when i drive i feel like im goingabout 15 miles slower than than cars next to me i can also tell by the RPM's....
what could it be, how can i fix it?
its really pissing me off, like i dunno how fast im going and if i should slow down when i see cops...

please help!!

 

you should use the online calulators for tire diameters and also the rpm one to check if your speedo is off or not. google them or look for them on the forums, i dont remember which one had all of it.

only thing i can think of that can cause speedo wrong, would be if your speedo teeth has gone bad, you have new gears, or your tire diameter is too small or too big compared to stock.