This is a very good method to keep tract of the miles that you are driving for the Mileage deduction with the IRS.
If you read more about the mileage log you will see that you can also deduct some other cost but not all of them. You can deduct parking but not fuel, that sort of thing.
Your IRS wants you to write the odometer mileage down when you begin a trip for either business, medical, moving or charity as long as you are deducting those miles.
When you finish the trip you should also write in the finishing odometer mileage.
Then you can do the math and get the miles driven on the trip.
If you want to download the Mileage Log as a PDF then do so here.
I also have the Mileage Log as a Bitmap file here.
You can download a JPG of the Mileage Log also.