A simple digital current sink for a fuel gauge?
USE serial control to octal MOSFET package

When you get bored with using up 6 of the arduino's output lines and realize that the ULN2803 won't quite work as well as you expect (the darlington outputs prevent you fully turning on a transistor to ground less than 0.7V), you might go for a serial MOSFET array like this: -



On resistance is 5 ohms so if the largest current in one FET is 25mA, it will turn on to about 0.125V giving you more range of control over your fuel gauge. If you didn't fancy choosing different resistors you can cascade several of these bad-boys (note the serial out line) such as 4 to give you 32 levels all using the same resistor value.

However, I'd go with Joe Hass's suggestion of halving the resistor value - with the device I've shown you can get 256 levels (\$2^8\$) from two lines, serial in and serial_clock.

Here is the data sheet