Have recently seen posts on MPG and durability/strength of the 300 I6 from new forum users. Wanted to share my experiences with my truck: 1990 F150 SC, 147K on I6, 3.08 rear end, E40D tranny and 235-75 15-inch steel belted radials. Bought this truck about a year ago when odometer read 130K. Learned how to check and read codes through dash check-engine light (http://www.dalidesign.com/hbook/eectest.html). Consequently replaced EGR valve, ACT and ECT sensors, also checked O2 sensor but didn't replace. Also cleaned throttle bore and air bypass valve and had fuel pressure regulator replaced when it failed about 8 months after purchase. Was getting 14-15 mpg in town on 88-90 octane gas at first. Went to Splitfire plugs and noticed a little better throttle response and mpg (not much). Tried a set of side-gapped Autolites (still running these after 5K miles) and throttle and performance boost was noticeable...mpg went up to 16 to 16.5 in town and on highway. Then read a post on this board referencing this site: http://www.himacresearch.com/books/hydro8.htm
Was skeptical of their claims, but tried the PCV jar they mention (cost me about $20 to build). Figured I didn't have anything to lose. After using the PCV jar for about a month (4.5 tanks of gas), mileage is up to 18 to 18.7 mpg...mostly in-town driving. Not a big improvement, but can now squeeze about 281 to 285 miles out of 15 gallons of 88 octane gas...always leave about 1 to 1.5 gallons in the 16-gallon front tank. Next plan is to test mpg on a road trip...maybe later this spring. Not plugging anything here...but am very happy with performance of sidegapped plugs in my I6. Won't offer an opinion on the PCV jar for another month or so, but it was easy to build and has boosted mpg a little. May also try the drop-in K&N flat filter next. All of my driving is in table-flat West Texas with occasional very high winds (20-40 mph not uncommon). I am also a conservative driver, so that may be a factor, too. This is my fourth Ford pickup, all with the 300-6 engine. Never thought I'd see/attain more than 15 mpg, but it is possible. Long live the 300 I6!

 

I would love to have 15 MPG on my I6! I replaced all the stuff you did...except for cleaning out my throttle body. could that be a reason for getting 12 mpg??? seesh! Some guys say that its normal to get 12mpg on my engine but many recent posts say 15 - 18 mpg is the norm. HELP! 1993 Ford F150 I6 EFI auto.

very conservative or no at all, highway or city...still 12mpg