My 2000 Mercury Mountaineer 5.0 gets around 9.5MPG with normal driving, 12MPG when I'm trying. How do I improve this.

 

Not enough info -- and no one knows

 

What do you mean by "not enough info"

 

One of the most important info necessary to calculate fuel economy is the CLIMATE and CONDITIONS that you're operating the vehicle in.
Temperature, Altitude, Humidity, Road Surface, Gearing, and Type of Vehicle are factors that effect fuel mileage, so must be considered in any attempts at improvement.
1995 Grand Marquis Restoration - Mercury Forum - Mercury Enthusiasts Forums (1)

 

okay so I live in south Texas and its very hot and humid all year round and a bunch of rain. So this by itself could make my MPG so poor?

 

South Texas !?? That explains a lot, but doesn't solve anything.
You say "poor mileage", but we don't know what that means -- and we don't know if you know how to measure your fuel mileage properly in the first place.
HOW TO MEASURE FUEL MILEAGE:
1. Fill the fuel tank until the nozzle clicks "OFF".
2. Record the number of miles on the Odometer.
3. Drive until there is 1/4th left on the fuel guage.
4. Fill the tank again until the nozzle clicks "OFF".
5. Record the number of gallons used to TWO (2) decimal places -- (00.00)
6. Record the new number of miles on the Odometer.
7. Subtract the OLD Odometer reading from the NEW reading, and divide by the number of gallons used. -- (00.00)
Simple math, and your answer will be dead-nuts accurate, and from there you'll be able to measure any improvements that you make.
The most common cause for poor fuel economy is IMPROPER TIRE PRESSURE -- Get a DIGITAL tire guage, and set your tire pressure to the numbers on the door jamb. I use and recommend the "JACO" Digital guage = 0-100 psi. (Amazon)
Next in line is the Engine Air Filter -- Avoid the oiled filters -- too much hassle with improper cleaning and oiling. I use a FRAM Tough Guard series, and change it every Thanksgiving week-end.
You didn't mention a "Check Engine" light, so no ignition misfires were suspected.
You'll wanna replace the FUEL FILTER, too -- a Clean fuel system is a happy fuel system. ;>)
That's enough to get you started -- do those things, check for improvement, and get back to me.
Good Luck !!
P.S. -- VOTE for BETO !!

 

okay thank you but my car reads out the mpg on a screen under my center console and btw I meant not a lot of rain. but I can now verify the mpg.

 

I had that same feature on a '92 Crown Vic -- after using it for 125,000 miles, I discovered that it's NOT accurate.
Like I said above -- there is only ONE method that is ACCURATE -- and that method works with ANY vehicle.
Of course, you are perfectly free to be as inaccurate as you'd like.
CHEERS !

 

Ha thx