MPG vs wheels
 

Me personally, I don't think this would change it that much but a second opinion doesn't hurt.

I recently upgraded from the stock LT trim 18" wheels and tires to a set of 20" with 275/40/20 wheels with a .1" larger wheel diameter. Each wheel maybe weighs 5 pounds heavier than my stock ones which I don't think would effect anything. Ever since I got these put on my mpg on the highway seems to have dropped quite a bit. Before I'd have an average of anywhere from 28-34mpg and now it Is around 18-24. Trying to figure out what could cause this since the only thing I've done lately is wheels

I don't think its the height as much as the width.
I went to 275 40 up front and 315 35 on the back same Diameter and my mileage has dropped on my SS.
there is just more contact with the road thus more friction, more weight yada yada yada..

Yeah I'm running 20x10 all corners and the stock were like 18x8 I believe so there's a lot more tire on the ground