Water and a soft cloth will usualy do the trick. If there is a stubborn film on the plastic try a dilute solution of water and commercial window cleaner(like windex). 50:50 should be strong enuf.
Slightly off-topic, but worth mentioning since we're talking about the sometimes 'sensitive' guage plastic. I've seen several reports of plastic/vinyl dressing products causing clear plastic over instruments to cloud or get hazy. Restoring with Meguiars #17 & #10 has worked in many cases.

dave and xtoy - mind the odo reset button