It depends on the interest rate of the loan and also what you expect in terms of depreciation/fuel savings by changing cars.

If you owe (for extreme example) $15k more than the truck is worth but are paying .9% interest then the 'hit' already came and you'd be better served making payments on this while aggressively tackling your high interest debt. Reducing fuel costs is great but even if you drive 1500 miles a month and go from 15 mpg to 30 at $4 a gallon you're talkking $200 a month which is potentially less than you'd be paying in interest on $15k on credit cards.