Pressure testing it is the only way I know to check it. That is the least likely problem though.

You are right, your tires could be skewing it a bit, but not much. Have you/can you check the speed with a GPS? I am not sure how much the difference is with 285's

IMO one tank of bad mileage is not really something to worry about. Could have been due to some lousy fuel or something. I would see what the next tank figures out to before I started worrying to much.

I average high 18s, empty. I think my lowest was 13. something, and my highest has been like 21.1. I have 265s and the speedo is dead on. (checked with GPS)

 

today I got to drive the truck at some high way speeds with a new fridge standing in the back. It seems the truck lacks some power as on some bigger hills it really seemed to struggle. The fridge was about 2ft above the cab and i was drivign into the wind but still. It seems that the truck shouldnt have noticed it was there. I changed the air filter plus oil and filter today and tommorow night I am going to try to replace the fuel filter. What else could be a culprit of power loss?
Thanks
Evan