I have a Mike Robinson overdrive on my
400 and it does 25-28 mpg
cruising at 70-80 mph, yet my heavy old 2L V6 Auto Rover does about 35
mpg.
More to the point a now dated Rover 25 with a 1.6 engines has a
similar performance to a
410 and also does 35-40 mpg.
Ash