I may not have followed your arithmetic completely, but it seems to me that the calculation would be as follows:
Kilometers in period = 1185
Days in period = 23 (inclusive)
average km per day = 1185/24 = 51.52;
Annual kms = 365*51.52 = 18,805
If that is the case, you can calculate it in T-SQL like shown below; simply replace the hard-coded odometer readings and dates that I have in there with the column names or variables.
SELECT 365*(45756-44571)/ (1.0+DATEDIFF(dd,'20120101','20120123'))