg first. BTW, forgot to mention it, but I've used Mobil 1 exclusively in the 1991 since I bought it with 12K on it. The 90 has (I think) Pennzoil.
I plan to switch it to Mobil 1 also, and maybe an application of Slick 50, I had good results with that in a different vehicle years ago.
I guess if I want mpg, I'll have to finish my 4.3 V6 trike.
Yeah, thats the ticket...
Thanks for your reply (and yes, AK is Alaska. Lotsa idling in the winter)

Bob

   As far as I know, Mobil 1 is not 100% Synthetic based on what an Amsil rep told me, but as for R.P. being Purple, I was told that it is just a Purple Dye, and will not harm performance. I had called their main # to find that out. I was going to ask them about whether it is 100% synthetic or not, but forgot, so I will call them again to find out.

   I just mearly wanted to bring up the subject of Synthetics because if they are 100% pure, mileage can improve on both Carb\T.B.I. I realize this does not directly connect to this subject, but I enjoy helping anyone who is looking to make their vehicle obtain better fuel efficiency without hurting muscle. I also found out that not all synthetics are Pure. In fact most that say synthetic use a process called hydrocracking. I think Mobil 1 uses this process. You see, the Hydrocracking process involes hydrocraking the molecular structure of petroluem oil down to a simple molecule, and then rebuilding it, yet from what I have read it still shows signs of being a less than perfect because petroleum oil is still it's base, and we know that the life of petroleum oil is the shortest next to mineral oils. That's why natural oils not only hold up better in an engine, but they last a whole, heck of a lot longer regardless of Carb. or T.B.I. setup. And they would keep the engine virtualy clean. I know that Carb\T.B.I. setup has alot to do with a vehicles efficieny, but I have heard that some Carb. cars like a 1950 Cadilac could obtail about the same mileage as most T.B.I. cars do today. Don't know how true that is, but it's what I heard.  

   While T.B.I. is better than Carb. Mileage still suffers a bit with T.B.I. And even petroleum oils are slowly revealing their outdatedness. I have found that many people prefer T.P.I., C.P.I., or M.P.I. as better avenues over T.B.I. I'm not saying T.B.I. is bad and should be thrown out, it's just that other avenues have been found that might improve efficiency even more. Sory for the indirect stuff, but I can't help it. I like helping people looking to increase the M.P.G. on whatever they drive.      

I like the 4.3 trike idea, good mileage, fresh air, Sturgis. Well, OK, Sturgis is a long ride for you. But that's what adventure is all about.

What you've done on the trucks sounds like all the right things, they're just big and hungry. Wet, cold weather doesn't help, like you said lots of idleing. Mobile 1 is good stuff. Synthetics start out as molecules that mostly come out of the ground. Then the boys down at Texas City rip 'em apart and make new molecules that mother nature would never recognize as her kids. Most synthetic stocks are based on Diester, Poly-ester, or alkyd glycol mixes with lots of additive chemistry to control everything from detergency (solvency) and water management. Synthetics for all their good properties make poor solvents thus don't flush gums, varnish and sludge well without help and they attract water molecules, so there's even more chemistry than found in straight mineral oils to help them over these problems. Not to say the're not good, and everything is headed that way. Every name brand oil on your parts store shelf is somewhere between less to more synthetic (RPM Delo a case in point) without actually saying so. Corn oil would probably be used to form alkyd glycol bases. Vegie oils wouldn't be recognized by mom nature by the time the chemistry department was done. Thinking back a time ago when caster bean oil (like where did Castrol get their name) was a very good lubricant save for one feature. That was after heating and cooling a polymerization process started and the stuff quickly became the consistencey of Jello. Not that you asked about oil and poor old EXTREME DRIVER has taken a beating, not the first time for him and even I've taken a poke or two. He just reads too much advertising and passes it on. To find out what's really happening he needs to get out of the hype zone and into the chemistry and physics.

Bogie

It is hard to expect good performance in Alaska. There is only 3 or 4 months in which to tweak your ride. I lived in Fairbanks from 1992 to 1995 and when it wasn't winter I was fishing and hunting. I didn't have any time to play with my truck. I did however, run all synthetic in it. Engine, Transfercase,transmission, steering box, Front & rear Differentials. It helps to crank at -60º F. I kept the truck outside. (My wife got the garage). Since then I have put a 350 Horse Vortec crate motor which I have breathing really well and a "turbo city" performance TBI Package. I only get 10 MPG in the city (Portland Oregon). It is very fun though. I still run synthetic lube ( Mobile One and Amsoil).  Have fun.