The most fuel efficient trip would be to get a bicycle and get rid of the car totally. Or just use your two feet and walk/run/jog to and from work.

Your gas mileage is highly dependant on several things. Like how fast do you accelerate when leaving a stop sign/stop light? Do you hit 60 before the next block, or only 25? Your mileage would be far higher with a lower acceleration. With a good sports car though, you can have a lot more fun with the higher acceleration, which in turn lowers your stress levels.

Checking your mileage over only one week is not enough though. You may accelerate more in one week than in another, or you may hit all greens for 10 days in a row followed by all reds for 10 days (less acceleration needed for all green lights). You will need to average this out over several months.

Also, make sure that your car is tuned up so that it is as efficient as possible.