The other obvious variable is how many kilometres the vehicle is driven.
Many vehicles do way below the average annual mileage, and pretty obviously, if they are replaced with a new vehicle the manufacturing emissions per year are a far larger proportion of their attributable emissions. Of course if you argue that they last a lot longer, it evens out. But this argument is suspect as well - economic life of vehicles is more often than not limited by the availability of parts or deterioration due to time rather than mileage, so it is not reasonable to suggest that a vehicle doing half the annual distance will last twice as long.
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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