anyone else noticed increased fuel consumption?

over last 2 weeks, i am going through a lot more petrol than usual.

In my 416, i usually get max 400 miles to a tank, only struggled 310 this time round.

Also found some water needed as well. Wouldnt be anything to do with filling my oil up with thinner oil from 10/40 to 0/40?

Check the fuel system for leaks. Any patches on the ground where you park?

does you car have a carb or is it fuel injection?

if carb then may be choke isn't knocking off when engine is warmed up, this would cause high fule consumption.

Is idle speed higher than usuall when engine is warm?