
Seems good with American gallons which are about 4 litres actually. Doesn't mention trip meter error which will exist unless a speedo healer is used.
I think that most of us get about 38 miles per gallon give or take a bit.
Now a Royal Enfield diesel (about 45mph top speed) will do a reliable 175 + to the gallon. Great for London commuting as 45mph is far faster than you'll ever likely to be able to do.