Fuel System: 13 gallon gas tank

[Rafe Hollister]

56 P-Hawk. I've had my car 4 months now, I drive it till the gas gauge gets to empty, then a little more till I hear the electric fuel pump crying of thirst. But the most I have ever got it to take is 13 gallons. I read capacity is 18 gallons. I did an inspection and removed sending unit for a look inside: no dents in tank, no sediment. I cannot see the end of the gas pick up line (will buy a mirror today and try to look again). I've read previous posts and others have similar experiences, but so far none that I've found have resolved it.
There is 1" of gas in bottom of tank now (elect fuel pump crying intermittently), figuring tank is 27" wide and 17" long at 1" depth (before it tapers up), that is 459 cubic inches, divide by 231 to get gallons and I come up with 1.98 gallons still in tank. But I'm still missing 3 gallons of capacity. On Tuesday I will fill it and check again to see if it actually gets full.
I've read previous posts and others have had this issue, but I have not read any resolutions to it.
This winter I want to weld in a sump; whether this tank, a new one, or an aftermarket from another car. It would be nice to find a bigger tank that would fit.
Rafe Hollister

[JoeHall]

If it is the OEM tank, and has not been modified, it definitely holds 18 gallons. With yours being a 56, if it has the OEM fuel supply line running through the sending unit on the tank, perhaps it has a shorter 'straw', for whatever reason, or possibly the straw has a tiny hole in it down near the end.

[bensherb]

I think its a question of how they measure capacity. I've never been able to get more than 14 gallons in our '62, K and the '53s (C and K) would only take 15; even after running dry and needing to call AAA for help.

I have no doubt the tank itself is capable of holding 18 gallons. If you remove it from the car, plug the fuel line, stand it on the right end and fill until it flows out where the filler pipe attaches.

However, as installed in the car you cannot totally fill the tank. Once you reach the upper level of the fill hole in the tank, it's full, but leaves a couple inches of airspace on top of the fuel.

Now consider how the tank fits in the car. Even with tail dragging, the front of the tank is slighly lower than the rear of it. One reason the fuel sender gasket tends to leak; it's below fuel level if you keep the tank "full". Add to it's natural incline, any "rake" your car might have due to larger tires, heavy duty springs or long shackels and you have a bit more air space on top of the fuel you can not get fuel into.

Both '53s we've had, have been stock ride height though the current one has slightly larger than original modern tires on it. The other one was a very low milage car, factory stock from the top of the roof to the ground and they wouldn't take more than 16 gallons. The '62 has had the rear end raised an inch by me and the previous owner replaced the springs with NOS heavy duty ones.

If somebody is somehow getting 18 gallons in one when they run out of gas I can't imagine how. When ours run out of gas there is still an inch of fuel in the tank too.

[Bullet]

I have a 64 Cruiser with new HD springs in the back and the moog springs in front. The most I have every gotten in my gas tank is 16 gal (2x). Typically it is 11 or 12 gal. I have owned the car for 11 years and have put over 30,000 miles on it. I used to think the same as you I had a gas tank problem. I no longer worry about it and just add gas when I need to.

Mark

[Rafe Hollister]

Joe, My Hawk has the fuel supply running out the front left of the tank, not out the top thru the sending unit. My 56 Sedan had it thru the sending unit. Has my Hawk been modified?
BenSherb, I checked the bottom of my tank for level, but now I will check the top. 2" of air on the top would be another 3 or 4 gallons or more, considering the top volume is the full width and length, unlike the bottom that tapers up at the back.
The filler spout is in the back, which you say is high? Wouldn't that tend to fully fill the tank? And my sending unit does leak, tho a new gasket last night may help.
Rafe

[Son O Lark]

I have always needed to rock the car to completely fill the tank.

[E. Davis]

When you rock the car you are effectively removing the air cushion. In other words you are "burping" the tank and that allows you to put more gas into what was air space.

[bensherb]

Thom, if I remember right the '56 is the only year that had the fuel pickup in the sender. So if yours doesn't the tank was probably changed at some point. Our current '53 Coupe has the tank from our '56 in it now. I had to modify a sender and add a pickup tube when the original sender failed. Other than the pickup tube location the tanks are the same. The '62 has the tube coming out of the lower left front.

OK, you got me thinking about it so I had to go measure stuff. On my '62 the way it sits now. The filler center is 7" from the rear of the tank. The main (flat) bottom of the tank angles down to the front 1.5 degrees; the rear foot or so of the bottom of the tank angles up from the main bottom 9.5 degrees. The long rail of my frame angles down to the front .4 degrees and my rocker panel measured at its center angles down to the front 1.5 degrees. The front of my rocker panel is 2.375" lower than the rear is. The rocker is 81" long and the tank is 28", making the rocker 2.893 times the length of the tank. So given that the angle of the tank and rocker are the same, 2.375 divided by 2.893 tells us the tank is only .821" (roughly 13/16") higher in back than in front. So all in all, it doesn't make a lot of difference; that rake probably couldn't make more than about a .348 gallon difference (44.54 ounces). If the rear was 1" lower it would be 25.6 oz. I guess we know how they won those fuel economy chalenges; just measuring the tank it couldn't hold more than 17 gallons, minus the two left in the tank (probably a bit different from car to car) when its empty, makes 15.

[JoeHall]

For many years now, on every Stude, at every fill up, I have jotted down mileage and gallons of fuel. I write it on old business cards and, when one card is full, use a paper clip to add a new card on top. I just looked in the wife's 63GT and, sure enough, there's about 40,000 miles of gas records, jotted down on business cards. The most fuel I recorded in that car was 15.7 gallons, and there are several other entries where it took over 15 gallons to top off. Will post pics of the cards tomorrow, too lazy tonight.

I recall once in the 62GT, mid 1990s, pulling a U-Haul trailer in Arizona, it took over 17 gallons to top up. These experiences are why I say, with certainty, the Hawk gas tanks hold 18 gallons.

As for fuel line running through the sending unit, that was 56 and 57. The 1956s used 5/16" line, and 57 used 3/8", IIRC.

[StudeRich]

The 3/8" Fuel Line was Only on '57 Golden Hawks with Supercharger, Probably Packard's, it requires a different sending Unit due to the larger Output Fuel Line fitting. I have no idea if 352 Engines in '56 GH 's also did.

The same thing was done on Avantis, More Fuel Usage, and Speed requires more volume.

But for sure all '56 AND '57's did have the Fuel Line in the Sender.

[altair]

I think the design of the tank allows for expansion, if you fill it up to the top of the filler spout and let it sit in the hot sun it can migrate the fuel out the filler cap.

[E. Davis]

Reading Bensherbs figures makes me really wish I had paid more attention in Math classes in High school. LOL

[JoeHall]

As promised, here are a couple of the gas / mileage note cards from the 63GT. Note at bottom left, the one entry is for 15.7 gallons, and several other entries for 15+ gallons.

[JoeHall]

On both the above cards, mileage is in the left columns, and gas is in the right.

I have never ran a Stude out of gas on the road, but have came very close, like the time in AZ. The wife ran out once, in her first 63GT, in the California desert. She's Japanese, and was used to Japanese car gas gauges, which will run way below empty, for many miles. She learned quickly, empty means empty in a Stude. LOL

[Rafe Hollister]

Conclusion: I have removed my gas tank and done thorough inspection. I have measured: how much gas is left in tank when fuel pump cries, total actual capacity, how much more you can get in it with SonO Lark's suggested burping. I did a 'burp' modification that should allow full capacity without burping. Tomorrow I plan to add a sump so as to get every last drop. And I have pictures of it all. After I get my tank installed and checked, I will start a new post on all this to bring you all up to date.
Thanks for all the comments and ideas... its what pushed me into full CDO mode!
Rafe
PS: CDO is the same as OCD, but in alphabetical order the way it should be.