A month or so ago I posted about the increase in fuel economy on my regular Aberdeen trip after putting 5/30 oil into my Focus instead of the 10/40 that had been in it for the last three years. Normal economy on that journey is
33 mpg but with the new oil I got 36 and 38 from the next two tanks.
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Unfortunately I didn't get to go back up to Aberdeen this month again to exactly repeat the test but I did do a few hundred miles of motorway driving last week to visit a colleague in Sussex. The first 107 miles of the tank were the usual short shopping trips to the village with a cold engine which always give 27 mpg and the fuel gauge reaches the first mark at 95 miles which it did exactly this time too. The trips to Sussex were 72 miles each way over two days, mainly motorway mileage on the M25 and M23 with about 12 miles at each end on A roads. Obviously that means the engine starts from cold a lot more than on a single long trip like to Aberdeen so I wouldn't expect quite such good economy. I also ended up in a 10 mile tailback on the M25 for over an hour which can't have helped.
Results over the full tank were 382 miles using 51.4 litres giving 33.8 mpg. That's still better economy than I used to get on a single long trip. Knock off the first 107 miles of short journeys at 27 mpg (18.02 litres) and we get 275 miles using 33.38 litres for the motorway journeys giving 37.5 mpg.
So it definitely seems that the economy has improved by a considerable margin to circa 38 mpg rather than 33 mpg once the engine is hot.