Without a computer - there isn't one...
However, the best way to tell is this:
Fill up (completely)
Reset the mile thing (is it an odometer?)
Do your 450 miles (Ok, I've got a TDi so I am actually bragging)
Fill up again (completely)
Take note of how many miles you've done
Note the number of litres you've filled up with and multiply it by 0.22 (to convert to gallons)
Then it's a simple case of Miles divided by Gallons, e.g. 150 miles, 2.2 Gallons = 68.18mpg
Nice and simple like.
Or use a spreadsheet that will work it all out for you (I've got one so if you want it, PM me and I'll mail it to you)