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Between Covid lockdowns, fixing everything necessary on a '76 I bought in kinda bad shape, etc, I'm just now working my way through the first full tank of gas.
For a while, I thought the gas gauge didn't work, it was above the full mark, then came down to a bit below. Now that I'm finally, (and happily), driving the car regularly, the fuel gauge has started moving downward, at a fairly quick pace.
It now shows a little less than half full, and I decided to try and make sure. So, I cut a strip of cardboard and dipped it into the tank until it hit the bottom. I felt down inside the filler neck and decided the bottom of this neck was the level of the tank being normally full.
The cardboard strip shows about a third full, but I thought this could be because of the shape of the tank, or the angle it's on, or whatever.
So, with these C3 gauges, is full actually full, and is empty actually empty?
I don't think there is anything more stupid than running out of gas, so when it gets down to this point, I'm going to fill up.
Any thoughts and ideas are welcome.
Last edited by hdeyong; Apr 26, 2022 at 08:58 AM.
Reason: spelling
I had a '76 for over 30 and never had a problem with the fuel gauge. You might want to check the sending unit and the gauge as perhaps one or the other is causing the problem. I personally would find and fix the problem as it could get worse leaving you stranded (YIKES).
I had the same issues on our '68 when we got it 3 years ago. Last year I siphoned the gas out of the tank (enjoy a good ride first and get it down to 4-5 gallons to make this easier); replaced the sending unit; and it works great now - an irritation gone. You should replace the rubber to metal fuel line from the sending unit while you're at it. Willcox has some great technical help on this (and other) subject. http://repairs.willcoxcorvette.com/1...-can-tell-you/ Best, Paul
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
The shape of the tank doesnt lend itself to looking inside to judge how much fuel you have. I drained the tank and added one gallon at a time to get an idea of how much is in it compared to what the gauge reads. I then bought a fuel gauge conversion gizmo to get the reading closer and its adjustable. Its better than before but I discovered looking in there wasnt as accurate as i thought https://shop.classicinstruments.com/sn34
Between Covid lockdowns, fixing everything necessary on a '76 I bought in kinda bad shape, etc, I'm just now working my way through the first full tank of gas.
For a while, I thought the gas gauge didn't work, it was above the full mark, then came down to a bit below. Now that I'm finally, (and happily), driving the car regularly, the fuel gauge has started moving downward, at a fairly quick pace.
It now shows a little less than half full, and I decided to try and make sure. So, I cut a strip of cardboard and dipped it into the tank until it hit the bottom. I felt down inside the filler neck and decided the bottom of this neck was the level of the tank being normally full.
The cardboard strip shows about a third full, but I thought this could be because of the shape of the tank, or the angle it's on, or whatever.
So, with these C3 gauges, is full actually full, and is empty actually empty?
I don't think there is anything more stupid than running out of gas, so when it gets down to this point, I'm going to fill up.
Any thoughts and ideas are welcome.
So Here's a picture of my 78 gas sending unit. In the empty poisition there will still be gas in the tank which the pickup will not be able to get to. Also the float is about 1 1/2" from the bottom. ~ 5 gallons of gas is required for my car to start.
Also since the float moves on a circular arc its not linear to the amount of fuel in the tank. Also the arc of the float doesn't extend enough to max fuel level. Giving you extra fuel past fuel reading.
So from full to 3/4 reading will move slowly. from 3/4 to 1/2 will move more quickly stalling near 1/2 full and then move quicker from past 1/2 full to empty.
Also when the tank is actually 1/2 full the float doesn't sit in the correct position. Mine reads past half full.
But based on design of the sending units / pickup empty is not empty. Everything else is a close approximation assuming the guage / sending unit is in working order.
The sending unit ohms range is 0-90 ohms. But notice the OEM vs Replacements available. The float range is not the same as OEM. Full is not the same.
While they are similar OEM (left) the float range is different. So swaping these units give different gauge readings.
Only thing you can do as another suggested, empty the tank and put know amounts of full and note the guage reading.
The 1975-1977 used the same Tank and Sending unit.
It was approximately 17 USG
All originally had an internal rubber bladder installed.
Anytime you lay the car up for extended periods of time make sure the tank is more than 3/4 full. These rubber bladders have a tendency to start to sag and even collapse.
My 77 has the low fuel warning module and was curious after repairing it when does it actually turn on.
So I started with a completely empty tank and adding a gallon at a time during the 3rd gallon the light came on.
I then cut a piece of edge protector made out of plastic and added another 1.25 gallons representing 1/4 Tank and scribed a line on the plastic.
I added another 4.25 gallons and made a scribe representing 1/2 tank.
I added another 4.25 gallons for 3/4 and another 4.25 gallons for a total of 17 gallons.
Then I did an ohms test on a new spare sending unit I have checking ohms to the marks I scribed on my plastic.
Empty the sending unit was 3.8 ohms to start with so a little off there and at a quarter tank 22.5 ohms the float was above the scribed line.
1/2 tank 3/4 tank and Full tank readings were real close to my scribed lines.
Using the marks I made on the plastic edge protector and a standard ruler these are my findings...
The 17 gallon scribe mark is just below the bottom of the fill hole.
So yes, you can put 17 gallons in a completely empty tank but there MAY be lags between where float is and actual readings at the gauge.
Look where the float is in relationship to 90ohms...
That is as far as the float will go up...
As usual, great info from everybody.
I'm going to keep running it lower on gas, then siphon out the rest, and then add measured amounts while watching the garage. This seems like the best plan.
Thanks.
My '71 bugs me every time I fill up. I have the full gauge sweep, no problem. I would hate to run out, so I'd fill the tank to the bottom of the neck. Gauge reads full. Drive around until it drops below 1/4 tank, I start getting nervous, and stop & fill up. It's always only about 7.5 gallons to fill it back to the bottom of the neck. That should have indicated about half full, not below 1/4. So at 1/4 tank on the gauge, I really have 1/2 tank left.
Fuel Gauge Readings
