Puzzling Gas Situation
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Elder User
Joined: Apr 2011
Posts: 917
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From: South Carolina
Puzzling Gas Situation
Hello again! I was doing research on here, and figured out my sending unit was bad in the rear tank. Grounded the wires, and the gauge pegged to full. So I got a new one. The past few days, the fuel gauge seemes to be going down rather fast. Monday, guage read 1/4. Filled it, and it took 8.5 gallons. Today, gauge read 1/8, filled it again, and it took 10.6 gallons. Were there 16 gallon rear tanks? Do I just have crappy MPG's? Does my carb need a rebuild? And I havent found any leaks.
My set up is: 302, 600cfm Edelbrock, NP435.
Thanks all!
My set up is: 302, 600cfm Edelbrock, NP435.
Thanks all!
Elder User
Joined: Nov 2004
Posts: 666
Likes: 5
You can't tell anything unless you record how many miles you drive and calculate your MPG. 1/8 reading and it took 10 gal to fill a 16 gal tank? Sounds like my truck after I put in a new sender.
The new aftermarket sending units leave a LOT to be desired. I watch my odometer and keep track of miles driven.
With the factory sender, at about 120 miles after filling up I was at 1/2. At 200 miles I was at 1/8 and looking for a gas station.
With my new sender, at 200 miles after filling up I am at 1/2. At 220 miles I am on Empty. Yep, 200 miles from Full to 1/2, 20 miles from 1/2 to empty. When my gauge gets to 1/2 I am looking for a gas station. If I let it go down to Empty, it takes about 14 gal to fill my 16 gal tank. So it's not far off on the Empty, it is just VERY non-linear.
The new aftermarket sending units leave a LOT to be desired. I watch my odometer and keep track of miles driven.
With the factory sender, at about 120 miles after filling up I was at 1/2. At 200 miles I was at 1/8 and looking for a gas station.
With my new sender, at 200 miles after filling up I am at 1/2. At 220 miles I am on Empty. Yep, 200 miles from Full to 1/2, 20 miles from 1/2 to empty. When my gauge gets to 1/2 I am looking for a gas station. If I let it go down to Empty, it takes about 14 gal to fill my 16 gal tank. So it's not far off on the Empty, it is just VERY non-linear.
Elder User
Joined: Nov 2004
Posts: 666
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I think he meant that on this fill up of 10.6 gallons, he went 139 miles. That's 13.1 MPG. Sounds pretty normal for an '84 F150 4x4.
I think the problem is not a fuel leak or a fuel consumption problem, but a gauge sender that is telling you that the tank is empty when it is still 1/3 full.
I think the problem is not a fuel leak or a fuel consumption problem, but a gauge sender that is telling you that the tank is empty when it is still 1/3 full.
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Laughing Gas
Joined: Oct 2010
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From: Rock Valley, IA
I misread the chain. You're right. I would be surprised if it got much better than that. I can squeeze 15mpg out of my 350 suburban if I have a strong tailwind and going downhill.
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Thread Starter
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Elder User
Joined: Apr 2011
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From: South Carolina
Sorry for my last post, but I actually drove 152 miles on those 19.1 gallons I put in. I think what I'll be doing is check for leaks, then adjust the carb. Also, I havent had the tank run completely out yet, or get to empty, but I'll see if I can verify the tank size.
Elder User
Joined: Nov 2004
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Wait a minute. The numbers keep moving around. In your first post, you said first you put 8.5 gallons in it. The next time, when the gauge was down to 1/8 you put 10.6 gallons in it. Then you said you drove 139 miles. Then it was 152 miles. Then you said you put 19.1 gallons in it.
Did you actually put 19.1 gallons into it at the pump, or are you saying it is a 19 gallon tank and you ran it until the gauge was on Empty. There is a BIG difference. I'm trying to figure out how you put 19.1 gallons into a 19 gallon tank and still drove it to the gas station. You typically start sucking air when there is a gallon or so left in the bottom of the tank.
Did you actually put 19.1 gallons into it at the pump, or are you saying it is a 19 gallon tank and you ran it until the gauge was on Empty. There is a BIG difference. I'm trying to figure out how you put 19.1 gallons into a 19 gallon tank and still drove it to the gas station. You typically start sucking air when there is a gallon or so left in the bottom of the tank.
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
The rear tanks were optional on these trucks.
Rear Tank is 19.5 gallon capacity, from the factory.
Front Tank is the standard tank, and capacity depended on wheelbase.
Front Tank: 117" wheelbase, 16 Gallon
Front Tank: 133" wheelbase, 19.5 Gallon
This will give you three different sending unit choices, not counting diesel and EFI models. You can still get the factory sending units last time I checked for 1980-1984 year models...
Rear Tank is 19.5 gallon capacity, from the factory.
Front Tank is the standard tank, and capacity depended on wheelbase.
Front Tank: 117" wheelbase, 16 Gallon
Front Tank: 133" wheelbase, 19.5 Gallon
This will give you three different sending unit choices, not counting diesel and EFI models. You can still get the factory sending units last time I checked for 1980-1984 year models...
Thread Starter
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Elder User
Joined: Apr 2011
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From: South Carolina
Sorry for the confusion. I didn't add some of the miles I drove, so I did a new total mileage count. The 19.1 gallons is the total amount of gas put in, during the 152 miles I drove, even tho I hadn't reached empty on the gauge at all. Does that clarify?
Post Fiend
Joined: Jul 2004
Posts: 8,786
Likes: 28
From: Northern California
152 miles using 19.1 gallons equals 7.96 miles per gallon.
This is assuming your odometer is dead accurate, which most are not due to oversize tires and the mechanical nature of the speedos could be off up to 5 mph from the factory legally. You can check this the next time you are on a road with mile markers. Mark the odometer from the start of the mile marker, and stop counting when you pass the next mile marker and subtract the difference. This should give you a good error correction estimate of plus or minus, 0.05
Say my trucks odometer will lose 0.1 of a mile for every mile traveled. Not much really, but after a while it adds up...
So if my truck burned 19 gallons in 362 miles... That equals 19 MPG.
Now factor in the error correction... 362 times 0.1 equals 36.2. Add 36.2 to 362 and that is 398.2. Now do the math... 398.2. miles divided by 19 gallons equals, 20.98 MPG.
Very near 2 mpg difference...
Your odometer could be adding more mileage than you think too.
This is assuming your odometer is dead accurate, which most are not due to oversize tires and the mechanical nature of the speedos could be off up to 5 mph from the factory legally. You can check this the next time you are on a road with mile markers. Mark the odometer from the start of the mile marker, and stop counting when you pass the next mile marker and subtract the difference. This should give you a good error correction estimate of plus or minus, 0.05
Say my trucks odometer will lose 0.1 of a mile for every mile traveled. Not much really, but after a while it adds up...
So if my truck burned 19 gallons in 362 miles... That equals 19 MPG.
Now factor in the error correction... 362 times 0.1 equals 36.2. Add 36.2 to 362 and that is 398.2. Now do the math... 398.2. miles divided by 19 gallons equals, 20.98 MPG.
Very near 2 mpg difference...
Your odometer could be adding more mileage than you think too.
Thread Starter
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Elder User
Joined: Apr 2011
Posts: 917
Likes: 1
From: South Carolina
Good point, I didnt think of that. Actually, I used google maps as I didnt pay attention to my trucks "mileage" from the beginning. I would hope that that map system is pretty close. I believe my problem is not a wrong sending unit, as there was only the one size rear tank, and that I need to adjust my carb. Correct?
And thanks to all who have responded.
And thanks to all who have responded.