just my opinion:
for the cost of a swap versus the savings at the pump isn't there
very basic math
80 miles a day divided by 12mpg= 6.6 gallons
80 divided by 15mpg=5.3
1.3 gallon a day diff x $3 a gallon= $3.90 a day diff
$3.90 x 5 days= round it up to $20 a week x 50 weeks a year= $1,000 a...