We calculate as follows
Avg MPG: 10 -> CO2 emissions per mile are 1067g *Note
Avg round trip: 40 miles – > CO2 emissions 40×1067 = 42,696 g = 0.043 tonnes of CO2 (per exhibit) *
Offsetting costs us £7 per tonne CO2. The charge per vehicle then becomes £7 * 0.043 tCO2 = £0.30
the 10MPG is an estimate for a typical classic vehicle as well as for an average loaded trade van.
The average return journey distance is based on actual data from a previous show.