Glen,
I have a resistance box (0-10k in 1 ohm steps, 4 knobs 0 - 9999 ohms)
I use it to test gauges in many cars, and you could use one like this to see what ohms you need for full, half, empty etc.
Of course, they are so many others out there now:
https://www.google.com.au/search?q=d...w=1536&bih=676
Gary