FWIW - I would hand calculate my fuel mileage like normal (odometer readings for miles traveled and then divided by gallons filled .... I then increased my MPG readings by the percentage of increase in wheel diameter .... I am thinking that yields the corrected numbers?
the thinking is my distance traveled would be greater by the percent increase in tire diameter....
That's actually pretty inaccurate...
The 195/65-15 is 25" tall and spins at 808 revolutions per mile.
The 205/75-15 is 27.1" tall and spins at 744 revolutions per mile.
With the larger tire, at an indicated 20 mph you are actually doing 21.7 mph.
At an indicated 50 mph, you are doing 54.2. At an indicated 80 mph, you are doing 86.7 mph.
You can see the difference gets larger the faster you go. And since you dont drive at a constant speed, you cant divide (or multiply) by a constant difference.
Why not just find someone with a VCDS (or use yours if you have one) and adjust the impulse multiplier that way your mph, odometer, and fuel mileage calculator are all correct.