Crappy MPG - help?

[nvrstuk]

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




So... 5mpg...I get that while snow bashing with a 500HP stroker...with SEFI now and Q-jet for 6 years before that... I get 2.5 mpg in sand.

9 of us drove our Broncos from the Seattle area to the Rubicon and back...I've told this story about 2 dozen times... it's appr 2,000 miles round trip...lot's of fill ups.

ALL of us checked mileage, 3 of us had 99.9% accurate odometers. We checked mileage by the mile markers on the road side to double check... what I'm saying is these numbers are 100% accurate... I got 14.5mpg average there and back... without EFI...

My rig back then--Qjet, 37" BFG tires, ZF, 4.56 gears, loaded for a 10 day trip. My Q-jet and properly tuned got better mileage at every gas station except one got better mileage than 3 other EFI and Exploder engines. Almost the same set-up as yours.

Is your engine properly tuned up?

What carb? Is it dialed in?

No O/D and what speed are you driving at that you're getting such miserable mileage?

Tires- some tires don't roll nearly as well as others...tire pressure, toe-in waay off, if brakes were dragging that bad you'd smell them 100' away...

 

[DirtDonk]

...9 of us drove our Broncos from the Seattle area to the Rubicon and back...I've told this story about 2 dozen times... it's appr 2,000 miles round trip...lot's of fill ups.

Haven't heard stories of The Horde down this way in a long time it seems!
Or maybe I just haven't been paying attention. But was fun meeting up with you all one time at least.

Come on back down sometime!

Paul

 

[Rustytruck]

Pull the plugs and see what they look like. Post a picture of the plug tips.

 

[blubuckaroo]

Speed is the biggie.
I can get 17 MPG all day long as long as I keep it under 60.
I hate that! Holding up traffic on the interstate sucks for everyone, but there's nothing wrong with staying in the slow lane. It gives the other drivers a better chance to see a really nice Bronco.

The other is tire size. Even if the gear ratio is correct for the tire diameter, wide tires have more rolling resistance and wind resistance.

 

[nvrstuk]

Blubuckeroo is right on...

I forgot to say in my post above that I lost more mileage going up than I ever did with engine size increases (stroker doesnt count)...every time tire OD went up or I increased susp lift or body lift I lost mileage....hence the reason even p/u's have valances to direct airflow...

 

[M@2s]

Hey guys,
Changed all fluids and gaskets and then took her to the wash. On the way home I lost all power and had a clinking noise under me. I initially thought it was the transfer case output shaft since this seems to be the weakest link, but after I got under the truck and started to disconnect the shaft from the drive shaft, I could turn the drive shaft by hand without the rear tires moving..... Um, problem.

So, I'm going to have to inspect a number of things, but should I start with imagining that a rear axle is broken? If one is broken then, depending on me rear end, I might have zero power if I have an open system.

Any suggestions beyond tearing into the whole thing?

I'll ask the PO what the rear holds, but wanted to put this out into the neighborhood too.

Thanks again for all the help!
I plan on changing my output shaft and speedo gear anyways, so I guess I'll have an extra output shaft for the trails.

 

[DirtDonk]

Yep, either an axle shaft (or two!) or some teeth on the gears, or a busted pinion shaft.
Then again, other than the snapping sound, you could get lucky and find that the pinion nut came loose and the yoke simply spun on the splines.
But then, that would be too simple and cheap!

With that size tire and half-way decent gearing and some torque out of the engine, the axle shafts are actually the weak link. Maybe even more so than the transfer case output at this point. It's all about the gearing as to what takes the most abuse.
Tall gearing and it's the gear boxes. Lower gearing and it's the axle shafts.

If they're stock 28-spline shafts and there is any kind of a locker in there, then a shaft end is right up there at the top of the list.
But hey, the good news is that, if you did not get a record of any gear lube or wheel bearing changes by the PO, this is your chance to check out all the components of the rear end. Brakes, bearings (common failure point with age and mileage), differential gears and oil.

Good luck.

Paul

 

[nvrstuk]

What Paul said !!!

If you're going to the expense of replacing the output shaft because you will be doing trails, I highly recommend upgrading to 31 spline axles because the 28's are a huge weak link. I think I went thru 8 of them before ARB finally introduced the 31 spline unit as I'd been waiting to bump up to 31 splines until they finally got their first generation 31 apline 9" unit on the market around '95 or so... (back in '87 they told me it would be on the market in 6 months...not over 6 years!!! Took them a while... Ford 31 spline was their largest market share back then...

 

[onpier55]

A couple of weeks ago I drove the bronco from Sacramento to San Diego. I have 3:50 gears. Used gps to track mileage. 65-70mph 15mpg 70-75-80mph 14mpg. I was pleasantly surprised. All miles before this trip were just local and hard to track mileage.

 

[M@2s]

Yep, looks like my chance to take it all apart, learn what I have and make any changes that are necessary. I'll ask the PO if the rear is runnin open or with limited slip before I do too much just so I have an idea of what I'll find. I don't have any self locking controls so I think limited slip is as much lock as I'd find.

I'll 100% move to 31 spline if they're broken and not already beefed up.
Thanks guys

Any suggestions on bearings and other things I should change out in general?
Loaded question of course. Images of my tail end are in my garage. I'll have to see if I have the large bearing settup or not already

 

[68ford]

If you have a small bearing housing, drum brakes and want 31 spline axles, check out http://www.9inchfloater.com/CompleteRrEnd/CmplteRrEnd_28_31.htm
You really can't even come close to his price for all you get.

 

[M@2s]

Wow
That is a cheap set up.

Looks like I have large bearings and haven't yet measured my drums. We'll see where this takes me. Thank you for the link and info! Gotta feeling it'll come in handy sometime regardless.

 

[M@2s]

Hey Nvrstuk,
Just re-read your post about mpg. I'm changing out all of the bushings and checking the rear end. Along with it, I'll look into the breaks since it appears I'm doing a full rebuild of everything under the cab. I kinda expected a project, but not so soon. I'm not one to half ass anything so it'll be a rebuild using existing parts but cleaning and fresh rubber.

Your mpg numbers are impressive to say the least, so I'm hopeful. The tires are currently BF G MUDs. AT tires will definitely be better and most likely pay for themselves. Also, there may be some toe.. Which I'll check once I have the bushings all set.

I got 7 degree c-bushings as a replacement but I'm not sure what's in there. The rig has a wild horses radius arm set up and I'm told they come with 7 degree bushings, so I'm not sure where else I can splurge to get better numbers on camber too.. But it's something I'm looking into. All in all, it's an adventure and I hope to get her road worthy enough to some day meet you for another Rubicon run.

When on the hwy, what speed was your crew going when matching mpg numbers? Just curious, as blubuckeroo is right, speed is a big factor here.

P.s. How do I post picks to this string? Do I have to post them somewhere else and have a link?

 

[DirtDonk]

I don't have any self locking controls so I think limited slip is as much lock as I'd find.

There are "selectable" lockers, then there are "set-n-forget" lockers.
Only selectables will have any sort of external control, so you could still have a locker and it not be visible from the outside.

I'll 100% move to 31 spline if they're broken and not already beefed up.

Good call. If you have, or desire to install, any sort of traction aiding device, whether it be limited-slip or locker, you'll want the beefier axle shafts.
With a limited-slip and not hard use, you'd be fine with 28's, but still 31's are a good upgrade. If you go with a full locker though, their life expectancy can be very limited.
Some live good long lives, while so many more die an early death even on the street. Had a friend go through three 28 spline axles with his Detroit Locker all on the street!

Any suggestions on bearings and other things I should change out in general?

Most aftermarket 31spl axles will have new bearings already. Some may stick with the standard sealed ball bearing setups, while others will have the Set-20 tapered roller bearings that Ford used on heavier full size trucks.

If you have to buy your own, BCA and Timken are two of the top brands. Most don't have bad luck with even cheap bearings, but since the costs are minimally different here, might as well go with the accepted good stuff.
I was pleasantly surprised when my recent purchase of some BCA bearings showed them all made in the USA.

Images of my tail end are in my garage. I'll have to see if I have the large bearing settup or not already

As you noted, you do have the large bearing. That's either a full-size Bronco/F150 housing, or a '77-only EB housing. You can tell by the wider center housing and more gradual transition to the axle tubes, as opposed to the original "banjo" style housings of all the other EB's.
These were only available (as far as I know) as big-bearing, big brake housings.

If yours is a '77 housing and was not modified to fit something else, you have the largest of the EB brakes, at 11" diameter and the wider 2.25" shoe width of the '76 and '77 Brakes.

I got 7 degree c-bushings as a replacement but I'm not sure what's in there. The rig has a wild horses radius arm set up and I'm told they come with 7 degree bushings, so I'm not sure where else I can splurge to get better numbers on camber too.

Do you mean our "bushing set" or actually our radius arms? As far as I know we never automatically included 7° bushings with any radius arm. In fact we would usually have tried to talk the customer out of them!
The radius arms have 4 degrees built in, so the typical bushing would depend on what your caster numbers were to begin with, but would generally be 2 degree offset bushings. With 4's as an option if you needed to gain more caster and were ok on the pinion angle.

P.s. How do I post picks to this string? Do I have to post them somewhere else and have a link?

The most direct way is to become a contributor for 12 bucks a year. That way you can post directly from your device right to the thread.
There may be workarounds, but not sure what those are. And we like to support the site generally anyway.

Paul