Is there a calculator that can approximate the expected MPG

The old 7.3 diesel averaged just shy of 13mpg but as low as 9mpg (into a 25mph headwind) and as high as 16mpg (30mph tail wind).

The new V10 doesn't have as many miles on it but seems to be running around 9mpg.

Way to much variation in rigs and how you drive to give a good estimate.