Ditto what Roger said. Q-jets are really pretty good on mileage as long as you keep your foot out of those giant secondaries. What is your normal drive? If it's a lot of stoplight-to-stoplight the mileage is not going to be great; however, unless you've got some really aggressive gearing I'd think that 15MPG on the highway should be attainable. Do you have a tach? if so, what does it read at 60MPH? if you can keep the cruising RPM down below 2,000 the mileage should certainly be better than what you're experiencing.

My 2 cents,
Glenn