A couple things to do.
One - Hand calculate your fuel mileage. You will need to know the difference of your stock height tires and the new ones (if there is any) Use this:
Tire size calculator
Enter your stock size and your new size. It will give you the difference in percentage. Bigger tires register less miles so you'll calculate your fuel mileage and then add the percentage difference.
Example: Your odometer says you drove 300 miles, you put in 20 gals of fuel. So, 300 / 20 = 15 mpg
There is, say 3.5% difference tire size, so...300 / 20 = 15 + 3.5% (.525) = 15.525 mpg
Two - Get a programmer that will allow you to adjust for tire size.