In an idle moment, I wasted some of my remaining life looking up numbers on t'interweb.
It turns out that an A320 type of aircraft, operating, say, LHR-MUC and return, would burn about 5 Tonnes each way. So if the return fuel burn is added to the outbound aircraft weight (guessed at 77T), it would be adding about 6.5% (these are ROUGH FIGURES!).
The CO2 produced on the outbound leg would be, according to the
ICAO website, 10,554 Kg. Assuming a direct relationship with aircraft weight, the tankered fuel generates 6.5% of that, ie 686 Kg CO2.
According to
"They Work for You" a road bridger vehicle weighing, say, 40-50 tonnes, would generate 950 grams per kilometer, ie 95 Kg per 100 Km carrying fuel to MUC. So if the return fuel were uplifted at MUC, those 5 tonnes would have generated 10% of that, just 9.5 Kg, per 100 Km road distance from the refinery to Munich.
Not to be taken as too accurate or even serious, but it would seem that within a wide margin of error, in environmental terms it's best not to tanker as opposed to uplifting at the destination, ceteris paribus, which of course they probably are not.
Mind you, my calculations could easily be outside even that wide margin of error. I haven't done sums like that for years.