I need a formula to correct my incorrect truck odometer.?

If the odometer in my truck is is out 300m every 5km, eg- when i reach a fixed 5km sign the odo reads 5.3km, how many km's have i really done if the odo says i have done 326km on this trip. Also can anyone tell me a quick formula to work this out. I am fighting a truck hire place for excess km's charge and i need to make sure im right with the numbers..cheers.