Yup, it's MY19 when they upgraded the tank, but given the OP is looking at MY16-17 I guess there must be quite a big purchase price difference.
I have an XC60 T8 MY19 with the bigger tank. On long journeys, at 80 mph (I do not live in the UK) I am getting 35 mpg. When towing (2 tonne caravan) I am getting 27-29 mpg at 50 mph.
I expect the XC90 would be a bit worse, but not much. Nevertheless, 38 mpg doesn't sound achievable
without charging the battery.