I second everything Graham has asked above - and add - are the tyres / pressures / wheels exactly the same as your AMF?
Quite apart from the external air temp sensors, how about the fuel temp sensor and the coolant temp sensor? You mention the erratic exterior temperature reading on the dashboard , but don't comment on coolant temperature (so reading between the lines this is deemed accurate) - but I'm also guessing you're going by the dashboard readings rather than an OBD2 reader or VCDS to check that the ECU coolant temperature channel is reading the same as the dashboard display.
Reason for asking this is that the coolant temp sensor has 4 pins and sends two independent temperature readings - one to the ECU, and one to the dashboard. They should be the same / near-identical, but if the sensor is older, it is plausible that one channel may have failed, and if it is the ECU side, this would only be evident by poor fuel consumption while the dashboard display is normal - effectively the ECU would be running blind as far as temp is concerned and it is like driving with the choke fully out (in old money!). Finally - is the thermostat in good condition?