I change my plugs now I get 8 miles per gallon & car shuts down after travling 20 miles

These are ideas only as there are many things that could improve gas mileage.
When was the last time you had a tune up? (Spark plugs check for bad sensors . When was the Air filter changed..
do you always check the air pressure in the tires. Low tires can bring your mileage down five miles per gallon or more.
Do you drive with fast starts and stops.
Do you run short trips? Short trips does not let the engine warm up to the right traveling temperature. I live in town and I get about 15 miles per gallon in town but if I drive 20 miles or more I get 24 or more with my Buick.
You will get less mileage if you drive in real cold weather.

You would need to check with a local garage for repair prices.
To find your average mileage, fill up the tank, write down the mileage, then do it again at around 200 miles. Divide the mileage traveled by the number of gallons used. I would recommend filling up after you have used about 8 or 9 gallons to prevent running out of gas. You can't go by the federal mpg numbers because everybody drives differently.

the fuel pressure regulator will not do it. you need to look at spark plugs ,wires ,air filter, fuel filter. tires pressure if tires are under pressure can cause bad gas mpg

It just depends on your tank size. You probably get about 15 -18 miles per gallon. 13 gallon tank would be around 200 miles. 15 gallon tank around 232 miles.

there is a book publish by the govnt or minister of transport on the exact milage as well if you know how many liters or gallons go in your tank like 80 liters and the gas is 1.00 a liter it cost $80 for 80 liters on a full tank you do 500 km but make sure proper conditions of 100km a hour tires all proper equal pressure the wind is also a facture there is a lot involved to calculate your gas proformance as well if you had a oil change done latly you can find out on a govt site for gas consuption or consumers report

Depends on how you drive, how long you let it warm up, etc. If it was getting say 20 mpg, it would use about 65 gallons to go 1294 miles (2082KM), Do you know how to check your fuel mileage? When you fill up, zero the trip meter, or write down the miles, next time you fill up, get a receipt with gallons (liters) on it and write down the miles, divide it by the number of gallons, this is miles per gallon. 200 miles divided by 10 gallons equals 20MPG . zero the trip odometer again and next tank you can figure it again. I write the miles on my reciepts so anytime I want I can check. Hope this helps.

When is the last time you had a tune up? Have you changed the spark plugs lately? An often overlooked item is the air cleaner. A plugged air cleaner can take 3 or 4 miles a gallon off your average. You can purchase a K&N cold air induction or a cheaper brand, this includes the cleanable air filter and chrome air intake hose, replacing the stock. This should improve your gas milage by 4 or 5 miles per gallon.

Your car should average roughly 32.5 mpg-US (7.24 L/100 km; 39.0 mpg-imp)

Lost gas mileage. I would have to assume that you are not getting the require mile per gallon. For example If the car book say 32 miles per gallon. You are getting about 20 to 25 miles per gallon.
If you want to improve the your gas mileage then you have to do the required maintenance
Oil change every 3000 mile
tune up as required
Air filter
If the wiring spark plug wire is old and crack then it need to be replace.
If you park out side and the batteries is about 5 years old have it check
If you buy the car new, you will have a car book follow the maintenance routine.
If you have done all the maintenance and the gas mileage is low than recommended then take it to a shop to figure out if some thing is wrong or not preforming as it should.
I do hope this help you.

Check the torque on your plugs, should be around 15 - 19 lbs - consult spark plug manufacturer on the torque - if you did not torque them enough, they may have become loose. Also check coil packs - make sure all plug wires are snug.