Ok....VB is 6000km/yr, 1.72/l for fuel, average 14L/100km. ($1440) VX is 11L/100km av., @ 1.55/L, 12,000km/yr. ($2046)
Now....rego for two cars is $1600/yr; comp. insurance is $1200 or so. NRMA road side assistance runs at about $300/yr for both cars. Combined service costs is $600 or so/yr.
Total fuel bill is $3500; total other items is $3700. $7200/yr total.
Now to go down to ONE car (i.e. selling VB AND VX) would halve the sundries bill - $1850/yr savings. If I were to get say a VW Polo and drive it 16,000km/yr (mileage less for I wouldn't be going on runs, cruises and to shows), fuel bill would be $1600 (assume 6.4L/100km, $1.55/L fuel) - save $1900. Total savings $3700/yr.
edit - if I wanted to go all greenie that would save 3.2 tonnes CO2/ yr