To answer your question, nothing.
Because that is average mpg you are seeing. Your 100 miles is only a percentage of your total miles since the computer was last reset, so your mpg will only alter by a percentage too and a very small percentage at that.
Say your car has done 3000 miles since you last reset, your 100 miles only relates to 3.3% of your total mileage.
30 mpg average for 2900 miles equates to 96.6 gallons of fuel used.
45 mpg average for 100 miles equates to 2.2 gallons used.
So, for 3000 miles you have used 98.8 gallons which works out at 30.36 mpg.
So, that gives you a grand total of 0.36 mpg increase.
i have never reset it ??? how do i lol ............ your 3000 is correct thats what my car has done ! thankyou very useful info there