yes, 12 is low.
Tank size for a 1998 ranger should be 17gal
But you calculate fuel mileage by replacement gas.
So fill up the tank and write down the current mileage on the odometer, or set the trip meter if you have one.
Then drive the truck until you need fuel again and fill it up, write down the gallons that went in to the tank this time, and then get the mileage on the odometer.
Subtract previous odometer miles from current one.
200,100 (current)
200,000 (previous)
100 (miles traveled)
So you drove 100 miles from last fill up
say you put in 10gallons of gas
100(miles)/10(gallons) = 10 miles per gallon
= sell the Chevy and get a Ranger, lol
On all engines with fuel injection there is an electronic "choke", if it has a problem engine can run rich all the time instead of just when warming up.
This is called the ECT(engine coolant temp) sensor.
It is a two wire sensor located near the 1 wire temp sender for the dashboard temp gauge.
Easy to test
http://oldfuelinjection.com/?p=28