Thanks, Armitage Shanks...

The US gallon is smaller than the UK gallon. Canada measure in litres so my MPG calculations here are accurate (multiply by 0.22) but no one in USA filling stations could tell me the equivalent volume in litres for a US gallon. Mildly frustrated by not being able to work this out, I was helpfully informed of the exact figures by an Armitage Shanks URINAL! The flush capacity is 1 gallon per flush, or 3.8 litres. Fantastic! The most informative visit to the men's "restroom" ever. Now i can tell you that the traffic jam MPG is 32, going up to 43 for town and country mix, and 53 for a long distance cruise. By the way, you can still hear the stereo at 110mph...