1972 Blazer fuel gauge accuracy question

[Tbruz]

My 1972 Blazer fuel gauge isn't very accurate.
At least it reads full when full and I can see it go down as fuel gets consumed.
When it reads empty or near empty truck only takes about 12 gallons to fill up; its a 20 gallon tank.

My belief is that the sender is %$#@* because the gauge goes thru the full sweep and moves correctly.
Any system check I can perform?
Thanks
Tbruz

[DustinU]

I think they are all like that to varying degrees, the one I just did is doing the same thing. Better than the 3 o'clock position I had while daily driving the past 6 months or so... lots of fill ups.

[Slinger60]

The sending unit in my 70 GMC pickup does the same thing. Gauge reads full when full and only drops to 1/4 and only takes 11-12 gal. to fill it up.Havent pulled it to see whats going on with it. So I am not sure what the deal is with it.

[Firebirdjones]

Have you guys left it at 1/4 tank for a while to see how long it hangs there before dropping some more?

Are you truly filling up the tank? I find that after the fuel nozzle clicks off I can still get another 3-4 gallons of gas in the tank if I slowly pump it in. Takes another 4-5 minutes though.

These sending units are your typical 0-90 ohm senders. They have all worked accurately for me. Only way to know for sure if your sender is working correctly is to pull it from the tank and check with an ohm meter.

[DustinU]

I will log my mileage per 1/4 tank next fill up (I know this will not be accurate MPGs or anything, just consistency in sending unit), I feel like the first half of the tank goes much quicker than the second.

[Firebirdjones]

Originally Posted by DustinU

I will log my mileage per 1/4 tank next fill up (I know this will not be accurate MPGs or anything, just consistency in sending unit), I feel like the first half of the tank goes much quicker than the second.

I know what you mean there. Each one of my cars are a little different in that respect. Some of them will hang at full for a very long time then drop down faster through out the middle of the gauge, then seems to stay at a 1/4 tank for a longer period before finally moving below that.

I think alot of that might have to do with the arc of the float throughout it's entire range of motion (different designs between models).

Keeping in mind that when a tank is rated for a certain capacity, they are figuring every single square inch of the inside of that tank to hit that number. Even if the float bulb touched the very top of the tank (and in alot of cases they don't) and it made the gauge register on the full mark, there can still be another 2-3" of open space in the top of the tank above the fuel level.
This might give you a false impression when the gauge reaches half a tank and it only takes 8 gallons to top off a 20 gallon tank to reach the full mark again, when in reality you can probably squeeze another 2 gallons in there after the gauge has reached full and/or the pump nozzle shuts off.

[7D2N]

Mine seems to work fine except empty shows alot... must be a big block thing.

[DustinU]

So I tested out my gauge this past week or so, topped COMPLETELY off well past the Full mark, all city, driving about the same route everyday... I know my odometer is not calibrated to tire size etc... so MPG's are not right.
FULL: 5764 miles
3/4: 5797
1/2: 5823
1/4: 5851
Empty: 5891 (as low as I wanted to risk it)

To fill it back up she took 17.92 gallons... so I think I ran her pretty dry.

All in all fairly consistent I guess... approx 30 miles per 1/4 tank and now I know she will go past "E" just a little.

If this is my actual mpg's I should of built a BIG BLOCK! lol

[SeventyOne]

7mpg?? Jeez dude what drivetrain are you running?

[DustinU]

Originally Posted by SeventyOne

7mpg?? Jeez dude what drivetrain are you running?

I know my odometer is not calibrated to tire size etc... so MPG's are not right.

[BBrendal]

Is it an after market sending unit? Ive found that most of the aftermarket sending units are always off. I have a 69 camaro and put a new tank and sending unit in it. The tank is around 18.5 gals. I had to put in about 5 gals before the needle even moved. It shows 3/4 of a tank when full. I drive it past empty then when I fill it it only takes about 12 gals. I had this with other cars as well and have asked people with after market sender units and the say there gage is off as well. My Blazer is original and is balls on accurate. I think a sending unit is one of those itemes you Must try to get NOS or OE to be right.

[Ole Beater]

Like firebird jones said the sending unit is 0-90 ohms. When it is full it should read 90 ohms across it with a multimeter. When empty it should read 0. But if you have any kind of poor connection it will cause a higher resistance and not read accurately. If the float was sinking the gage would always read lower than what is actually in the tank. I would check the ground from the sending unit and sand to bare metal where it is attached to the truck.

[SeventyOne]

Originally Posted by DustinU

I know my odometer is not calibrated to tire size etc... so MPG's are not right.

That can be figured out with some simple math.

(New tire height in ") - (Stock tire height in ") / (Stock tire height in ")

Add 1 to this number then multiply by your MPG. This will give you your corrected MPG for tire size.

An example if 31" is stock tire height and you're now running 35" tires:

(35-31)/31 = .129

+1 = 1.129

X 7.087

=8.001 mpg

[DustinU]

Originally Posted by SeventyOne

That can be figured out with some simple math.

(New tire height in ") - (Stock tire height in ") / (Stock tire height in ")

Add 1 to this number then multiply by your MPG. This will give you your corrected MPG for tire size.

An example if 31" is stock tire height and you're now running 35" tires:

(35-31)/31 = .129

+1 = 1.129

X 7.087

=8.001 mpg

Well according to that calculation... I should have put in a big block!

I didn't expect much more than 10, but 8.4 w/ my 33" tires is ridiculous.

[SeventyOne]

What size is stock? 29"? 30"?