Different gas gauges for 25 vs 31 gallon tank?

[goathearder]

Hey Guys,
I am having a weird issue with my gas gauge. A couple months ago I swapped some TBI on my 73 and rather than running some inline pump, I decided to do it the better way and get an in tank pump. I bought a replacement 31 gallon tank and pump/sending unit from amazon,

http://www.amazon.com/gp/product/B000C7TXO0/ref=oh_details_o09_s00_i00?ie=UTF8&psc=1

and

http://www.amazon.com/gp/product/B000VB0ZQ2/ref=oh_details_o09_s01_i00?ie=UTF8&psc=1

And ran fuel lines pulled from a junkyard to the TBI unit for a nice, factory style appearance.

The TBI works GREAT and I love it but for whatever reason when my gas gauge reads E, the tank only takes 23 or 24 gallons. Originally the truck had a 25 gallon so I am trying to figure out if maybe there is a difference in the gauges?

It doesn't make sense though because the sender would be 0-90 ohm regardless of the gauge so I am thinking it would work either way.

Anyone ever run into something like this before?

 

[dyeager535]

Shouldn't make a bit of difference. I have to assume the gauges themselves were 0-90 Ohm as well from 1973-1991.

Does the full side read correctly?

I've played around with the gauges quite a bit, they can be "tuned", but realistically if the sending unit is working/made correctly, 0-90 Ohms is 0-90 ohms.

Assuming you used factory wiring/similar to factory wiring, I'd check resistance when it reads "E" on your gauge, at the connector near the fuel tank. If the sender resistance is what it should be at "E", I'd say maybe the float isn't at the right height, and bottoming out before it's actually running low on fuel.

Pre-post edit: You did notice that the dimensions of the tank and sending unit appear "odd" (neither matches the other for what should be depth), which makes me wonder if you have a 25 gallon sender on a 31 gallon tank.

 

[sweetk30]

gm is 0-90 ohms .

possible bad sender or loose ground for sender.

 

[Snoozer]

I'm 99% sure that the gauges aren't different. Rather, GM used different sending units depending on the gas tank size. Are you sure the sending units are the same for a 25 and 31 gallon tank?

My humble (and probably ignorant) opinon?... I think GM purposely wanted the gas gauges to read "E" even when there was still 5 gallons of gas left in the tank. Trucks didn't come with "Low Fuel" warning lights on the dashes like today's cars do. And when a truck only gets 8 mpg, 5 gallons isn't that much gas in the tank.

I think the more realistic "variable" might be that you bought a non-GM sending unit. Can't expect it to read as accurately as an AC Delco one.

 

[goathearder]

Yea, I am sure they don't have the same sending unit cause there are different PNs for the sending units, which is why I bought the 31 gal one.

Your theory about reading E way early might be a good one though.

I did test the sender before I put it in (to get the wiring right) and it read 0-90 so I doubt its a bad sender, especially because it goes from full to E.

Maybe I should throw my 5 gal tank on the back and see how far I can drive it on E and see if the needle drops below E, if that is even possible.

 

[sweetk30]

also the old trucks of ours tend to read e faster than normal .