Fuel mileage update

[TrainD5115]

Well, after running a bottle of Amsoil through the system and a fresh air cleaner on top, Big Bird turned out 215 miles this past week on a full tank (approx. 19 gals.) for an average of 11.3 mpg. Thats six trips to work through town plus some short runs of a couple miles here and there. I am hoping I can pin down the ghost in my engine this weekend if the bloody snow and arctic air let up some. I am confident that my mpg can go up some more with a little more work. By the way, I looked at a new GMC truck the other night at work to see what the mpg estimates were and was shocked to see 15 city/ 19 hwy. Of course if my 79 was getting that I'd be stoked!

ps: Think Spring....like the picture!

[plumbcut]

If you put an aluminum intake and headers and a decent exhaust system, you'd probably be up over 12mpg no problem.... what are you doing for ignition timing? if you're running the stock 4 degrees or whatever, Id mess with that.. will free up a lot of power and give you better mileage.. the 3.73's outback arent doing you any favors but it could be worse.... Electric fans would probably bump up your mileage a bit too...

keep us posted... IMHO anything over 12mpg with a stock or close to it, truck is pretty dang good

pc

[79gmcc15]

yeah not too bad, considering last time I checked I was geting about 7 mpg in town on my truck

[mitch1965]

My '96 gets 15-17mpg, my '85 gets about 10. I'm still working on the '85 to find all the little things that need attention.

[JonnyTechMech]

Originally Posted by plumbcut

the 3.73's outback arent doing you any favors but it could be worse.... pc

Why is that? It was my understanding that low gears in the rear would increase wheel revolution, in turn, use less engine power to move the vehicle; due to the tranny and the engine not working so hard. I figured that most of us were striving for 3.73 - 4.10’s, so we move that steel a little faster without the use of the lead foot.

I believe I’m running 2.73 that pushing a stock 87’ 305wTBI/TH400 and my mileage is a serious drag….. on my wallet . Please enlighten me before I spend some money on new gears and regret it.

JonnyTM

[purple gas]

The higher your gears, the higher you engine needs to rev at highway speed. So, if your not into scorching the pavemnet at every stoplight, or do a lot of highway driving without having to haul/tow anything , lower gears should give you better fuel economey.

[Prerunner1982]

jonny, it also depends on the tires...

i am not for sure what size trainD is running, but the pic makes them look decently tall.

a 3.73 ration and lower profile tires would be good around town, but highway speeds your RPM would be pretty high and MPG would suffer. On the other hand, a 3.08 ratio and say 32"s are dogs around town, but highway i get pretty decent MPG..

[purple gas]

Prerunner has a very good point. Also, radical changes from stock tire diameter will also affect your speedometer accuracy. Try as we might we can never escape the laws of mathmatics.

[1FaastC10]

Originally Posted by JonnyTechMech

Why is that? It was my understanding that low gears in the rear would increase wheel revolution, in turn, use less engine power to move the vehicle; due to the tranny and the engine not working so hard. I figured that most of us were striving for 3.73 - 4.10’s, so we move that steel a little faster without the use of the lead foot.

I believe I’m running 2.73 that pushing a stock 87’ 305wTBI/TH400 and my mileage is a serious drag….. on my wallet . Please enlighten me before I spend some money on new gears and regret it.

JonnyTM

low gears actually decrease wheel revolution, bringing your RPMs higher, and in many cases lowering mpg.

[1FaastC10]

Originally Posted by purple gas

The higher your gears, the higher you engine needs to rev at highway speed. So, if your not into scorching the pavemnet at every stoplight, or do a lot of highway driving without having to haul/tow anything , lower gears should give you better fuel economey.

you're confusing low gears and high gears. 2.73 high, 4.10s are low.

[TrainD5115]

All I know is that I could use an overdrive transmission. I don't scorch pavement I just try to take care of the truck as best I can.

[86-C10-V6]

I would definetly check the speedometer for accuracy.
Large tires will make your mileage appear to be worse than it really is if the speedometer is not accurate from having large tires. The standard stock tire on 1/2 tons was P195s. A lot of trucks were ordered by dealers with larger tires with the correct speedo gears. Check the door sticker for the tire size the truck was delivered to the dealer with.

Also the large tires will most likely cause your mileage to actually decrease.
An overdrive transmission with most likely help, but a 2.73 rear axle should give you the best mileage if you are running the truck with no load.
The nice part of the overdrive transmission (700R4) is its low starting gear that help get the truck moving along with the high overdrive ratio for mileage.

[Grim Reaper]

Originally Posted by 1FaastC10

low gears actually decrease wheel revolution, bringing your RPMs higher, and in many cases lowering mpg.

Not always. 2.73 lug the engine and causes people to put their foot in it to try to overcome it because its such a dog. In most cases a 3.23 on a 31 inch tire is about right if you have no overdrive. You have to balance the weight with the gear and a 5.5k truck needs a LOT more then 2.73 unless you live in Florida and are a little old lady driver.

I have owned three no OD trucks with 3.73-4.10 gears.
My 79 K5 was bone stock and ran perfect with a crate engine. Best I ever got was 12mpg.

My 75 was on 35's weight 6600lb with all the armor and was tuning 4.10 and got 9mpg

My 88 Burb with TH400 3.73 FI454 and hwy tires gets about 11.5. It gets 10.5 with 5500lb in tow.
'
These trucks have horrible aerodynamics and with no Overdrive you are doing good to get 12mpg. Overdrive with a 350 and 3.73 gears most people will report about 15mpg. The low gears help it get moving then the OD kicks in the hold low rpm for steady cruise.

[marvxyz]

TRAIN D 5115- What size engine do you have? I always thought that the larger the engine the less mileage, however I'm learning that, it isn't necessarily the case. Also I always thought that if you had a standard that you could get better mileage than an automatic. I run a "80" GMC with a 6cy auto stock, and lately I've been getting 12 1/2 mpg and I noticed yhat it went down to 11 1/2 at about Nov.(presumably because of the winter formula gas.) I wonder if a rebuilt carb could increase my mpg.

[C-10volvo]

i get 17ish with a gm "stock" crate 350, stock q-jet, 700r4, 3.08 gears and 29 inch tall tires. sorry, had to rub it in.....

[TrainD5115]

I've got an original 350 motor with about 128,000 miles on it. Tires are close to stock size, 265/75/16 without actually looking at them. I would have to ask the PO if the carb has ever been rebuilt. Sometimes it will help and sometimes it could be another component of your truck that is causing bad fuel economy. I think I still have a vacuum leak but I could be wrong. People tend to blame the carb right away when it could be something else you are overlooking.

[tkeduk8r]

well i figure ill add my two cents here .... from what I have researched your vehicle will get its best milage from keeping the engine at its torque peak for the greatest length of time... thats why the 700r4 and 2004r are better due to the multiple gear changes and including a deeper gear (tourque multiplication, aka 3.07 1st) to start the vehicle out from a stop and .70:1 on the highway to reduce engine rpm.... so for the best milage you want to match the torque output of your engine (cam selection, gearing, etc.) with the cruising RPM or rpm where most of your driving occurs... i hope that made sense, i had to type it between classes im teaching...
--Mike