Gas Mileage

[MBRZ71]

still waiting on proof from OP. spoiler, it won't happen because he's not getting 20s mpg

this is as bad as the cummins guys claiming 20mpg while towing

Just wondering what is acceptable proof? I think it may vary from person to person.

 

[RichLo]

What’s the classic quarter mod?

Please read the thread before posting, its not that hard to read through 3 pages. It was explained a few times already.

 

[Caman96]

Just wondering what is acceptable proof? I think it may vary from person to person.

IDK, but an “88 Squarebody” full size Blazer from what I see got 11/city - 16/highway and came with 235/75/15’s. So can 33” tires, 3.73 gears, new plugs, Acdelco plug wires, Accell coil and a K&N oiled air filter gain you 5-7 mpg on a 36 year old truck? Remember, having new clean fuel lines is irrelevant compared to a brand new one, that when new, only got 11-16 mpg.

 

[L31MaxExpress]

I have owned several TBIs. Every single one was better on fuel than the EPA estimates on long trips. My Vortec was better than the EPA estimates as well. That was running 70-75 mph as well. I find 55-65 mph actually kills fuel mileage compared to 70-75 in the typical undergeared modern vehicle.

 

[0xDEADBEEF]

Not only was your engine 'acting' smaller due to the lower air density, but also your Pathfinder was acting like it
had a virtual smaller total frontal area at elevation in CO as compared to driving the same speed while pushing
through 'closer to sea level air density' in TX. And of course the faster you go, the more that
frontal area x Cd x Air Density matters.

****

I've always had an interest in MPG/efficiency. Once you spend just a little time rooting around
in the MPG rabbit hole, the fact that rolling resistance is a constant, but that wind resistance
(ie: punching a hole in the atmosphere) goes up with the square of your speed, this means my
preferred method of rapid automotive travel directly affects my wallet. :-(

Of all the wind resistance graphs out there, I find the following graph to be the most helpful to me:

You must be registered for see images attach

(Compare the total resistance at ~45mph {~17KW} vs 90mph {~70KW} -- Speed thrills...but also sux the gas!)


I like to use the following curves when trying to map a real-world 20 or 30 mpg vehicle to fuel consumption at speed.
(Note: For accuracy, you have to include any headwind in addition to your ground speed in order to find the
most accurate spot on the curve below. (If you've ever headed West and pushed all day against a steady 20+ mph
headwind you know the difference it can make in a tank of gas.)

You must be registered for see images attach

A thumbnail sketch of what a given MPG # for a tank of gas represents would include:

* Wind Resistance x coefficient of drag for your ride (see squared resistance curves above)
* Rolling Resistance (Tire inflation = how much energy lost to flexing tire sidewalls + contact patch friction, size & shape)
* Pumping Losses (the bigger the motor x the higher the cruise vacuum {the worser} x the more Revolutions Per Mile {the worser}
* Parasitic losses in drivetrain: Power consumed to drive water pump & fan + stuff on serpentine belt, engine oil pump, auto trans ATF pump, & power through 90° angle in diff)
* Total number of changes in vehicle speed per tank.
* Total vehicle weight vs number of accelerations per tank. (more work performed per mile driven + 4 tires acting as large flywheels consuming energy with every speed change.)
* Total Number of Elevation Changes per tank (!)

****

Given all of the above, if I wanted to post a 'repeatable at will' hero MPG number for my Built to Work big block chore truck,
this is what I would do:

1) Tires aired up towards the max rating. (Temporarily trading long term goal of even wear across the tires for min rolling resistance.)
1) Engine warmed up, fill up tank (slowly + 3 clicks) at gas station located on Highway 36 at the Colorado/Kansas border.
2) Head East, come up to speed once, and drive at the Rt. 36 speed limit, taking advantage of any prevailing tailwind I might encounter.
3) ~24 gallons later, stop driving smoothly, pull over to refill. (slowly + 3 clicks) Note: No hypermiling/getting in the way of other folks on a mission.
4) Post photos of both time stamped receipts + before/after odometer readings, and resulting hero MPG for the beast. (And yet still instantly lose credibility
with some because they've never seen anything close to this # in a similarly equipped GMT400. :0)

I know, all of the above is concentrated boredom when gas is hovering at $2.00/gallon. But as gas prices are closing in on $4/gallon yet again,
I offer the above as food for thought. And from the way the original post reads I'm thinking that @Nick88 has honed his GMT400
supercruise skills and has figured out how to minimize the effects of those big tires acting like 4 large flywheels that resist changes in speed.

It's quite possible to achieve > factory MPG ratings. But the driver has to work within the confines of the top 7 MPG variables listed above. And that same driver
will never get the 'KS Rt. 36 nonstop cruise MPG' down on parallel I-70 at 75-80mph heading westbound, or when employing heavy pedal action during the
cut & thrust I see when folks are rushing to/from work, or driving up/down the mountain roads of Vermont.

Given all of the above, I hope that Nick88 and others will continue to share what they've been able to eke out
of their GMT400 vehicles. FWIW I've driven everything from 70+ mpg 2-seater aluminum Honda Insight hybrids to
a lean-burn Civic VX on the far side of 60mpg...all the way down to the big block C2500 chore truck, so from firsthand
experience I am comfortable to share the above.

I'm all about the data. Together let's see what we can discover about the GMT400 vs MPG calculus.

EDIT: Years ago I use to traverse Kansas on I-70 with the goal of minimizing the time spent doing so.
Oh so boring. And then there came a time where I just didn't want to face the monotonous sameness of that
driving chore yet again, so this is how I was motivated to discover the Rt. 36 experience. Sure, it takes a bit
longer in terms of sheer wall-clock time, but it actually felt a lot shorter behind the wheel, and I enjoyed the
little towns I drove through along the way.

As a matter of fact, on the rare occasions that I still get to indulge in a road trip I almost never use the
Interstate system anymore. And no regrets. :0)

Cheers --

Great post!

It takes a X amount of power to go a given speed. You might be able to squeeze out a little efficiency here and there, but does anyone believe GM would leave 5 mpg on the table when they could just run a different air filter or put a different spring in something? It's ridiculous.

 

[Caman96]

And this is only a 32” tire compared to a 235/75/15.

You must be registered for see images attach

Edit:

This information was bugging me. Showing the tire diameter as 23.33. It’s wrong…should be 28.8.

 

[L31MaxExpress]

Great post!

It takes a X amount of power to go a given speed. You might be able to squeeze out a little efficiency here and there, but does anyone believe GM would leave 5 mpg on the table when they could just run a different air filter or put a different spring in something? It's ridiculous.

Well GM did leave a ton of efficiency on the table with the TBI due to the lack of technology available at the time to meet the emissions requirements. The lack of an IAT sensor and un-heated 02 sensor causes the fueling to drift all over the place. The heated sensors were installed after the Y-pipe as well for a more accurate representation of what the whole engine was doing AFR wise. The single wire sensors have a ~30K mile life and fall off drastically in accuracy after that. When they fall off, they fall off in the overly rich direction causing much higher CO and HC readings and atleast a 2-3 mpg loss. The pellet box cat in addition to being overly resctrictive was only capable of reducing HCs and CO, not NOx thus EGR, richer mixtures and limp wristed timing advance. The pellet bed also has a tendency to clog and become even more restrictive or the screen material inside fails and fills the muffler core with the beads, choking it. It is fairly easy to get 3-5 mpg better out of a TBI especially an earlier one. I added long tube headers, dual in/single out high flow 3 way cat, a K&N filter and tuned a ~200K mile L03 305 years ago. Made more power at the wheels than GM rated it at the crankshaft and gained about 4 mpg. That 305 made 18 ft/lbs more torque than my stock L31 made at the wheels and only 8 hp less.

Higher fuel pressure does offer a better spray pattern and promotes better fuel atomization and distribution in the manifold. GM went to smaller injectors running at 30 psi to help this later on.

 

[L31MaxExpress]

And this is only a 32” tire compared to a 235/75/15.

You must be registered for see images attach

Edit:

This information was bugging me. Showing the tire diameter as 23.33. It’s wrong…should be 28.8.

You have the 2nd tire listed as 235/45 not 235/75, FYI.

 

[Caman96]

You have the 2nd tire listed as 235/45 not 235/75, FYI.

Good catch!
Here’s correct comparison.

You must be registered for see images attach

 

[glock20]

Just wondering what is acceptable proof? I think it may vary from person to person.

not really important because we haven't seen anything except a bogus claim