gas gauge/sending unit troubleshooting

[RICH MUISE]

Found this info at Tank's Inc. I think it would be really helpful tracking down issues with fuel gauges not working correctly. It's generic information, but includes some info on early Fords as well as popular aftermarket gages.
http://www.tanksinc.com/index.cfm/page/ptype=results/category_id=159/home_id=-1/mode=cat/cat159.htm

and some universal sending units available in different ohms ranges
http://www.tanksinc.com/index.cfm/page/ptype=product/product_id=276/category_id=62/mode=prod/prd276.htm

[RICH MUISE]

I've been thinking about this gas sending unit problem so many guys are having problems with, and it's something I'm going to have to address pretty soon. Yep, I know...I get in a lot of trouble when I think too much, but.......assuming the ohms range of your sender matches up with the fuel gauge's ohm range, and you're still not getting correct readings, the problem very well may lie in the float adjustment. I know, that sounds like an over simplified and obvious statement, but think about it...the float has to go thru it's top-to-bottom travel, and be coordinated with the sender's rheostat going thru it's full range. In other words, if the float hits the top or bottom of the tank before (or after) the rheostat reaches the top or bottom of it's range, you'll get an erroneous reading. Obviously the same erroneous results if the rheostat reaches the end of it's travel before the float reaches top or bottom.With that in mind, the position of the float on the rod has to be a calculated distance from the pivot point of the rheostat. The depth of the tank (= float travel) is one of the factors used to calculate that distance. In the link I posted above, if you click on the installation guide under the list of senders, it gives a chart for those dimensions based on different tank depths.
edited 10/13 to change "potentiometer" to rheostat

[jseth83]

Thanks for the link Rich.  I actually have a fuel tank from a Mustang in my '57, so that was easier than most.  I've worked on some street rods where the setup is a real pain though.  One bad thing is that a lot of the aftermarket gauges don't have a fuel pickup which is sometimes needed.  And like you said, the actual physical sweep can get blocked by tank geometry, and you can definitely get erroneous reading, even if your gauge and sender have matching ranges.