For those that think that a 4% decrease (or equivalently a 5% increase) is not much, you should go ask your boss for a 5% raise and see what they think.
I think the 1m mile savings figure implies a much higher cost that's harder to quantify -- vehicle wear, accident reduction, traffic congestion, labour, etc.
Isn't it more like tell your boss you'll take a pay cut, all it will cost is hiring your personal firm as efficiency consultants for £2M ... But look 5% reduction off your portion of the wage bill.
The contest prize was $15,000, there were probably some ancillary costs related to running the event but still quite cheap.