light led at specific resistance?

As long as you can handle not being able to set the value above 5vdc I would use a voltage regulator on the pot. 

The 7805 has three pins - in, out, and ground.  Connect the in to switced ignition, connect the ground to ground, and then connect the out to the "voltage in" pin on the pot.  If you want to "do it right" you should also add a cap across the input and a cap across the output (check the data sheet for the recomended type and value).

This will limit the range of the pot from 0-5vdc and, more importantly, it will eliminate the flucuation on the op amp input caused by the unstable voltage of the vehicles electrical system.

If you are having trouble dialing the pot in you may want to look for a higher quality pot or a pot with more turns.  On the last circuit I built I used a pot from Alpha that was a single turn (270 degrees from end to end) and I could adjust it to the hundreth of a volt between 0-5vdc rather easily.

If you go the 7805 route, you will have to buy a new pot and also have to sink and mount the regulator.  Or we could use 1 or 2 resistors and a different value pot.  No heat sink no worries about mounting the heat sink and the IC. 

Why would you need a different pot and why would you need a heat sink?  I just did this the other day with a 10K pot and a 100mA 7805 with no heat sink.  Works great.  The pot will only draw 1/2mA, WAAAAAAAAAAYYYYYYYYY below the point of needing a sink (typically 100mA is the point of adding a heat sink).

If you want to eliminate the pot and just use a voltage divider circuit you should still use a 5vdc regulator to power the voltage divider circuit.  Your battery voltage will bounce around from ~10-~15vdc while driving depending on a multitude of conditions.  Having a voltage regulator will insure that you ALWAYS have 5vdc going in to the voltage divider (or pot) and will eliminate the effects of the unstable voltage.  I would guarentee the guage has a voltage regulator inside it to provide consistency in reading the level sensor, so why not include this "feature" in the circuitry to light the light?

If you can not tell me what voltage the resistance lead of the meter has on it when you are at the desired level of fuel, I can not answer your question.   You earlier said that the range of the gauge was from 6.22 volts to 2.75 volts.  Then you said you wanted it to come on at 2.5 volts.   That is not within the range of what you said the gauge operated at.  The following diagram includes a resistor and a zener diode used as regulation.  

The pot has been replaced with 2 fixed resistors.  They will make the light come on at 2.5 volts.  If you need to have the light come on with less voltage, change the bottom resistor to a slightly lower value than the 1K.   If you need the light to come on at a higher voltage, replace the 1K with a resistor with a slightly higher than 1K.

If you insist on using the 7805 here is how you would do so.  The left leg of it needs to be connected to power.  Notice the A in the 2 boxes, connect those together.