One thing I noted when pricing switched to litres was the change in people's perception of price fluctuations.
They see price increases in monetary terms per unit, not realising that the old unit was 4.54 times greater than the new and that an increase of 5p/litre is 22.7p/gallon. £1.50/litre is expensive to most people, but I can't help wondering what their thoughts might be about fuel costing £6.81/gallon.
On a different note; when I see people in the US complain about the rising cost of fuel per gallon, I can't help wondering how they'd feel if they used proper, full sized gallons