In my never ending quest to figure out my poor gas mileage (12-13 MPG), I've decided to take a second look at my ignition timing. Initial 14 degrees, mechanical advance is 24 degrees and my vacuum advance is adjusted fully clockwise and it's producing 12 degrees max starting at around 9" of vacuum.

I do not get any pinging and it feels "fine" but I've read about others getting a full 18 degrees of vacuum advance, can't help but wonder if that would help considerably?

Quick background on the truck might help...

25k mile rebuilt 300
NP435
4x4
3.50 gears
31" tires with 35psi
6mo old National Carb rebuilt YFA
New cap, rotor, plugs and wires (disclaimer, I got the cheapo cap and rotor with tin(?) contacts instead of brass)
New air filter and fuel filter
New harmonic balancer
All vacuum leaks fixed
Idles at 19" Hg
Cat is of unknown age/condition
New EGR valve
Air pump bypassed
New thermostat

At one point (before the carb and tune up) I was getting 13-14 mpg (still not great) but after the carb and ignition "tune up" I'm only getting 12-13 with equal driving habit and route. Granted it has become hot as hell here in AZ so the A/C has been cranking, so I can attribute the 1 mpg loss to that. So really I'm at a wash after all said and done. The truck feels "ok" to drive, plenty of torque to get around but I don't have a well sorted (mpg) 300 to compare it with. It drives me nuts whenever I read that a decent carbed 300 should be getting at least 15 mpg and when people can top out over 20 I feel even more ashamed.

 

Tell us about your vehicle.
Body style
weight
2 x 4 or 4 x 4
tire sizes
drivetrain specs

Your mileage may or may not be typical.

 

Hey man, thanks for replying. Most of the specs are in the OP, but the body style is an 82 Bronco.

 

I do not think you will ever see 20 mpg with that package. Two vans I've owned got 19 and 21 mpg highway with AC off, 55-62 mph, 4 spd toploader OD, 3.00 gears, 2- wheel drive, smaller tires, flat and level roads. The van has better aero and likely weighed less or the same.

Be sure all the little things that can significantly influence mileage are nominal, like choke come-off, float level, fuel pressure, oil levels, etc.
I would install a cat test pipe take it out on a highway tour, maybe at night when no AC is needed and get a good baseline mpg.

 

I'll assume you meant to say NP435 (granny low).. With all of that in top running shape including the cat test pipe that has been mentioned, I would test it at a maximum of 50-55mph and see what you get.

31's epecially 10.50's can be most of your problem especially in a hot climate where ur gettin maximum "bite". it's akin to pedalling a fat tired mtn bike in 100 degree heat on black pavement...just another take on that problem.

 

Hey Parsedout. I know how frustrating that can be!

First off, what type of conditions is this mileage under?

Is it stop-and-go around town?
35 - 45mph?
Interstate at a smooth, constant 60 - 65mph? 80mph?


I have an '81 Bronco with a 3.00 rear end. It's had 2 different tire sizes, 2 different transmissions, and a multitude of carb/intake/exhaust combos. I can't believe how vastly the gas mileage has changed and danced around throughout its life.

When I first got it:

1bbl carb, SROD, 215 75R15 tires.
16 in town. 21 open road.

Had carb professionally rebuilt:
12 in town 17 open road

Swapped to EFI manifolds, new cat, muffler. No EGR/Smog pump.
11 in town 18 open road.

Swapped to a new YF carb (given to me by Harte3) so that I could pass emissions.
9 in town 22 open road

Swapped to Holley 390cfm 4bbl
12 in town 12 open road

Swapped to 31" tires (also stopped using the SROD overdrive because it was too low)
12 in town 12 open road

Swapped to Holley 600cfm 4bbl (model 1850)
15 in town 19 open road

Swapped in an NP435 transmission
15 in town 19 open road

Swapped to a different Holley 600cfm 4bbl (model 80456)
12 in town 16 open road

Swapped to Holley 80555c spreadbore
11 in town 14 open road

Swapped to Holley 465cfm and installed a DUI ignition
14 in town 17 - 18 open road



From my experience, when it's running well, the 300 is one of those engines that will simply consume however much fuel you dump into it and almost always return the same performance. It can be really frustrating and difficult to tell why yours is using so much more fuel then someone elses without any real reason why. I feel your pain because as you can see, lots of the changes I made above either netted me way more or way less mpg and it doesn't really make any sense why.

However, also note the differences all with the stock 1bbl carb. When I first got it, it'd been sitting in a field for 4 years and when I drove it 700 miles home from California, it got 21mpg. I had it professionally rebuilt and it dropped by 3 - 4mpg. I swapped to another one of the exact same model and my around town mileage tanked while my interstate mileage went way up. Maybe it's simply the tune on the carb.
Those little 1bbls can be finicky. A finicky carb on a finicky engine can be a really frustrating combo.


On a side note, I would recommend taking it down to an exhaust shop and having them check out your cat. Most shops will do it for free.

 

If when Arizona summer hit you only lost a mil or two per gallon, I would say you are doing well.


One other factor - does your gas formulation change in summer? Especially around big cities they tend to reformulate gas in hotter weather as a mans of pollution control, and some of those formulations can be high enough in alcohol to put a hit on your mileage.


A friend I worked with always said that a nice carb rebuild really opened all the passages and ports so you could REALLY suck gas.

 

All good advice, I'll check some of the basics around the fuel delivery and make sure it's all as it should be. Test pipe is also a good idea.

Yeah, NP435 sorry about that. The tires have a pretty high treadwear rating but absolutely they could be a factor. I try to keep the PSI on the higher end.

Hey Abandoned, driving conditions are about 50/50 highway/city. Here in AZ, if you're not going at least 70mph you'll be run off the road so that's about where I keep it. With my non-OD tranny, I'm spinning about 3k at that speed. Can't be good for the MPG?

Well I did do most of the work right before the summer hit, so it's possible I'll be up to the 14-16 range once the heat dies down. We'll see!

I think they reformulate the gas for winter, not summer here but I could be totally wrong. There is definitely an ethanol blend year round though, hate this gas.

 

Check out, pure-gas.org, its a site that lists all the ethanol free stations by state and the app will even navigate you to the closest one. There are 15 stations in Arizona but looks to be mostly located near the bigger cities or near the lake where boaters like the ethanol free too.

 

No, 3000 RPMs isn't going to net you the best mileage. My '84 is in the same boat with an NP435, 3.55 gears, and 31" tires. It's around 3000 RPMs at 70 - 75. Even at 65mph, it still only gets 15 or so because the engine is running at some pretty high RPMs. It's begging for a good overdrive.

Understandable about having to go 70. Our interstates are 80 here and I'm often inclined to keep up. In my '81 (with the 3.00) at 60 I'll get around 18, 70 around 17, and 80 around 14. It drops off fast as you go faster.


If you really want to know how you're doing, you need to do a baseline. Go somewhere where you can go a decent speed, like 60 - 65 or so (I think 50 - 55 is too low since most public transit roads are at least in the 60s, especially in AZ) and where you can travel for 100+ miles or so. The less hills the better. Top off before you leave and again when you get there. Check your mileage. Repeat on the way back.

That'll give you a lot better idea of what kind of mileage you're getting than combing stop and go traffic with fast speed driving.

 

Parsedout,


Have you checked the brakes closely to make sure one or more are not dragging?


Gas mileage is a hobby of mine and I will provide a link to a thread where some changes to my truck are discussed. I hope it will perhaps give you some ideas.


https://www.ford-trucks.com/forums/1...-9l-300-a.html