I know these trucks arent known for fuel efficiency but how much do you get out of both tanks?
Combined I get under 250 miles. I that doesnt seem right? My guage does bounce around due to sloshing and I cant seem to get an accurate reading but 250 miles out of both fuel tanks doesnt seem right? Ive checked for leaks and nothing. I have put new plugs, wires, injectors, fuel filter, new exhaust, while I have a lot more power I can only seem to get 231 miles combined. I am running 4.11 Gears i thought about going to taller gears but wouldnt that hinder towing? I usually run at 2300 RPM when I am cruising at 65.

I dont daily the truck I have a 30 minute commute to work, Maybe I am too used to the milage my F150 got and My Fusion.

 

Assuming you have 16 gallon front & 18 gallon rear, you're getting around 7 mpg. Yep.... That's "almost normal" for a 7.5 V8, depending on a few factors. How heavy is your truck? How hard do you accelerate? Other members on here report anywhere from 7-12 mpg from a 7.5. My mom's 77 Lincoln had a carb'd 460 & a C6 trans. It got 10.1 mpg on the average, year-round, weighed about 6,000 lb.

 

That is a little low but only just a little.. the big block loves to drink the juice. Any codes present? Check engine light doesn't have to be on. What is your fuel pressure? What transmission does it have? Bigger tires produce the same results as taller gears but if you don't have an OD trans swapping in one that does would be the the best mod.

 

I think 7 MPG is pretty far off from good. It's quite bad. Even for a 460. The problem with "MPG" numbers is that they don't readily give context for just how much fuel is being used. You may calculate 7 MPG and think "well that's only 3 mpg off from 10, that's not too bad". The reality is, 7 MPG is actually 30% lower than 10 MPG. 30% is a huge difference. The difference between 7 MPG and 10 MPG is the same difference between 40 MPG and 28 MPG.

Start tracking your fuel economy. Fill up both tanks and reset your trip meter. Drive around until you need fuel again, and then fill up both tanks. Add up the total amount of fuel you put in the truck, and then divide the miles on your trip meter by that total amount. This will get you your average MPG. You must completely fill both tanks for this to work.

Do that over a number of fill-ups to get an average. Then we can work out if you have a problem or not.

 

Good advice LeadHead. My truck holds close to 34 gallons. Towing a big boat, I used to be on/near empty of both tanks after ~250 miles (according to gauge), but it typically only took 25-28 gallons to refill for an average around 9.5mpg. Empty on the gauge is pretty far from truly empty, so estimated fuel economy from the gauge is nearly useless.

 

Check codes as suggested above...check fuel pressure as well, if the rail doesn't hold pressure when the engine is not running, then you have a leaky injector. Leaky injector would explain your poor mpg too.

 

Is this the E40D or the C-6?

I have the C-6\7.5\4.11 set up, can't remember what RPM it runs doing 65 but it only gets about 8 (though I think I'll be squeezing some more out of it soon) The 4 spd should get better than that..

Also what tire size are you running?

 

I have an f150 2wd with a 302 in it and I get 150 miles to a tank... bite me.

 

My 351 averages about 400 miles to a fill of about 34 gal. and that is with a slide in camper driving about 60 mph.