Based on,
100,000 miles per year
Fuel mileage going from 16.8 to 19.1
Todays AAA reported prices for 87 and 93 octane
By using 93 you lose $753.55 per year. You lose more if you drive more.
The break even point (at today's prices) is to average 20mpg. My guess is if you drove the van to...