Why is my car misfiring

@letlhogono92979 there just isn't enough information. We need a bigger story :-)

What exact model and year? Engine size? transmission ( automatic/manual)? so it goes to 80 (mph/km/h?) and then what happens? How does it idle? How does it run? What have you checked?

You can never give to much information :-))

It's a Runx 1.5 , automatic 1NZ engine,2005 model, when it reaches 80 it decreases the speed and start having a loud rough idle when I press the accelerator,fuel consumption is very high,I changed the oil,the sparkplugs,air/oil filter and I even replaced the petrol pump still no difference