18.5 mpg is pretty good for a truck with a V-6. You might see a little better with cleaning they system out but I wouldn't expect more than maybe a .5 - 1 mpg gain, if anything at all.
The best I've been able to get with my manual four-banger was 24 mpg and only occasionally. I usually see around 22 mpg, which is about average from my experience.
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2011 Ranger XLT, 4X4, 4 door Super Cab. 4.0L, 5 Speed M50D-R1HD, 3.73:1, 8.8" open differential, Jason Pace cap
1999 Honda CR-V EX, RT4WD, 2.0L, 5 Speed MT, 4.56:1, open differential