I was driving with my friends and we were checking our speedometer. while they were going 65mph, I was making 58mph [&:] at same speed.

I think it is because of 18" rims that I have. Is there any place that I can get this problem fixed? or can I do it myself ?

That's why my car is so slow [8D]

If you got new wheels but didnt get the right profile tire compared to your old one, your speedometer will be off, either slow or fast.

the gearing changes as well, your final drive is now higher.

so how can this be fixed? or it cant?

you can fix the speedo if they make a gear to correct for taller tires. The taller gearing can only be fixed with smaller tires, or change the gearing in the tranny.