LRO do a real world test. They have a set route that includes M’way, A&B plus a couple greenlanes. They do it on secondhand stuff as well as new. Usually gives a good indication.
Tbh though, my old Wolseley 16/60 auto (with a BW 3spd slush box) avgd about 30mpg as did a Volvo 144 auto and many others. Even the 3.0 Capri got 26ish sometimes! So I’m not that impressed when twenty plus years later a Kuga diesel and Sharan diesel only AVG about 44!