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improving gas mileage

M

mwood1985

1/2 ton status
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Marietta Ga
ok silly question being that we drive big steel V8 powered awesomeness i know but im gonna ask it anyhow. ive got a 86 k5 with a stock 350 other than a 1405 edelbrock 600 and headers with 2 1/2 inch pipe no mufflers turned out at the rear axle, 8 inch lift 4.56 gears and 36 tsl's with a 4 speed manual. she wants to cruise at 50-55 and burns a hell of alot more gas when i have to go to highway speeds ie 65mph+. any ideas on getting a little better miles per gallon? maybe change gears or go to a taller tire? im gonan add mufflers but i was wondering if that will improve milage any. its not a money issue its more of i get tired or having to go to the gas station when im driving from marietta ga to rockmart. seems like its burning 50 bucks every 125 miles lol
 
A gear vendors OD unit (and I suppose an NV4500) would get you to around 2000RPM at 65MPH.

At 65MPH I average right around 16MPG, 2200RPM.

I have a feeling after the underside of the truck gets X inches above the pavement, economy doesn't get any worse. Everything matters a bit. There are about a million differences between a car that gets 40MPG and a truck that gets 16MPG. Weight, drag, and displacement are the big three, none of which you can minimize on a truck enough to get "good" mileage. Heck, look at the hybrid full-size truck GM was making. Was still in the 20's for MPG.
 
im short shifting it and staying out of the throttle if i can help it. maybe its just my setup being wrong. the gears being 4.56s duh im gonna run out of top end. i was kinda thinking leave the setup like it is and go to bigger tires like a 39.5 just to get the gearing closer to the original 31s 3.08s it came with. 4.56s are supposed to be for 40-44s but ive got 36s so maybe my problem is undersized on the tires compared to what size the gears really call for? any thoughts?
 
4.56's with no OD and 36's is pretty deep for optimal MPG, the 8" lift means that much more wind resistance as well.

Did you tune the carburetor with a dyno or O2 sensor?
 
no we tuned it by feel and ear really well an old GM tech did it for me. mixture screws are 2 1/2 turns out and the idle is set at roughly 500. i think the timing is advanced way too far honestly. its off the tab on the timing cover but the old guy said his "ear knows" slack in the timing chain might cause that problem too
 
38377k5 said:
4.56's with no OD and 36's is pretty deep for optimal MPG, the 8" lift means that much more wind resistance as well.

Did you tune the carburetor with a dyno or O2 sensor?
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what do you think wind resistance aside would a taller tire get the gears closer to stock? i feel like it can turn great at low speed but the truck runs out of the ability to keep accelerating past 65 due to gearing. like down a hill on the interstate the most i could get was 68 out of her in 4th gear she just quit pulling
 
A taller tire would get your gearing closer to where you want it, but then you have more rotating mass and extra leverage working against the axles. I doubt you would see any gain in mpg and you would definitely see a reduction in braking power. I know you didn't mention that as a concern, but it is something to consider.
 
I wouldn't change the gears. Slowing down on the highway will give you better mileage. Every mph above 55 will reduce your mileage 1/2 mpg I've been told. On my 465 I don't run it over 60 and get about 12 highway mpg on 35s and 410 gears. So that won't help you much either. I'm running my dizzy at about 10 degrees. If you run better fuel it helps a lil as well. Try 92 octane and see if it helps. When I cheap out on fuel and get regular and have to back off the timing to stop pinging I'm only getting 10 mpg. Not much you can do with these big pigs. Also are you checking speed and distance with GPS. With your speedo its going to be off a l bit.
 
Just slow down. Your truck wasn't designed to go 65, it was built when speed limits were 55. That, or drop some money in an economy beater to DD.
 
Most everything I have has the timing "off the tab". Also I have never heard a ear that also works as a good a/f meter. Look for a chassis dyno and see what they charge. You will need a tuning kit for that carb since I would bet that most of the dynos only have holley stuff. It is the best money I have ever spent, for power and for economy.
 
TerryD said:
Just slow down. Your truck wasn't designed to go 65, it was built when speed limits were 55. That, or drop some money in an economy beater to DD.
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ive got a 2006 duramax tow rig and a 95 s10 4 banger i drive to work so the K5 is the weekend toy. i had debated just towing it with the dmax because i know its cheaper lol but i kinda like driving it sometimes. theres a dyno shop about 10 miles from my house so that might be the next stop to have them dial it in perfectly and see what i get
 
The difference between running 13:1 AFR and 15:1 AFR is 15%. There is an assumption that the motor is just as efficient at both AFR's, which isn't going to be true. But...

Assuming 30 gallon tank, 300 miles @10MPG (obviously), and 345 miles @11.5MPG.

Too many unknowns to accurately solve for the RPM difference either, but theoretically, assuming no other change, you would see a ~33% reduction in fuel consumption with .70 overdrive. That'd be 402 miles on a 30 gallon tank.

When you lift a truck (suspension, body lift would make a diff) you aren't exposing anything to "more air" except the tires. The undercarriage surface area/contour does not change. Factory, the trucks air dam is probably already 8-10 inches off the road. With a suspension lift you ARE changing the u-joint working angles (normally) and if steeper than factory, would result in efficiency loss.

If anyone doubts the lift statement, I can pull my air dam off, which would be the equivalent of ~4" of lift in regards to exposing the undercarriage.
 
Getting some sort of OD is going to be your best bang for your buck. No other way around that. You could try a complete tuneup, and make sure your carb is running at peak efficiency. But still you will be hindered by the lack of an OD at 65mph. Do both and bam, done.
 
TerryD said:
Just slow down. Your truck wasn't designed to go 65, it was built when speed limits were 55. That, or drop some money in an economy beater to DD.
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This is the ticket.

The best ways to save gas in our trucks is:

1. Start off the line slowly. Very slowly. No more than 1/4 of the gas pedal. Be slow.

2. Drive slower. Wind resistance is CUBED as you go faster. It's not doubled, tripled, or even squared ... CUBED. The faster you go, the more fuel is needed. Driving 60mph will save you more gas.

Power

The power required to overcome the aerodynamic drag is given by:

e31430f0898268091f410282a89503b1.png


The power needed to push an object through a fluid increases as the cube of the velocity.

A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower to overcome air drag, but that same car at 100 mph requires 80 hp. With a doubling of speed the drag (force) quadruples per the formula.
Click to expand...
 
how about looking at it from this angle. would the injection system make a difference? ie do fuel injected tbi blazers and suburbans with a 465 transmission and the same setup as mine with the difference bing a carb or a tbi style. power i know is gained through the fuel injection but would it also gain any noticeable milage increases?
 
No.

I gained nothing from a carb to TPI on my 350, except driveability, when I tune it right. There might be some more there if I cheat (run it lean at times) but all in all, no.

Trucks are just too heavy and un-aerodynamic for injection (or any other single change) to make a huge difference. I alluded to it with the Hybrid Silverado comment, I'll be more blunt: manufacturers have 25+ years of technology under their belts, and still struggle to get 20MPG out of a gas powered truck.

Again, that is ALL assuming the carb is running optimally. If it is, injection will gain little if any at cruise speeds, all else equal.
 
I went down this road with a 1978, 0 to 2 " lift, sb 400, new edelbrock carb, th350, np203, 3.73s and 33s, d44/14bff. Tuned it as much as possible, had a mallory hyfire, changed to a 700r4 for the overdrive, blah, blah - nothing. still 5 - 6 mpg. 200miles on a 31 gal tank every 2.5 days regular as clock work.


Best you can do with gas:
Injected
cam for lowwww end torque
tire size and gears to get it a little on the high end
get tires that have a low rolling resistance
stay off the skinny pedal
forget hp , more hp = more fuel burned

Still don't expect more than 16 maybe with a gas engine in the heavy blazer.

=========================================

Changed to a 4BT/NV4500/NP205/4.56 gears/38.5 tires/ 4" lift/d60/14bff 25 solid on the highway. The 4.56 with the 38.5 are a little tall which helps with the highway. VERY low rolling resistance tires.
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YMMV (no pun intended)
 
Any money spent on upgrades will only pay you back pennies on the dollar as far as mileage is concerned, which means you'll need to drive tens of thousands of miles just to break even..........

Even buying a crappy used econobox could pay for several years worth of fuel.

Drive decent, and keep your ride as it is. It seems counter-intuitive, but that's the only way you'll actually SAVE money on fuel, brother.
 
im not spending any money major to save penies at the gas pump. its 3.09- 3.20 in atlanta right now so it sucks either way. i was kinda wanting to squeeze more miles out of the tank so i dont have to fill up so often but the pretty much truth is the only way to drive a k5 farther is to put more gas in it to begin with lol.
 
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