Alright I've been wondering this for awhile. First off I have a 1980 f100 4.9l auto and if I'm right it has a 19g tank. When I shut the truck off the gas guage pin or needle goes below the empty mark. So I'm wondering how many gallons are still in the tank after the empty mark. On my truck the needle goes about a quater below the empty mark. But I'm to scared to drive it once its at the empty mark cause I don't want to run out of gas but if there's still a couple gallons in there then I wouldn't have to worry bout it.

 

Is your truck short or long wheel base and does it just have one tank?

 

It has one tank not sure if its short wheel base or not

 

Is the bed 8 ft long or about 6 1/2 ft?

If a side mount tank [driver's side under the bed near the driver's seat], it will hold 16 gallons if it is the 6 1/2 ft bed and it will hold 19 gallons if it is the 8 ft bed.

 

It's 8 ft long

 

You could drive it until the needle reaches the empty mark, then hit the trip odometer and keep driving until you run out of gas, then record how many miles you went after "empty". Just make sure you have a spare gas can thats full and some starter fluid to start your truck back up. Then you'll know roughly how many miles you can go before you're completely out of gas.

If you know your average mpg then you can divide the number of miles after "empty" by your mpg and that will tell you approximately how many gallons are left in your tank after you hit the empty mark. For example, if you went 50 miles after you hit empty and your truck gets 10 mpg, then you have 5 gallons in your tank after the empty mark.

Hope that helps.

 

If still factory correct, it should be 19 gallons. If you have been filling it up at 1/4 tank and it take 14 gallons, you should have 5 gallons left. Carry a gas tank [the one you use for your lawn mower] with you for security [just in case you go too far] and drive 30-45 miles after you reach the 1/4 mark and then refill. If now on empty and it takes 17 gallons to refill, you have 2 gallons left when indicating empty.

Oops! I'm too slow. I see that BullyBlue85 beat me to the punch!

 

I think my guage is messed up cause last week I put 40$ in it and that gave me 9.6 gallons and the guage was showing it was full

 

Forgot to mention tht it was on empty when I put the 40$ in it

 

When you turn your truck off and the needle goes all the way down, this is not indicitive of where your "empty" will be. This just means you removed voltage from the sending unit and therefore the gage goes away....

I hope this is not what confusing you. With the truck off, pay no attention to what the gage reads.

 

Haha yea the forum doesn't show new posts in real time, it's all good advice though.

 

Mine does something similar. It will read empty unless the tank is more than half full. I need to check the sending unit.

 

Use FTE Live for that.

 

These fuel systems are not all that linear. My gauge stays on full for 75 miles then drops like a rock. This kind of behavior is not all that uncommon for any make and model of car though some are better than others.
Due to space constraints most tanks can not be made into a perfect square or cylinder. That makes it difficult to design a sender / gauge to work perfectly with the typical odd shaped tank designs.

They also do not take into account overfilling the tank. You can get a couple of extra gallons in the filler tube.

You could drain your tank and then put fuel back in 4 or 5 or so gallons at at time and watch where the gauge ends up with each inclement. If you do this be sure to wait several minutes between each addition of fuel.