[Regulus]

Originally Posted by The_Laird

You'll never get an accurate fuel consumption figure if you use "roughly" and "estimated" values. The only way to do it is to zero the miles before the trip when you fill the tank. Then keep filling the tank and setting the miles to zero, noting the miles each time. And the end of the trip, fill up and note the miles. I've been doing this on every tankful for years (I know I need to get out more!).

I second that
I keep a driver's journal since April 2001

edit: another advantage you will have is you can calculate the total cost per mile over time.
I discovered that my BMW 530D -99 in fact was basically as expensive to drive as my Jeep Grand Cherokee 5.2 -96. Despite the Jeep being twice as expensive on the fuel