Rear axle ratio is listed on Drivers Door label, read here:
http://www.therangerstation.com/tech_library/axle_codes.shtml
Headers do not add horsepower or torque, they change WHEN you get HP and torque in the RPM band
Rangers have factory mid RPM range headers
Most want lower end power so get low RPM range headers
Racing headers are high RPM range
Scavenging the exhaust flow is what 'headers' do
The velocity of the exhaust leaving a cylinder is used to lower the pressure at the closed exhaust valves on that bank of the engine
The smaller header pipes dump exhaust into a larger 'collector' pipe, this is where velocity comes in, in a certain RPM range this faster flowing exhaust hitting the larger pipe causes a pressure drop in the other smaller pipes on that collector.
This pressure drop means the pistons on that bank do not have to 'push as hard' to get exhaust out, so more power is left in the crank for the rear wheels.
This is the scavenged power headers provide
The diameter of the smaller pipes and the length, and the diameter of the collector pipe are how the RPM band is tuned, it is a science, lol, and above my pay grade
3.0l Vulcan was designed as a high RPM engine, making best torque above 3,400RPM, most engines make best torque at 2,400RPM
So you need to drive it like you know this, keep engine in the 3,400-3,900RPM range for best power, you are NOT over REVing it
2003 3.0l actually has best torque at 3,900RPM, and best horse power at 5,200RPM
Torque is what gets you up to speed
Horse power is what keeps you at that speed
Good read here on 3.0l and power ranges:
http://www.therangerstation.com/tech_library/3_0performance.shtml
Also some performance add ons
Switching to an electric radiator fan and removing the mechanical fan always adds HP and increases MPG
Mechanical fan is always drawing engine power whether it is needed or not, fan clutch helps but power draw is always there.
E-fan only draws power(alternator) when extra cooling is needed, like when driving slow or idling, when driving at highway speeds the speed/air flow cools the rad, e-fan is off, not drawing any power, so best power and MPG when you need it
HP increase will not be much or the MPG gain, but it won't be 0