Well...not really sure what to think here. I know the 460s are thirsty, but didn't know they were this thirsty. I just took the first road trip in the '86 F250. 5300lbs dry weight, longbed, 4x4, C6, 33s @ 67lbs per tire, about 200lbs of stuff in the bed. I've rebuilt the carb and tuned it as best as possible...spark plugs look great. Alot of other work done to the truck to get it running its best. Was thinking it'd be closer to 13mpg...and am kinda shocked that it turned 7.3mpg. Verified with GPS app on phone, not the odometer since it's off not that it's on 33s. Didn't run A/C and used cruise the entire trip. So, my question is...considering that the truck is as I've detailed, is 7.3MPG that bad or is it somewhat ok or?
Really interested in what ya'll think. Now with all that negative out of the way...the truck otherwise ran great. Not a single problem in the 700ish mile round trip. Very pleased with it overall.

 

Try dropping back to 60mph just to see what it does to the mileage. My '79 F350 with a 33 inch tire and 4.10 gears was turning over 3000 rpm @ 70mph. It would get about 10 when running at 62. Now after I built an E4OD for the truck it'll get a little over 12 mpg running 70. I'm getting ready to go through the engine and I'm going to change the cam because I think that it's a little over cammed now when it is cruising at 2200rpm.

 

If possible, get yourself a wideband/AFR gauge, it's use can tell you and/or allow you to see where a carburetor needs the most attention. I use a simple AEM unit for my Ranchero, REALLY helped me achieve not only better mileage, but also better drivability. Here is a link to some helpful folks that will aid you with tuning a carburetor. Racing Fuel Systems ? Index page Hope this helps.

 

Some of the milage per gallon can be adjusted with the skinny pedal as eluded above. Back down a bit on the speed and you will get some improvement. Big tires, 4X4, speed, higher profile vehicle, and deep gearing all have an effect. Not running the ac does not make that much of a difference at highway speeds, in fact with the windows down you are making a turbulent passage through the are even more turbulent and that is costing you some MPG with the extra drag. Windows down and around town may help a bit but on the highway it hurts.

 

I used a wideband O2 on my truck quite a bit too when I was messing with this and it is helpful and interesting. A vacuum gauge is pretty essential too. What's challenging is figuring out which carburetor circuit is the one that needs attention but on my truck the idle feed restriction was the most important thing. Start by taking a piece of stranded copper electrical wire, strip off about an inch or so of the insulation and use one strand of the wire in each idle feed restriction. Then put the carburetor back together, readjust the idle mixture and road test. If the engine will tolerate the leaner mixture and not develop a lean misfire. If it does try opening up the plug gaps to .080 and try again.

Too lean can be about as bad for fuel mileage as too rich but you'll probably find that the engine will start to lean misfire as soon as you get much leaner than about 14.7:1. It'll run just fine and tell you everything is ok even if it's two ratios or more over rich which I've always found odd.

 

I guess it depends on the truck and the tune of the engine. My 78 Bronco, that I haven't really tuned yet to get the best mpg out of it (Holley Sniper and Hyperspark ignition), gets 10 mpg on the highway at something around 60-65 mph and I have to be very steady with the pedal, no goosing it or flooring it for fun if I want to get some "decent" mpg. I don't know the weight of it, but that's with a rebuilt 460 to around 450hp, NP435/NP205 combo, 35" tires, and 4.56 gearing and I had the windows rolled up, but with the wing windows open for some air flow, because my Bronco is one hot pos in so many ways.

My old 2wd LWB '75 F150 got similar mpg, I think, with a similar build to the engine (C6, 850 cfm carb, no EFI). I only ever really calculated the mpg once when I did a long trip and that was towing a 4-door '65 Coronet behind it. It got 11 mpg towing that car about 220 miles.

I'm hoping, once I get the 460 in my Bronco tuned a bit, I can get another 1-2 mpg on the highway. If you were getting that 7-8 mpg in town, I'd figure you were just doing some city driving, maybe spirited. I haven't calculated the mpg for my Bronco while driving it in town, but there's no way I'm getting the 10 I see on the highway. I guess it's good it's not my daily driver.

 

I'm thinking about doing an E4OD swap from my 4 speed in my 1986 F350 4x4.

 

My E4OD replaced a C6 so some things stayed the same and some things changed. Flexplate stayed the same, shift linkage stayed mostly the same. Trans cross member changed, driveshaft length changed, yoke stayed the same. I used a Quick 2 controller and harness from Bauman and it works great.

 

Also make sure ignition system is all sorted out, simple tweaking can sometimes give back big or at least decent rewards. I used these instructions to tune my Duraspark distributors, one in my Ranchero the other in my old F250 4x4 on 38.5's, both gave noticeable improvements. Duraspark_distributor_recurve_instructions_index Hope this helps

 

Looking into an AFR gauge, I found the Innovate 3891. Tuning each bank seems like a better idea than just tuning the mixture between the two(just using 1 sensor). It's kinda expensive at $300, but might be a good investment in a fuel thirsty motor like the 460. The gas bill round trip was about $250.

 

With a dual plane intake you'll be getting the average of both planes with either one or two sensors. I think that one in either bank would be good enough especially for tuning part throttle running at high manifold vacuum.