Ok so my 86 f250 hd 4x4 with the ho 351w gets a whomping 10-11 mpg
This is my set up

351w HO
600 cfm carter afb competion series
1inch 4hole carb spacer in place of egr
Smog pump removed
MSD cap and rotor
Accel 8mm wires
MSD blaster 2 coil
Stock exhaust with cherry bomb
New tires believe there stock size I think 235/75-r16 may be worng

Over all its a basic stock set up runs really strong only 84, 000 miles but I feel like I should be getting better mpg my 94 f250 hd ext. Cab 7.3 idi gets a good 18-20 but gotta keep in mind that has overdrive

Now do you guys think I would see any gain in mpg with any of these mods witch ones would work best. I was thinking

285/75-r16 bigger tire by about 2 1/2 inches
Electric fans instead of the belt driven
180 thermostat so I can run more intail timing with out ping
Possibly electric water pump "might be going to far"
Roller rockers
Headers
Does this seem to be a good start any one have any thing to add

 

Replace the whole exhaust system with headers and whatever you like behind them for big gains in torque which will translate into improved milage. Outside that larger tires or a trans with overdrive are the best upgraded for milage.

 

You haven't mentioned your axle ratio......but my F250, with 3.54 gearing, lost 2 mpg when I dropped down from 30" tires to 27.5"

It also gained a good bit of power, but I changed back for the mileage.

 

Bigger tires weigh more = worse mileage

 

Your idi gets better fuel mileage because a diesel engine is more effficient, and like you said the overdrive helps. When I changed from a c6 to a zf on my idi, I gained 3 mpg.

 

If you are running stock tires they should be 235/85/r16. what transmission are you running and what is your rear axle ratio. going to a 285/75/r16 might not help you as much as you think becuase as oneowner said larger tires have more weight. the bfg Rugged Terrain T/A in 235/85/r16 weight 40 pounds each and are 31.7 inches in diamiter. The same tire in a 285/75/r16 weight 51 pounds each and are 32.8 inches in diamiter so you are turning an extra 44 pounds but are only gaining just over an inch in diamiter. Also with that the tread with is also about 2 inches wider on the 285/75/r16 so you have more rolling resistance. If it was me i would put the money i was gonna use for tires and find an overdrive tranny, but that is just my two cents.

 

What gears? My '86 Supercab when stock height/tires with 351w and C6 with 3.55 gears got me a best of 16 MPG highway. Stock engine, no smog pump, Holley 600 1460 carb jetter lean and timing advanced, I forget where I had it but likely 12*-14* initial with factory curve. Forget what jets it had.

Have you tuned the carb? Jet it as lean as you can. On an AFB there's more involved, maybe jets or maybe just changing rods would do it. You want to go lean enough that at steady highway cruise it'll just start to lean surge, then go a bit richer. Set timing to 12*-14* initial and ensure you're advance is working smoothly and all in by 2500 RPM. An adjustable vacuum advance can will let you fine tune that part. You want as much advance as it'll take without detonating or power falling off once you're at speed and cruising.

Biggest thing you can do to increase MPG is swap in a ZF5.

Electric fan, water pump and roller rockers you're barking up the wrong tree. You won't see any practical MPG increase and will never get ROI if you're just doing it for MPG. I always ran a 180* stat, but on a stock low compression engine you shouldn't have any trouble running the initial timing it wants. It'll more likely become hard to start (around 16*) before you're detonating on pump gas. 12*-14* initial and listen for detonation.

Headers into dual exhaust will help MPG. I like Dynomax Super Turbo mufflers. Flow very well and sound nice without being obnoxious.

Bigger tires are a crapshoot, you may see slight gains but more weight takes more fuel to turn as well. I would go bigger along with a lift to have ground clearance and better traction, or just 33" tires with no lift if I needed new tires anyhow and didn't wnat to lift it. I would not replace tires with plenty of life in them in a quest for MPG, you'll be dollars behind.

 

A lift is bad for fuel mileage. These trucks are nothing but a brick moving through the air, and moving the brick higher up doesn't do anything but make it worse.

I have a 4 inch lift with 35x12.50 tires, because it came with all that when I bought it. I still get 15mpg, but I bet if I removed that stuff I could get 17+mpg.

 

Have you considered possibly going to a smaller carb? if it is just a normal daily driver and not seeing high RPM's for a consistent sustained period of time, then a 500 CFM carb would be more than enough. Might pick you up a couple miles, and might even feel a little more responsive.

 

Not only have you not told us the axle ratios, you haven't told us what driving conditions give that MPG. Both of those are extremely important. You have to get the RPM down to something like 2000 at 65, which will surely take an OD tranny or very tall gears.

Second, Dave was almost right with his brick analogy. A brick is actually more streamlined than our trucks since the trucks have recessed grilles and headlights that trap air - which is probably why the next generation of Ford truck when to flush grilles, headlights, etc. Anyway, the way wind drag works every time you double the speed you get four times the drag. And going from 60 on the highway to 70 will reduce my MPG by about 2 - and my truck has been tuned with an air/fuel ratio meter. Plus I'm running a ZF5 tranny with OD.

Third, if you've not tuned that AFB then you have some gains right there. Edelbrock's manual on their carbs is an excellent resource for that, outlining how they work as well as the jetting. Go to the stock jetting for their 1406 carb and I'll bet you see a good gain.

 

One other thing is that switching to electric fans won't save you much. You're just adding that drag to the alternator. Maybe it is more efficient so you aren't losing as much energy with an electric fan vs. the mechanical one, but you won't see anything dramatic. I know that for a fact. But with a nice push/pull fan setup it might cool even better, so there's the bonus!

 

My truck does have 235/85 r16 with a good old c6 and the axel de codes to a 4:10 rear gear and I'm getting 10-11 every where around town and high way but I never take the truck over 55 after that the motor just sounds like its winding up to much and where eould I find carter jets and rods

 

This is how mine does.


Moderate '75 351W, 3.73s, 85 F250 (base), C6, lot of extra weight (bumpers etc). Gets about 6mpg on 235/85r16 and somewhere around 3-6 on 33s. This is with us not going anymore than 55, on backgrounds. Now i will say it runs real rich though so thats part of the issue. Truck is a pig. Part of the reason we took it off the road.

 

http://www.summitracing.com/parts/edl-1487

Several other places sell them also.

 

That will work fine with the carter