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First of all make sure what you are getting in the way of mileage just looking at the gauge isn't very accurate. Second eight mile per gallon is about the best you will get if it ever gets ten count your blessings. 3.55 rear gears is about as tall as you will have more that likely it has 4.10 gears in it.
I have an 86 F250 460/C6 combo which is very close to your truck.
You mentioned that your truck took 9.5 gallons or half of your back
tank... my truck has dual tanks as well... it was my understanding
that the back tank is closer to 11 gallons not 19 with the front being
around 15 gallons.
If your truck took half the back tank, it would be 6.5 gallons which
would bring your mileage up to par.
I have a loaded 1978 F 150 Ranger, XLT, Lariat, Towing Special, 273 Locker. It has duel tanks and the rear is much smaller. It is the auxilary tank and holds around 10 gallons and it is located high up under the bed at the rear of the truck close to the bumper. Above the spare tire. Where the front tank is the main and takes close to twice the fuel. I think around 19 gallons to fill and it is located along the inside of the frame. Take a look under neath and you should see there is a big difference. On the basis of that you may have just doubled your mileage. My 460 with the AC on when it isn't under load gives me 14 MPG mainly highway and I am very easy on it. I set the cruise at 55. Under load towing I am luck to get half that.
Ric
I'm just going by how many gallons I can get in the tank when
the gauge reads empty - 10 gallons rear, 15 gallons front.
I've read where you can get an "extra capacity" fuel tank, but
from what I've seen, the tank is the same size, but the filler neck
doesn't go as far into the tank, giving it the extra capacity.
Clifford, It sounds to me like you have the same problem with your fuel gauge as I. When I fill the tank, the gauge reads full, but when it reads empty, it only takes 9 - 10 gallons to fill it. I ran the tank completely empty (the beauty of having two tanks) and when I filled up, it took 19+ gallons. I can run both tanks on empty (or so the gauge tells me) for quite a while.