Completely understandable. But you must understand that you're not going to be able to draw any useful conclusions because of the inaccuracy.
My mileage, fill-up to fill-up, varies as much as 2 mpg with NO changes to the car or route. The only way to get a reasonably accurate measurement is to average mileage over SEVERAL fill-ups, the more the better.
If you can't do that, then it's just not feasible to measure improvements with any accuracy after each modification. You'll have to see what the net result is when you're done.
