If you don't mind sharing, what car is that, and what year of manufacture?

ooooh ... you know what, maybe i'll avoid highways and stick to roads with 30 mph speed limits, too, actually. i drive a hybrid so it might be pretty chill and cost next to nothing.

Hot weather does not help with fuel economy precisely because you have to turn on AC (or arrive at your destination a sweaty mess, everyone will LOVE that smell).

Mild weather helps a lot with fuel economy though, engine still warms up faster compared to the winter and you don't need AC.

The most economical speed was 30mph in 5th gear with cruise control on

That can't be too good for your engine's health unless you have a fairly short 5th gear (e.g if you have 6 or more gears anyway or just a short final drive). Most of my manuals have been 5 speed manuals and they aren't too happy in 5th at 50 km/h.

Since you mentioned below that it's a diesel, that will cause real issues over a few hundred thousand. I had to clean turbo vanes on a 415k km 1.9 tdi, all gunked up from eco-driving. Vanes don't get exercised and they don't get any proper heat either. You'd ideally want to be cruising around 2000 rpm and you should be using most of your rev range, not just the bottom end.