12 gallons for 118 miles is just a little under 10 mpg - I'd hazard you ought to be hitting at least 20 mpg with a V6 Ranger, if not more.

How did the truck run prior to changing the plugs? IOW, is this a recent development? Knowing a little more history of how the truck has behaved would help.

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Just say "NO" to Ethanol - Drive Diesel

Mitchell Oates
Mooresville, NC
'87 300D 212K miles
'87 300D 151K miles - R.I.P. 12/08
'05 Jeep Liberty CRD 67K miles
Grumpy Old Diesel Owners Club