With 35" tires 13-15 mpg would be correct. If you were getting 20-21 something was wrong. When you were calculating the MPG at 20-21, was that based on 1 fill up or 10 fill ups? The best way to know you MPG is by using fuelly.com (see my signature below). If you have a smart phone it easy to do every time you fill up. I have heard plenty people claim 20-21. Go to fuelly.com and look at all the 7.3 trucks. You can do a search by any vehicle year and fuel type. No 7.3's on there are getting 20-21. May be a few 18 mpg trucks but they are 2wd speed limit only highway trucks with a manual transmission. Not wanting to knock your claim, but think you may have made an error on your calculation or fill up procedures when you got the 20-21 mpg as its not going to happen.
this is how i have always checked my fuel mileage i reset my trip then when i go to fuel up i take my miles on the trip then divide them by how many gallons of fuel it took to fuel up so if i had 330 miles on the trip and my fuel up was 21.965 gallons my fuel mileage would be 15.023902 i just round to nearest number which would be 15 mpg
also this was going by numerous fillups so more than 10 fill ups
my truck is auto tranny but i also use the dynatrac free spin system so no front axle drag
my truck when i first got it had over 200,000 miles and i was getting 412 miles to the tank and normal fuel up was 21 gallons which is 19 and a half mpg
that was normal for me from may 2011 till feb 2012 then went to 380 miles for about a month then 300 miles thats when i started using 30oz's of diesel kleen instead of 16oz's which brought me back up to 375 now i can put 40oz's or even 60oz's and i still get 300 max
as far as no 7.3 can get 21 diesel power mag or it may have been 8 lug back in 2008 maybe 2009 proved that a 7.3 using syn oil,aftermarket cool air intake,dyna trac free spin and a few more add ons could get up to 23mpg as a daily driver
so there are ways to achive 20+mpg with a 7.3 turbo 99 and newer
my other F-250 aways got 18mpg but only after i installed a dyna trac free spin on it before the free spin i was getting 15 to 16mpg
I did notice when i was in illinois back in march that running there bio diesel mix made my fuel mileage drop to about 10 to 12 mpg when i got home i changed the fuel filter thinking this would help i also noticed that since then my fuel mileage has been not so great, i have ran bio diesel before and gotten great fuel mileage but this could be different because of the precentage of bio mix i think before when i ran bio no problem it was a 15% mix where this last time it was a 40% mix.
I dont think its the bio its self to blame however i do think this has something to do with 1 or more sensors that may be failing but arent throwing codes yet as the only sensors i have replaced so far have been the ICP and back pressure i also cleaned tube leadign to back pressure sensor as i was told this could also cause a problem.
If somethign was wrong with the truck i was unaware as it ran as good as it ever has even when pulling a 30ft travel trailer so i'm at a loss if it was something going wrong as far as i can tell it never hurt the truck so i wish it would break again lol