Fuel economy

[mrtazwrench]

Seems silly to post about FSJ's and fuel economy, but with the trend in fuel prices I'm looking to do what I can to get what I can for fuel mileage out of my truck before I start driving it for the season. Its an 87 J20 360 stock except for Edelbrock intake, street demon 625 carb, HEI dizzy, manifolds, Y-pipe performance muffler, 3.73 gears and soon to have 315/75/16 tires(almost 35's at 34.6"). Right now with 265 tires I get 10 MPG. Right now I would maybe change carbs, someday I might do EFI but not yet. Open to whatever setups have resulted in the best fuel mileage.

[MysticRob]

You're pretty much in line with EPA estimates, and EFI won't net you much unless you're running badly now, which doesn't sound like a problem.

The engine, transmission, gearing, and weight is the biggest detriment to these beasts. If you're stockish, then you're on a 3-speed transmission and boat-anchor iron block. Going to an AW4 will be slightly better than a stock 3-speed auto, but unless you spend a lot of cash doing an LS swap with a much better transmission, you're going to be pretty screwed.

Car and Driver magazine had an article years ago about how each added gear in modern transmissions increased MPGs somewhere around 8-12% for each of those added gears, but don't quote me on that.

['89_Wagon]

Best bang for the buck ways to gas... good tire pressure, conservative driving style, good alignment, and a well tuned engine.

FWIW, I did a 4 speed conversion + EFI on my AMC 360 GW on stock tires and suspension and those changes netted about 2-3 MPG. So I end up getting 16-17 MPG on longer trips but still terrible gas mileage doing errands. I did these changes with reliability and drivability in mind, knowing I would never get my money back on gas savings.

Here's my little rant... It's hard to justify major FSJ upgrades/changes on the based solely on $ saved on gas. The dollars spent to save gas money take years to recoup (if ever), all the while those upfront dollars could have spent/invested in something else that would perform better from a money building perspective. I remember in a college course hearing what sounded convincing at the time, how an home investment in solar panels would pay for itself in 7-10 years. Initially, I thought "wow, that makes a lot of sense! Why isn't everyone doing that?' Then, later on in life, I learned a payback time of 7-10 years is terrible. I guess the point I am trying to make is, don't spend dollars trying to save pennies.

[babywag]

with gearing/tires you have the only thing that would improve it is better transmission swap or modern efi engine+trans swap.

lose any removable weight and drive like a granny may help

[mrtazwrench]

Back in the early 90's I had a big block Mopar ended up with a 600 Holley on it and got near 15 MPG, but like the one reply it was on a road trip, anyway I was thinking someone might have a carb combo that did good like that. If this Gear Vendors kit was not $3,100 I'd be game to try one of those even though they are for dodge truck applications. At least it's just a summer toy and not a daily driver.

https://www.gearvendors.com/product/automatic-3-speed-727-np-208-aluminum-case/

[goldhammer]

My 81 wt, 360, all stock, t18, 3.31's, turning 31-10.50's would get me 12 in town and the 10 mile trip to work. 14 on the freeway at 65,just at peak torque.

Think that's where you'll find a sweet spot

[mrtazwrench]

My 81 wt, 360, all stock, t18, 3.31's, turning 31-10.50's would get me 12 in town and the 10 mile trip to work. 14 on the freeway at 65,just at peak torque.

Think that's where you'll find a sweet spot

That might be more apt to happen when I get the new tires on, I did the numbers on spedo correction and found out I better not push 5 MPH over the limit like I'm used to as it will be 5 MPH over with the tire change, so I will be going the same speed at less RPM.

[goldhammer]

I tended to drive by tach. Once I got over that 2300-2400 rpm spot, mileage took a nosedive off a cliff.

Had a waggy owner ask me at a gas station what I got for mileage, told him. He complained that he only got 10 no matter what. Balanced, blueprinted, carb overhauled and running as lean as he could. Told him, yeah with an AT, bout all you're going to get, mine had a 4 sp. He liked to give me a concussion sticking his head in the window to see. He never knew you could get wags and cherks with a stick.

[babywag]

That might be more apt to happen when I get the new tires on, I did the numbers on spedo correction and found out I better not push 5 MPH over the limit like I'm used to as it will be 5 MPH over with the tire change, so I will be going the same speed at less RPM.

larger/wider tires will kill mpg
efi netted me several mpg(3-4) city driving after tuning
efi gains maybe 1mpg hwy driving if that
the lack of 4spd and zero aerodynamics make these things gas pigs

[mrtazwrench]

I guess I'll get a tach and try Goldhammer's RPM trick, I do have an auto though which won't help.

[letank]

35" tires and 3.73.... better get an early 90's econo box, it will pay itself in 2 to 3K miles... of course in your area, they might already have been devoured by rust.


Yaris, corolla, civic... protege... now the prices are thru the roof... I scored a $500, 265K miles protege last fall for the kids... beside a loose battery cable and worn off tires, it runs.... and stick shift, so no issues w weird auto trans

[tgreese]

...
the lack of 4spd and zero aerodynamics make these things gas pigs

These old-design automatics are inefficient. They are strong and smooth, but not efficient. I suspect you could get nearly the same improvement from a modern efficient automatic as from a manual transmission.

IMO aerodynamics is the main thing these Jeeps lack for better mileage. This goes along with the big tires and wheels reducing mileage, partly from rolling resistance and partly from a larger profile in the wind. Modern design optimizes every surface and edge in the vehicle for less drag. Also, drag goes like the square of velocity, thus the speed you drive make a big difference in mileage when your vehicle has such poor aerodynamics.

[Crankyolman]

I think the best solution is to jam a tennis ball under the throttle pedal

[goldhammer]

Aero is a cast iron b....

Once brought home a J truck cab on a trailer with around a 30-36 inch deck. 65 miles, got about 5 mpg. Winds not kind that day, fought headwind all the way from Boardman to Portland on I-84.

[rocklaurence]

Your truck/J20 is almost exactly what I have [same carb/gears/tires]. Except: I may have more lift and Race Shift kit with 2000 stall TC. I get bad fuel mileage--not horrible, just Bad