Is it possible to add a custom calculation for a top level row total in a pivottable?

Question

My Pivot Table has the following format

Manufacturer           MPG 
-   Nissan              10
      - Petrol          35
          - Model A     30
          - Model B     40
      - Diesel          45   
          - Model C     50 
          - Model D     40
+    Toyota              5
+    Chevrolet           7

It displays a list of car manufacturers, each car is classified as either Petrol/Diesel (i.e. the car type) and the miles per gallon is shown for this car. At type row level I want to see the average of the miles per gallon. For example the average of petrol Nissan cars is 35mpg (ps. these numbers are not real). This is the part I can do. The part I need help with is the manufacturer level totals: Here I want to see

Diesel Average - Petrol Average 

instead of the average of the two. Is that possible?

Answer 1

I would use the Excel Data Model for this (aka Power Pivot). I would use the IF and HASONEVALUE functions in a DAX Calculation to switch between the 2 calculations depending on the level.