my v6 mustang, (98) will not go above 90 miles per hour. it gets to 90 and it stops accelerating

. Has the engine got restrictions in it. If there is i want to remove them.
Could there be a fault somewhere. I have only had it a few months and i am still learning about it. i have a slp exhaust on the way, i hope this will help it perform better and sound better.
What is the largest size wheels i can fit on my car(and tyres)?
how often does it need a tune. i think the previous owner only changed the oil, I took out the original spark plugs which had done 48000 miles. Some people wrap up there car. Me, I love driving my car, named "The Sheriff"
Thanks if anyone can help.