Outboard gas mileage dropped

Alright y'all it's a 97 evinrude 115 with a reman powerhead and an sei lower unit. E115eleua.I've started to notice the boats mileage has gone to sh*t. I'm aware that outboards don't get "good mileage" anyway. But i think this is ridiculously less then before. Boat has the same prop as it did back in the day, much lighter load then before,and I've been easy on the throttle. Also fishing the same fishing holes as before. Before we could run to these 3 spots in an entire weekend and only use 10 gallons of gas, this weekend i've fished the same spots, only running 26 mph (boat tops at 42) and have used 35 gallons of gas. What in the world could possibly make me use this much more fuel?

Do a cylinder drop test on it.

Do i need backpressure? Should I do it on a dyno?

Dyno or in the lake.

Should i just do it in neutral while on the water? Ive only done cylinder drop test on water muffs (and passed) but I guess it can run on all 4 on muffs and not on the water?