First of all, range depends on conditions. During the peak of the skip cycle, you won't get crap for range. Now that it's declining I can get about 10-25 miles on the 40 cb channels or *ahem* slightly more on some near by frequencies that aren't exactly legitimate to be utilized to their potential... /forums/images/icons/laugh.gif
Get that antenna up absolutely as high as possible. You want at least 2/3 of the antenna over top of the roof for best results. I use magnetic mount Wilson 5000 centered on my roof... Did you get your SWR's set? That will be important for a few reasons, the biggest two being that you radiate the power you have most efficiently, and it helps keep the radio from being damaged...
But it's mostly conditions, nobody can specifically say how far out you'll get, could be a mile, could be 100 or more miles... Sometimes nature is very strange when it comes to radio transmissions/communications... /forums/images/icons/laugh.gif
If you do decide to ever run any power, make sure it's an AB biased amplifier. Of course any amp is illegal but the AB will help ensure you have the cleanest output possible (less IMD (bleed over) and so forth)... Also if you want to run any power make sure nobody has removed any limiters from the radio, and so forth... And make sure they match the settings in the radio to the capabilities of the amp so it's not over driven (even an AB amp, when over driven will result in IMD/bleed over)... Also even with a "pre-amp" (which boosts received signal into the radio) it probably won't help to increase your reception very much, only distance you can transmit)...
Is there any engine noise present? It'd be in the form of a whine, or other sounds that vary with engine speed... Might want to track down the cause of said noises and that should get you a lower noise floor so you can hear better... If it's an issue... /forums/images/icons/crazy.gif