so several things have changed at once, and the mileage in my truck has dropped by 4.8MPG. (avg of 15 to 10.2 overnight)

tires switched from 245/75/16 to 265/70/17, calculating a 1.1" increase in diameter. (also the rim/tire combo is heavier, but not sure by how much)

temp dropped from mid 40's to below zero

changed the oil (?)

im thinking the only thing that factored my bad MPG was the tires. have you dropped MPG that much from switching to bigger tires?

also the truck is a 1989 4x4ECSB 5.7, 700r4, 3.73. driving style has not changed, and measurement technique has not changed. earlier this summer i replaced the following

CTS
MAP
02
Plugs
CAI
Fuel Filter

so i suspect those are not the issue. my engine is burning some oil, but has been for a while, and am planning a rebuild soon. i guess this could be a factor for my MPG, but like i said, its been this way for a while. any thoughts??