Use a credit card for fuel and KEEP the receipt
Take a photo of the dashboard with fuel guage in it and then a photo of the receipt (I tend to misplace pieces of paper)

It's a pain to dispute the charges but that's just how it works

I have been picking up more and more vehicles in Australia that are not really full lately. I find that I drive 20km or so and the guage is creeping down fast.

One tactic I have used, is to drive to a gas station as soon as I leave the airport and fill up the vehicle. In some cases it's been $12-$20 of fuel to fill it to the first click of the pump.

When returning the vehicle I explain this, and have received a credit against the rental.

It's a pain but I refuse to be short changed up to $20 on each rental, and when I am renting once or twice a week it adds up!