Patrick,
As others have stated; Those 20-22MPG mileage figures are on longer trips...not day to day commuting mileage.
The best way to verify you actual highway mileage is to get on the open road when there is not traffic around (like very late at night or in the wee-hours of the morning, and do an accurate mileage test, and a long stretch of flat road between 65-70 mph with the cruise control on. A good distance would be between 50-100 miles at minimum.
Be sure your tires are properly inflated, and even better if you add an additional 5-7 PSI to the normal tire pressure...just don't exceed the Max PSI indicated on the sidewall. I would recommend a tire pressure of about 33-35 PSI for minimal roll resistance without too harsh of a ride.
IMO One of the best investment to improve mileage is an Ultra-Gauge or similar device. This gauge simply plugs into your OBDII port and gives average and instantaneous mileage readings as well as access to other computer sensors.
Your vehicle may have an instantaneous MPG readout, but it gets its reading from the vehicles speed sensor and they are often inaccurate and the calibration cannot be adjusted by the owner. The Ultra-Gauge lets you accurately calibrate the gauge to your speed sensor. I picked up an additional 1 MPG just by calibrating my gauge because of the error... I did a test run of exact 7 miles using mile markers and my GPS and the odometer and gauge only detect 6.63 miles? The Ultra-gauge allowed me to calibrate the gauge with a 1.034 adjustment factor and now my ultra gauge speed and mileage prefectly match my GPS and the highway mile markers.
Also, the Ultra-Gauge will show you exactly what speed, and driving habits are saving you fuel and what eats into your gas mileage. That trains you how to drive and save the most gas you can. It also shows the impact of of things we cannot control and how traffic, wind and weather can effect your fuel mileage.
The best part is that the Ultra-Gauge only costs about $60 and you can order it online at: www.ultragauge.com
...Richard