Here's the details: 98 Explorer, 5.0L, AWD, 130K miles

Just installed new Accel plug wires and Bosch Platinum plugs, just regular maintenance.

No codes on the computer.

He's the gripe: gas mileage went from 350 miles per tank to 275 miles on the last 3 tanks (each).

I have rechecked all wires and plugs, all vacum lines and all electrical connections. Any thoughts as what would have caused the sudden drop on mileage?