I get 10 to 12 town driving or highway the exhaust is 5" straight pipe the intake needs work the pump is turned up only enough that I get a slight haze when I put my foot down. no turbo.

 

unless you put in the 3.73 gears, you dont have them. ford never used them in our era. it will be either 3.55 or 4.11

 

yeah 15.5 to 20 mpg's for an idi is good..... i average 16+ most of the time.... and mines turned up 1 1/2 flats or so, add atf to the tank, i put half a quart in each tank every other fill or so..... keeps the injectors cleaned up..... in the summer i run a gallon or filtered ( 5 micron) to the tank every fill up... smokey but cool... .....

 

Unloaded I average 18-20. Pulling a 9,000 lb horse trailer mixed hills and highway, I average 13-15. But I don't drive fast either. Mine gets better on backroads usually. She gets scared over 60mph. 4.56 gears ain't ideal for highway running. But she will pull. Pumps stock. Dual 3"to glasspacks cut behind the cab. Soupbowl removed. I baby the old girl until it's time to work, then she hammers. I just gotta get an over-drive of some kind.

 

Before i put the 7.3 in the 6.9 got 17-20 easily in city and highway. The 7.3 as long as it stays below 60 gets around 18. 86 reg cab 4spd 4x4 with 3.55s

 

My 87 with a 6.9,4speed and 3.55 gears is averaging 12 in town and 15 on the hwy with the cruise set @ 70.

 

i have a 93 idi 7.3 non turbo but on the road just drivin i can average 22-24 mpg and thats with the pump turned way up

 

Im runnin between 16 and 18 as long as i keep my foot out of it.