My thinking is this, not sure if it's right, but it kinda makes sense to me.
His car has lower gearing for good acceleration at low speeds, with a 350. However, his gearing doesn't go up as high as mine. Now, logic will tell you that eight cylinders at 1500rpms uses the same gas as four at 3000 rpms. Once his car gets up to 75mph, let's say 8 cylinders at 3000 rpm, it can shut off half of them and as long as it stays at 3000 rpm, it's enough to keep the car going.
My car has higher gearing, poor acceleration at low speeds, with a 307. However, that gearing allows it to cruise at 75mph with all eight cylinders but at only 1500 rpm.
Both use the same amount fuel (roughly 17/25 EPA estimated) but his uses newer and more expensive technology for better acceleration. His car would blow mine away off the line.