I've read all the advice about economical driving but one question is never raised or answered - too complex artihmetic?

Say you drive at 60 for 100 miles - it takes 100 minutes
Drive the same route at 40 - it takes 150 minutes

Which drive used less fuel?
In the first, you're using more fuel per minute but travelling further per minute and so for less time with the engine running
In the second, you're using less fuel per minute but travelling less far per minute and so for more time with the engine running

I've got a feeling (no evidence though) that it makes little difference either way.