also you can try this: go to Autozone and get one of those red 5 gallon thanks. Wait until your low fuel light turns on then go to your gas station. Tell them you want diesel for $22 start pouring diesel into your 5 gallon thank. Monitor gallons meter on the computer to see if 5 gallons on the display screen match with your red 5 gallon tank. Put the rest in your vw. Drive until your low fuel light turns on again, then stop somewhere and put all 5 gallons from your red tank into your vw. Now you know that you really put in 5 gallons. Reset your trip odometer and drive until your low fuel light turns on again. See how many miles you get for 5 gallons.
For faster results you can find 2 gallon tanks or save 2 empty milk gallons. This will be the most accurate measure.