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Crappy MPG - help?

M@2s

Newbie
Joined
Mar 22, 2019
Messages
11
Hi guys,
I'm new to the forum and have searched it out a bit looking for some answers but I decided to post for suggestions anyway.

I bought my rig about a month ago and I'm figuring it out, making changes and tuning things. I'm new to 4x4 and broncos but not old cars.

After a 20 gallon tank refill, I got 100 miles. I'm thinking something is a bit off here.

4 barrel carbed 351W with small RV cam, 4.5" lift, 37" tires, 4.56 gears, NP435, running topless and cage doors.

I'm going to check for dragging brakes and other obvious issues, but shouldn't I be a bit closer to 9 MPG instead of 5?

Looking for real guidance suggestions and not comments of me being a nube.

Thanks for the help in advance.
 

LilMixedUp

Full Member
Joined
Feb 27, 2016
Messages
163
Loc.
Bonaire
I would think 9-11 at least.... my last 351 with 4.11 and 33's got 14. If I babied it I could squeeze 15+. As you said, tune up to include timing and do a good check up of brakes, tire pressure, fluid levels; etc... and a lot depends on how you like to drive:) petal to the metal or "pappy" like.....
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,821
Are you going by the gauge? Or are you saying that after a second refill you were able to put 20 gallons back in the tank?
Is your speedometer and odometer corrected for the tire size and gear change?

I would not be surprised if you were more in the 7-8 range, rather than the 9-10 range that many get. Your 37's are really holding things back. Even with 4.56 gears.
And a 351 with an "RV" cam is an unknown too. Hard to say how much effect that cam is having. But yeah, at 5 there is something else at work.
But it might be the filling method in the end.

Does yours fill easily and consistently? Or do you have to fight it?
Are you able to run the tank dry because you have dual tanks? Or do you have to just estimate and re-fill?
Running dry then refilling then running dry and refilling while checking miles traveled would be the sure bet, as long as you know for sure how many miles you just went.
If your odo is correct, or if you've calibrated it via GPS or someone else's vehicle, then you're good there.

Paul
 

jagbucket

Full Member
Joined
Aug 9, 2013
Messages
251
if alignment is off, with larger tires especially it could increase rolling resistance . worth checking the toe anyways. Push by hand to see if it rolls easily or in a bind. My rebuilt tc was very tight at first and i should have taken it apart but with a few miles on it now rolls easily i can feel a change performance and mileage. What color is the exhaust pipe ? black sooty or tan ? If black move to the plugs and read them.
 

sprdv1

Contributor
REBEL
Joined
Mar 8, 2007
Messages
81,823
Welcome aboard.. Have you asked the PO what they were getting, if they had a norm that you could work to improve on?

Hi guys,
I'm new to the forum and have searched it out a bit looking for some answers but I decided to post for suggestions anyway.

I bought my rig about a month ago and I'm figuring it out, making changes and tuning things. I'm new to 4x4 and broncos but not old cars.

After a 20 gallon tank refill, I got 100 miles. I'm thinking something is a bit off here.

4 barrel carbed 351W with small RV cam, 4.5" lift, 37" tires, 4.56 gears, NP435, running topless and cage doors.

I'm going to check for dragging brakes and other obvious issues, but shouldn't I be a bit closer to 9 MPG instead of 5?

Looking for real guidance suggestions and not comments of me being a nube.

Thanks for the help in advance.
 

blubuckaroo

Grease Monkey
Joined
Jun 11, 2007
Messages
11,795
Loc.
Ridgefield WA
Welcome M@2s...
Low mileage rairly has just one cause.
Your Bronco, as you describe it, isn't a candadate for excellent mileage, but should do way better than 5-6.
How well does it run while cruising? Does it run smooth? Does it smell like it's running rich?
How much weight are you carrying?
What are your driving conditions? Hilly or flat? Cruising speed? Good mileage stops around 55 mph with these rigs.

If it were me, I'd do a quick check of the timing and make sure the vacuum advance is working.
Check the running fuel level in the carb.
Check tire pressure.
Make sure the plugs and plug wires are in good shape.
 

SHX669

Bronco Guru
Joined
Jan 9, 2009
Messages
1,997
How accurate is your odometer ? These things came with 28" tall tires.and the odometers were calibrated for that.
 

sykanr0ng

Bronco Guru
Joined
Aug 11, 2014
Messages
5,363
The OP has not been back on here since the 3rd, is he even checking this thread?
 

Pa PITT

Contributor
Bronco Guru
Joined
Jul 15, 2005
Messages
11,257
Loc.
Stephenville TEXAS
.... Sorry but I think 5 or 6 is about correct for you rig.
..I had a 1986 351 Holley 7 ABOUT 4 WAS ALL IT EVER GOT .. I sold that thing.
 

Digger556

Sr. Member
Joined
Aug 8, 2013
Messages
793
Hi guys,
I'm new to the forum and have searched it out a bit looking for some answers but I decided to post for suggestions anyway.

I bought my rig about a month ago and I'm figuring it out, making changes and tuning things. I'm new to 4x4 and broncos but not old cars.

After a 20 gallon tank refill, I got 100 miles. I'm thinking something is a bit off here.

4 barrel carbed 351W with small RV cam, 4.5" lift, 37" tires, 4.56 gears, NP435, running topless and cage doors.

I'm going to check for dragging brakes and other obvious issues, but shouldn't I be a bit closer to 9 MPG instead of 5?

Looking for real guidance suggestions and not comments of me being a nube.


Thanks for the help in advance.

I had the same combination. Got about 9mpg.

Check for fuel leaks, open holes in the fuel tank. What do your spark plugs look like?

Have you confirmed your speedo / odometer are accurate?
 

73azbronco

Contributor
Bronco Guru
Joined
Nov 11, 2007
Messages
7,842
How fast were you going?

With 4.56 gear and 37 tires, you are already into sub 10 mpg territory on a good day.

With my well broken in 351, stock cam, I got 11 mpg with 3.75 and 31 tires with a top doing 55-60.

Add higher speeds above 60 and fogettaboutit.

Seriously, try a 100 miles doing 55-60, see what that gives you if no other obvious issue pops up.

Oh, and it's spelled noob:)
 

valveamp

Full Member
Joined
May 30, 2011
Messages
287
Lots of good ideas here. I'll throw my recent experience in here. I recently had a string of carb problems that weren't all related to the same root cause problem. After replacing my carb, an Autolite 4100, with a carb of the same type. My gas mileage was horrible. I didn't actually measure it because that carb wasn't on long enough, but I had to drive about 20 miles or so and I used about 1/2 of a tank. It was really bad. I took it to an engine place that had a dyno (that was the 20 mile drive). They ran it and the A/F ratio was waaaay off. It was so far off, it max'd out their equipment. They significantly changed the jets (sizes) and the A/F ratio came back to normal. The other evidence of this was the spark plugs (as Digger556 mentioned). They were black and oily looking, indicating the engine was running extremely rich. The shop replaced the plugs and all went well (for a while). So, I would pull the plugs and see what they look like. That would be a good indication of how it's running fuel wise.
 
OP
OP
M

M@2s

Newbie
Joined
Mar 22, 2019
Messages
11
You guys are amazing!! I seriously appreciate all of the helpful insight. I didn't even consider that my odometer gear is off. That's a pretty large culprit already. I'm sure that takes me into the 7-8 MPG range alone. I think that I' have a few other issues to work out too. I have a feeling that my steering is off and there might be something funky going on with that, every once in a while I get a bind and the truck feels like its fighting one tire being crooked or something. When I look under, everything looks to be in good shape and not broker, but I have some more tinkering and greasing to do.

I had ran the rear tank down to where I thought was empty, but it only took on 20 gallons and I'm pretty sure it's a 23 gallon tank. I refilled till the the pump quit... so I recorded 100 miles on the 20 gallons I refilled.

The engine has about 30K on it.. just nicely broken in. It runs great, sounds great, smells great, but I need to check the plugs and compression to really know what the doctor thinks.

I have been doing little run about in town, some hilly portions, and been on the freeway at about 65.. so all recipes for worse mileage in general. But I'll tell you guys, you've given me a lot to work with for checking and I really appreciate that there is a place like this to get some real workable knowledge. I'll let you know how it turns out.
 
OP
OP
M

M@2s

Newbie
Joined
Mar 22, 2019
Messages
11
Another thing I found too, unrelated to MPG.. I heard something snap while I turned a corner the other day. I looked under the tail and found that I have a broken Leaf Spring keeper on both sets of springs (both sides, rear of the axle and first keeper near axle). I'd like to remedy this issue as I feel that there's more play and sway from it. Any suggestions since it was the rivet that broke and I'm not sure of how to get a new keeper attached to the bottom spring. Welding springs is a bad idea, and a u-clamp will not let the springs slide any longer. Anyone dealt with this?

Thanks again!
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,821
I didn't even consider that my odometer gear is off. That's a pretty large culprit already.

Very possible. But easy enough to check and, maybe correct. It's all correctable of course, but some might take more doing since you're into the really big tire size range.

I have a feeling that my steering is off and there might be something funky going on with that, every once in a while I get a bind and the truck feels like its fighting one tire being crooked or something.

Definitely don't want that binding.
Here would be a great excuse to show us some pics of the front end and steering linkage. We might be able to pinpoint a few things right off the bat. Or at least give some directions to look.

Whenever I hear "binding" or "catching" with regard to steering, I immediately go to the lower trackbar bolt catching on the tie-rod adjusting sleeve. This happens when the tie-rod has been switched to what's called "TRO" (or tie-rod over) configuration. It's done fairly often when taller suspension lifts (anything above 3.5" basically) are installed and something is done to help correct steering angles.
Check yours out, but shoot some pics too and let us take a look.

Besides, we love pics! Gotta' see what everyone is running after all.

When I look under, everything looks to be in good shape and not broken, but I have some more tinkering and greasing to do.

Definitely greasing, since most PO's (previous owners) don't seem to think that's important. Or they're so frustrated at some things that they forget the rest.
One area that most forget to do is the centering ball in the double-cardan section of the front and rear driveshafts. Most of the fittings are standard male Zerk fittings, but each CV head has at least one female grease fitting that requires a needle-like adapter to the grease gun.
So they get ignored a lot!

Another area is gear lube. People might change their engine oil religiously (and too often most of the time too) but never change the gear oil in a manual trans, or differential, or transfer case. Not to mention changing brake fluid every few years too.
Most new-ish Bronco owners run into these issues.

I had ran the rear tank down to where I thought was empty, but it only took on 20 gallons and I'm pretty sure it's a 23 gallon tank.

That's actually pretty good. A proper reading fuel gauge typically leaves 5-15 percent left in the bottom as a sort of "unknown reserve" for when you let it go to empty without a station in sight. And Bronco gauges are not often known to be accurate anyway!
So if you had three gallons left when the gauge shows empty, that's perfect in my book.
However, if you mean you ran it to empty and then some, and were in fact almost empty, it's possible that your "23 gallon" tank might not quite be able to load the max in there. I've heard of that being the case more than once, but it doesn't seem to be universal.
What is universal however, is that Broncos can be very inconsistent. Even more so now that they're all at least 42 years old.

I have been doing little run about in town, some hilly portions, and been on the freeway at about 65.. so all recipes for worse mileage in general.

Yep, definitely check it when you have more consistent driving ahead of you. When I was young and had all the time in the world, I used to actually take mine out late at night and head up the highway for a loop of about 50 to 60 miles to check my highway mileage.
Of course, I used to test it on trips as well, but I wasn't patient enough to wait for that after some mod or another, so I made my own trips just to check.
Late at night the traffic was non existent, so I could hold a steady pace on generally level ground. Thought it gave me a pretty good representation of the max I could expect. Not "every day normal" but at least the best I could do.
Can't do that now though. These days there's more traffic at 3 am than there used to be at 10 pm!

For uploading pics, to do it directly from your device you need to be a contributor for 12 bucks a year. To get some basic shots though you can use the "garage" or "gallery" features here as well. Also some members use other image hosting sites and link the images to their discussion. Whatever works best for you.

Good luck. And enjoy the new ride!

Paul
 

68ford

Bronco Guru
Joined
Dec 26, 2004
Messages
2,710
Where is your toe setting on your front axle? You should be able to get over 10 cruising at 60 easily. Engine ignition timing?
I get 12mpg with 4.88 and 35s doing 3300rpm at 60mph. I am less aero dynamic than most EB and weigh 5400 pounds.
Your issue is likely several small issues that add up to quite a bit of inefficiency
 

Budman23

New Member
Joined
Feb 15, 2017
Messages
1
Just read your post. I also have 351w and 4.56 gears. Less lift and 33 inch tires. With AX 15 typically get 9 to 13 miles per gallon. On the highway at 2200 RPM going 63 miles an hour able to get almost 15. Very happy with this. Do run about 33 pounds in tires on the highway.
 

jckkys

Bronco Guru
Joined
Mar 15, 2012
Messages
5,198
Do you have any REAL reason to use 37" tires? I can't think of one. The idea that they look cool to some, doesn't qualify as a real reason. I can think of many reasons to not run 37"s. Horrible fuel economy is just one of them. The 351 isn't part of the problem. With the right gears 351s get the same or better mileage than 302s. Exactly what 4bbl are you using?
 
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