My installed VDO 301-107 fuel gauge works but is inaccurate, reading almost empty with a 1/2 tank of fuel remaining. 

Can this accuracy issue be resolved by adding a resistor and if so, would the resistor be added on the lead near the fuel gauge or would it have to be added at another location such as in the tank (hopefully not). 

Lastly, given the level of inaccuracy, does someone have an idea of how large of resistor should be added?

I could be wrong here but you could add a variable resistor then dial it in. Once yo have it dialed in measure it's resistance and substitute it for a resistor of equal value.
Regards Martin.

Within a certain range there's a screw on the side of the VDO gauge that let's you adjust it.

Excellent suggestion on the variable resistor. Just another great way for me to dig myself a deeper hole - but will be a good learning opportunity.

I'll first see if I can figure out where/how to make an adjustment at the gauge itself.

Thanks to both of you.

It's more likely that corrosion on the fuel sender itself is causing the problem you are seeing with inaccurate fuel level readings. Before fooling with the electronics of the system, I would recommend refurbishing the fuel sender and testing again. I provide instructions in my article Messing with the In-tank Fuel Sender.

Yes, it appears removing the sender unit is my next step. The little cogged wheel (a potentiometer?) seems inaccessible (at least to me) without removing the sender unit. Visually, the sender unit appears to be in very good condition (bright, shiney, no visible corrosion). 

Q: Can the existing gasket sealing the sender unit be reused if in good condition? Are there any re-sealing issues I need to be aware of?

Not a gasket per se but a very large O-ring.  I've never had any issues reusing them although they do seem to get pretty hard with age.

Part # 62161459608  about $8

There's just 4 large Phillips screws holding the sender on.  You'll need a big Phillips screwdriver for those.

Thanks! And this answers my wife's question of what I am going to do today.

Generally, these senders are 30 years old and very cruddy. If the sender looks in good condition when peering into the tank, then maybe first testing the sender is in order. Separate the tank connector from the harness. Use a multimeter on Ohms (resistance) setting and attach the leads to the Yellow and Brown wires on the tank lead. Reach into the tank and slowly move the float arm up and down, full motion, while observing the reaction on the multimeter. If the indicator also moves smoothly with the action of the float arm, not skipping around or indicating "OL" on the display, then most likely the sender is in good condition. You're looking for smooth change in resistance as the float arm sweeps across the resistor plate. If the meter indicator jumps around in value, then that is a sign of corrosion. Remember the range of resistance is from very low, like 1-2 Ohms, to 135 Ohms.

Robert,
Appears the sender is in serviceable condition. It shows a smooth transition from 17 - 117 Ohms.

The plan at this point is to pull the tank and drop the sending unit and move the cogged wheel to see if this will decrease the resistance sufficiently to allow me to use my existing VDO gauge.

Dave

Sorry, but the cogged wheel will do nothing to adjust the resistance values you've just confirmed. The wheel is an adjustment that determines when the other wiper on the float arm intersects a grounded copper pad; it has no connection to the resistance plate. I think you need to start looking at the condition of your gauge as being the culprit in this mystery.