Dual Fuel tank capacity?

[SunSoaked]

Is there a code on the SPID that denotes the capacity of the saddle tanks?
85 K20. I assume it's 16 gal but since installing my Dakota Digital guages it's seems to be off about a quarter tank when below full. For instance, when it hits 1/4 tank, I fill it up and it takes 15 gallons.

[dmack91]

Not sure on the spid, but my 83 K20 has dual 20 gallon tanks.

[SunSoaked]

Short of running it dry and refilling it, how do you know that?

[Keith Seymore]

Nope. Capacity is not noted on the SPID.

K

[1967KaiserM715]

all short bed trucks used 16 gallon tanks, not enough room physically for a 20 gallon tank.

you can measure the tank, a 20 gallon tank should be roughly 56 inches long and be almost touching the front rear spring hanger. a 16 gallon will be roughly a foot shorter...43 inches if i remember.

[sweetk30]

0-90 ohms is the gm sender spec when they work in a perfect world .


and short bed tanks in a long bed get special filler tube steel pipe extension or if someone else swapped them in it would be a looooong rubber hose to reach the tank from the filler .

[DieselSJ]

Fuel gauges are always just an approximation. On one side, filling from the half tank mark it will take ~11 gal, and the other side it is about 9 gal on my truck.

[Dead Parrot]

^^ This. On my 75, with the original gauge, I had 4 gallons left on E. Had to replace the gauge. Now E pretty much means expect to switch tanks real soon. Same tanks and senders. On a tank that is run dry, I can sometimes get as much as 21 gallons into the nominal 20 gallon tank.

[SunSoaked]

Thanks for the info.
So is it safe to assume that shortbeds have 16 gallon tanks and longbeds have 20 gallon from the factory UNLESS someone swapped the shorts into the longbed?

I'll probably intentionally run one tank out of gas on purpose and then go to the pump on the other tank. I'll fill it and notate what the guage says at each 1/4 mark and full. I just want it to be on my terms, not in the middle of the freeway.

[sweetk30]

1987 r/c-10 i parted out for the body was factory twin tank 16 gal on a long bed . had the factory extender pipes for the fillers .

so do NOT think long = 20 and short = 16 ........... or you will be surprised .

also this truck had 56" rear springs in a 1/2 ton . hangers were still factory riveted to the frame rails .

[DieselSJ]

Originally Posted by SunSoaked

Thanks for the info.
So is it safe to assume that shortbeds have 16 gallon tanks and longbeds have 20 gallon from the factory UNLESS someone swapped the shorts into the longbed?

I'll probably intentionally run one tank out of gas on purpose and then go to the pump on the other tank. I'll fill it and notate what the guage says at each 1/4 mark and full. I just want it to be on my terms, not in the middle of the freeway.

I just run a tank until it is just about at E then switch tanks. I'll fill the empty tank when convenient. Keeps the gas fresh in the tanks. Plus the only place that has hoses long enough to reach both tanks is Costco so it is a pain to try to fill both at the same time.

[SunSoaked]

I just want to know when it is actually about to run out by the guage. My Nissan goes another 100 miles when it hits E, my motorcycle runs out at 3/4.

[Keith Seymore]

Originally Posted by DieselSJ

I just run a tank until it is just about at E then switch tanks. I'll fill the empty tank when convenient. Keeps the gas fresh in the tanks. Plus the only place that has hoses long enough to reach both tanks is Costco so it is a pain to try to fill both at the same time.

For a manual trans vehicle you can actually run them completely dry; switch tanks and it will catch and keep going.

I used to do that with my '80.

K

[tantank79]

Originally Posted by Keith Seymore

For a manual trans vehicle you can actually run them completely dry; switch tanks and it will catch and keep going.

I used to do that with my '80.

K

I do this all the time in my 76, but it's an auto. Why the manual trans stipulation?

[Keith Seymore]

Originally Posted by SunSoaked

Thanks for the info.
So is it safe to assume that shortbeds have 16 gallon tanks and longbeds have 20 gallon from the factory UNLESS someone swapped the shorts into the longbed?

I'll probably intentionally run one tank out of gas on purpose and then go to the pump on the other tank. I'll fill it and notate what the guage says at each 1/4 mark and full. I just want it to be on my terms, not in the middle of the freeway.

Almost.

Short bed will always be 16 gallon tank(s).

Long bed could be a 16 gallon tank (depending on GVW and other powertrain considerations) but will usually be 20 a gallon tank.

K

[nekkidhillbilly]

ive seen them only have one tank also just pointing that out. lb with a single 20 gal tank.

[Dead Parrot]

You will usually have several seconds of partial power before the motor really dies. It only takes a second or so for fuel from the full tank to reach the carb. As long as you are at cruise, you will feel the loss of power well before the engine fully dies. You don't want to be in the middle of a high power merge onto an interstate when a tank runs dry. Between the low fuel level and the high front to back slosh factor for these tanks, you can go from full power to off almost instantly.

[dagnabbitt]

I recently put some NVU digital gauges in my 81 swb, when the fuel gauge hits 1/4 tank, I pull over at the nearest gas station. Earlier this year I actually ran out of gas when I decided the gauge was lying to me, and I just got one little 16 ga tank, so there was no switching for me.

I am guessing that it is more of an art than a science to get a digital set up to work with an analog sending unit and most of these companies figure close enough is close enough.

[Factory Guy]

Seems to me, back in the 70's there was a rule? that stated GM could only put on 39 gallons worth of fuel tanks in any long bed pick up as a MAX capacity?

Maybe somebody can clarify this?

[Keith Seymore]

Originally Posted by Factory Guy

Seems to me, back in the 70's there was a rule? that stated GM could only put on 39 gallons worth of fuel tanks in any long bed pick up as a MAX capacity?

Maybe somebody can clarify this?

No. I don't remember that.

Plus - Suburban had a 40 gallon tank, in the rear.

K

[gmachinz]

LOL fuel capacity limits...blah!! I just recently provided a customer with a custom rear chassis harness and control wiring kit to run 2 Pollak selector valves to operate 3 tanks....a 40-gallon rear tank and both 20-gallon saddle tanks in a CC dually! A 502/NV4500 is a thirsty beast!!

[DieselSJ]

Originally Posted by gmachinz

LOL fuel capacity limits...blah!! I just recently provided a customer with a custom rear chassis harness and control wiring kit to run 2 Pollak selector valves to operate 3 tanks....a 40-gallon rear tank and both 20-gallon saddle tanks in a CC dually! A 502/NV4500 is a thirsty beast!!

Dang. I need that. Actually now that I'm getting 12mpg with the NV4500 (instead of 9 with the TH400), I think I can live with the 2x20 saddle tanks. But good to know that is an option.

[gmachinz]

It was a build that required dual Pollak 7-round trailer add-ons as well as a cool mod to the rear markers lights-this required the use of a larger, 1” frame loom and related frame clips w/rosebud clips. Factory style routing and worked out slick!

[hatzie]

Originally Posted by gmachinz

It was a build that required dual Pollak 7-round trailer add-ons as well as a cool mod to the rear markers lights-this required the use of a larger, 1” frame loom and related frame clips w/rosebud clips. Factory style routing and worked out slick!

Stake body with the side markers?

[biggsteve97]

Look underneath at the back of the tank. If there is atleast 8-10 inches from the tank to the front spring hanger then its a 16 gal tank. If there is less than 2" its a 20 gal tank. The replacement 20 gal in my 87 is so close to the front hanger that I had to bend the flange that goes around the seam of the tank just a little to clear the hanger properly.