You have a problem in your script.
#1
In your for statement you are using a variable variable. While this can be done, you are setting it with a number, which a variable cannot be. I'm assuming that you just messed up and put an extra $ in there.
#2
You are passing the inputs as litres<?php $i;?> which doesn't really do anything. So you are posting a value from $_POST['litres'].
You should be passing the inputs as litres<?php echo $i;?> which would pass it as $_POST['litres0']. $_POST['litres1'], etc.
#3
You are not retrieving these same litres variable right, you are only retrieving $_POST['litres'] which gives you the LAST value submitted. (*note* this applies to litres and to hours).
#4
You should be retrieving your litres and hours variables from inside the for loop. Otherwise you cannot automate the count.
Side Note. Please sanitize your data, some very good sanitize functions floating around this forum. This will help your database from getting screwed, and protect your user's from the boogey man.