Interesting Fuel Economy Observation

I have been tracking the fuel economy on my 98' 12 valve Quad Cab 4x4 Long Bed Automatic ever since I purchased it in early October of last year.
**Truck weighs 7150 without driver.
**I built up the automatic transmission with a stock stall torque converter with billet triple disk cover, a few misc. internal upgrades, basically my version of a 600 HP transmission.
**3.54 gear ratio, 285/75/16 Bridgestone Tires, no leveling kit, AFE panel filter in stock air box, boost elbow set to 40 psi max.
**It has a 2095 rack plug, no plate, and modified AFC. Stock governor springs, 60lb marine valve springs, stock valve lash, and unknown mileage marine (330) 4x.013" injectors.
**I checked the timing and was surprised to find that it was not stock and is set at 26.4* BTDC.

I have been trying everything to get my fuel economy to come up, played with tire pressure, adjusted the valves, changed the fuel filter, altered driving habits, etc.


No matter what I did, fuel economy unloaded always came in around 16.5 to 17.7 MPG, usually upper 17's when coasting to stops, easing into the pedal, etc. In fact, my running average since I purchased the truck is 17.3 MPG.


Today I drove over 500 miles on a work trip, and since my employer was paying for the fuel, I drove faster than my normal conservative 65-70 MPH.


One the first leg of the trip, I drove 72-80 MPH, probably averaged 76 MPH and blasted up every pass exact opposite of someone trying to set fuel economy records. Averaged 19.53 MPG, huge difference.

On the second leg I purposely drove faster and drove 76-85 MPH, climbed several passes from 1000' to 4700', some head wind and cross wind as well. Averaged 19.51 MPG.


All that I can come up with is the combination of injector spray angle and high timing makes this motor most efficient in the 2000 - 2150 RPM range because it's a well-known fact that the wind resistance exponentially climbs with cruising speed which should make the truck get much worse mileage at 80-85 MPH.

I feel confident on my documentation of MPG in the 60-70 MPH range at the current timing so I'm now going to retard timing back to 20* BTDC and see if I can get 20- 21 MPG in a normal cruising range of 65-70 MPH.


Thoughts, experiences, theories?


And for those not interested in fuel economy, why did you click on this thread after reading the title???
My 95' Junker Drag Truck averaged 8MPG and 1-1/2 sets of tires on the last fill-up so I do have a real oil burner in the garage for those who only care about performance.....

I only read the highlighted parts, but it makes me love our trucks even more.

I've also noticed that i get better mileage at higher speeds. Not as dramatic as yours, but a little better.

And I agree with your rpm/timing/efficiency idea.

My truck can average 21mpg 70-75 and it was down around 21* (hand calculated no dial) now its up around 26* probably closer to 30* with how hard it starts In the summer lol. Not much mileage on it with that timing...yet

Good info

I think its a combo of the timing and boost presure coming together to make a sweet spot. My personnal combo is a stock block rebuild thats still breaking in, 180 pump (4k, no plate, 022 dvs, rack plug, 17.5* timing), small injectors w correct spray angle, 3.55s 4x4, 265/75r16 tires, 362/475 twins, ealy .79 od G56. Blasting up the interstate everyday at 2200 RPM and ideling through a 6 mile traffic jam each morning for 15-45 minitues is netting me a constant 19.4-19.6 MPG. Stock the trucks best was 19.7 MPG and running out of steam on a hill at 70MPH. From my experience 12v's need to be making 5-8lbs of boost to get the fuel economy. Maybe the low boost numbers is where the turbulence in the cylander is enough to give proper atomization/mixture of the fuel resulting in the best bang for the buck. If I try to keep RPMs low and off the boost as much as possible I can knock about 1.5 MPG off the average fuel economy. Thats right a lower milage! Hit the throttle enough to accelerate with around 15lbs of boost, 1/4-1/3 throttle where I just start to haze, and milage is right back up to mid 19's. With the trends I have been seeing I cant wait to try the cam That just showed up at the door. More spool and air in the motor earlier should net me more milage along with the ability clear that haze. This should also get my primary making more than 1-2lbs of boost when just daily driving. I would love to break 20 MPG with this truck one time in the 6 years I have owned it! When I had my timing up around 19* I lost alot of off idle power till the DVs were installed and milage was about the same. One of these days I am going to split the difference and see if 18* does any better for me.

Big Blue24 said:

All that I can come up with is the combination of injector spray angle and high timing makes this motor most efficient in the 2000 - 2150 RPM range

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I think you pretty much got it $.02

Just for giggles, take it down to around 18° and see what changes.

Or up to 30

I've always thought of it the same way; if you run your timing higher, it puts your "power ban" up in the higher RPMs, allowing you to still get good mileage, but able to cruise at higher speeds. $.02

i unhooked my afc tube from the head, and made a straight shot run from calgary alberta to regina sask.... so far on this tank im @ 380miles and still just under half. held it @ 2000rpm 70mph for 8hrs straight with a bit of stop and go.... tons of wind and the odd rolling hill

i find i get better miliage at higer altitudes also. seems the thin air is easier to push a big truck through.

Kronic_187 said:

i unhooked my afc tube from the head, and made a straight shot run from calgary alberta to regina sask.... so far on this tank im @ 380miles and still just under half. held it @ 2000rpm 70mph for 8hrs straight with a bit of stop and go.... tons of wind and the odd rolling hill

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Pretty much running like valet then?

12valvePwr96 said:

Pretty much running like valet then?

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Yep.

My 99' 24 valve got 1.5 worse MPG when I lived in Utah for a few years at 4500' vs my hometown in Washington State around 2200'. I was a college student so I had several tanks to compare at the various altitudes and it definitely was worse in Utah.

I also noticed a solid 1 mpg loss when I added a simple 2" leveling kit.


Back to the topic at hand, I didn't end up retarding my timing last night, instead i installed some 5K springs in the Junker Drag Truck so that I could use it's 4K springs in my daily driver.

I will get the timing retarded and report back on 65-70MPH cruising mileage but I need a tank or two of just the GSK difference to make sure it doesn't adversely affect my current fuel economy test results.

I'm getting 24.7 mpg cruising at 70-75. 60-65 I'm getting 25.2, 80mph+ I haven't seen over 19.6 mpg

Sabbath5.9 said:

I'm getting 24.7 mpg cruising at 70-75. 60-65 I'm getting 25.2, 80mph+ I haven't seen over 19.6 mpg

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And you're running 16*, correct?


Interesting......

What size tire and rear gear ratio, what I'm really after is the info so I can calculate your cruising RPM at 62.5 MPH vs 72.5 MPH.

33 and 3.54

my rpm at 72.5 is just about 2000-2100.

My 4x4 on 35's was getting about 19-20 with the timing at 19*-ish and an opened up turbine housing and ported head. I deffinitely think you need to drop the timing to the 18*-20* (max) range.

the lift in my truck makes driving it thru saskatchewan much like trying to steer a sialboat in the wind...... but it looks sick.

Kronic what size tires do you run and what kind of mileage are you pulling?