Do the math. Most cars on the road won't get better than 30hwy like the Bville, but for comparison'* sake, let'* say you decided to replace the Bville with a 4-banger Honda getting 35mpg.
To drive 300 miles, your cost comparison would be:
$32.13 for the 35mpg car on regular.
$37.50 for an L27/L36
$39.50 for an L67
Less than a $7 difference for a 300 mile trip from a 35mpg econobox to an SSEi. THREE HUNDRED miles.
This is based on my current gas prices of $3.75/$3.95
Trade-in worth it? Or even selling? Read this:
And to get better mileage:
The price difference to fill Reg vs. Premium? About 3 bucks per fillup. I usually have that much in loose change in my ashtray.
You may argue that over time it adds up (based on the number of miles you drive per year), but I'll argue in return, that with gas prices being high, your Bville will sell for alot less now than it would have last year even without depreciation. You'll LOSE or just break even in the long run.
Keep the Bonneville, drive easy, and fill your tires to the max rated pressure, and you'll get better mileage than 95% of the cars on the road, even those advertised as fuel-efficient.