whats wrong with my gas mailge

[84AZbronco]

I recentley purchased a 84 full size bronco.(351) (2 barrel motor cafrt carb) I was was aware this big rig would not get the best gas millage on the road but what i have experienced in my first couple of tanks is outragous. I was always told to expect about 12 mi a gallon. my first full tank i drove till i ran out and it turned out to be a little over 10 mpg. but in my last couple tanks its been coming out to more like 8. my engine is re-built with less than 5k mi on it. my tires are well inflated, the spark plugs and air filter are realitivly new and clean and i regularly use the fuel system cleaner. I can not figure what would **** my gas milage so bad. there are no gas leaks or hloes in my mufflers as far as i can tell. there is one verry small hole at the verry end of my left exhaust pipe right where it curves 90 degres to the left but besides that the rig has no obvios problemes. im looking for sugestions for posibilys to hidden things that can **** my gas milage. perhapse the card is just sucking to much gas and needs ajusting? any oppions and suggestions welcome! also i curunly have 31 inch tires and want to get some 33's will that size tire rub with no lift of any kind? thanx , 84azbronco

 

[Broncoholics]

Do you know the CFM on your carb? I like 4 barrels myself with small primaries for milage and larger secondaries for power. The problem here is the secondaries will really **** the gas if you have a lead foot. Try a 600 cfm single pumper Carter Carb I found it to be the best out of the box, no adjustments. Edelbrock is OK for milage too, most have same size primaries and secondaries so the milage stays consistant. Most others seem to **** the gas from my experience.

When coasting does the truck slow down quickly? try it in nutral to see if something drags like a brake.

Adding 33's should fit and might help gas milage if you do lots of freeway driving. Once up to speed the RPM's will be lower at freeway speeds with larger tires which will help. In town use might get worse due to off the line power loss with larger tires but not much.

The 33's will fit the rear no problem. The front might need a 1" lift coil. Some mfg's sell them if you use a snow plow, winch, heavy bumper or want to level the truck.

 

[highboy73]

he's right about the 4bbl carb the smaller primaries do help as long as you can keep your foot out of it,I had a dramitic change in gas mileage when I changed to a 4bbl(holley 600).especially on the highway.

check your timing,fuel filters,pcv valve,the works go thru it all.

I always do one thing at a time and see what if any diff it made,and as always I start with the cheapest simplest stuff.

I'd check the brakes for a drag like broncoholics said also.

is your 4x4 disengaged?something to check.

 

[repo_man]

i agree with highboy and start with the cheapest things.. just curious does your gas mileage get worse when it's colder outside and better when it's warmer, the reason i ask is that's what's happening to me. when it's below 40 here i get between 6-8 and when it's warmer i get between 10-12 all in town. of course my speedo is off due to my tires but those #'s are my best calculations.

 

[xsbronco]

I'm no expert but I have heard that an oil change can help if needed. Also for colder weather, I have a bud who has engineering degree in chemical stuff and he says the fuel supplies change in volitility between winter and summer months. Alttitude changes make a difference. I have a 429 with edelbrock 750 and I can get 12mpg or more sometimes. Fuel additives aren't always good either I hear. There are so many choices out there, I use one that goes in with every fill-up. Saves me hundreds annually plus octane booster, cleaner, the works. Used in USAC and CAPT sanctioned races. All that along with the previous posts. (Just a thought/ question) Ignition coil bad? Hope that help.

 

[xsbronco]

Plus, make sure you have quality spark plugs. Plug wires aren't jumping arc.

 

[White Horse]

Exact same problem here. I have an '86 Bronco with the 5.0 EFI motor. I figured I could get 18-20 mpg on the highway. Nope. The thing absolutely guzzles gas -- worse than any truck I've ever driven. The rear-end is a 3.55, which is pretty conservative, and the truck is stock, so the problem is not because of modifications or big tires or gears.

I've changed the spark plugs (holy shit that was difficult!) and air filter. Mileage improved marginally. But, I can't afford to drive this thing if it continues to guzzle gas. I have purchased a new oxygen sensor, but other than that, I don't know what else I can do to improve mileage -- short of sending it to a mechanic and getting robbed for the repairs.

Would a fresh distributor cap, rotor and wires make much of a difference? Could the transmission be the problem?

Any help getting mileage back to what it was when new would be GREATLY appreciated.

 

[Seabronc]

White Horse,

Do a search on milage in this forum. There are tons of posts on the subject, just don't get depressed /emoticons/[email protected] 2x" width="20" height="20" /> . 18 - 20 is a rare thing witha Bronco. What are your RPM's at say 60 MPH. Could be your choice of tire size and if you changed tire size you need to change the speedo gear to get an accurate reading on your odometer / speedometer.

Check Jeff's Bronco Graveyard catalog for 80 and up htp://www.broncograveyard.com page 30. It has a chart that is designed to help pick the best gearing and tiresize match for your needs. It is calibrated at 55 MPH running at 65 shifts the shaded area up a couple of notches. However, you can use it to see if the tires that you have pushes you toward the economy range or power range. for example 3.55 x 65mph x 336 / 31 inch tires should give you a 65mph RPM of about 2500 RPM . Looking at the chart, 2500 RPM, even though in the optimum area is in the high end toward the power side. Therefore, either larger tires or a gearing change would be needed to push the RPM down toward the higher milage range, 33's with no other changes would bring the RPM down to around 2350.

A gearing change to 3.08 and no other changes would give a 31 in tire a RPM of about 2170 RPM at 65. 3.08 gears matched to 33 in tires will bring the RPM down to about 2038 RPM. Improved milage due to running the engine slower at 65 but a reduction in power.

The light shaded area of the chart is a comprimise between economy and power, sort of the, " have cake and eat it too", area.

The bottom line of all that dribble is that if you want improved milage for daily running, pick a tire gear combination that fits your requirements. Above that, there are lots of suggestions in the forum adressing milage.

Geese, did I get carried away or what :blush:

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[Seabronc]

PS, In regard to RPM, I was referring to 3rd gear where your transmission has a 1:1 engine to driveshaft ratio. OD only improves on the economy end. Ya, gotta know I think primarily in carbed, non-computer controlled, non-EFI, non-OD terms, just the basics , so you need to forgive me if I don't prequalify my statements 0 . Some times I think I'm writing what is actually going on in my head ) .

Communication: I know you think you know what I said, but you need to know I didn't say what I ment. :-"

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