Mitsubishi Repair: 94 3000GT SL SPEEDOMETER, speedometer cable, stock rims

Question
I HAVE A 94 3000GT SL.. AND IM HAVING PROBLEM WITH THE SPEEDOMETER.. EX: GAGUES SAYS IM GOING 60 BUT IM REALLY GOIN 55 AND IT REALLY PISSING ME OFF.. I FOUND OUT CAUSE I WAS ON THE NJ TURNPIPE AND SPEED LIMIT ITS 65 AND I WAS GOING 70MPH BUT EVERYONE WAS PASSING ME EVEN OLDTIMERS SO I WAS LIKE WTF I GAIN SPEED TO 80MPH TO KEEP UP WITH EVERYONE SO I GOT OUT ON A EXIT TO CROSS A BRIDGE AND THEY HAD A SPEED RADER AND CHECK MINES AND IT SAYS I WAS GOIN 15MPH BUT MY GAGUES READ 25MPH... HOW COULD A FIX THIS PROBLEMS? I HAVE STOCK RIMS WITH RIGHT SIZE TIRES

Answer
Gustavo,
There are a couple reasons why this can be happening.  If your tires are under-inflated or are excessively worn, your speedometer can appear to be faster.  Also, with a 14 year old car, the gears in the speedometer cable can wear and make your speed look faster than it is.  Lastly, many manufacturers purposely configure the speedometer to appear faster to reduce the chance of people actually speeding.  Some manufacturers do this purposely, and some do it unintentionally.  

If this is really upsetting to you, have your tires checked and your speedometer cable replaced.  If the speed still appears faster than you're actually going, you can adjust the needle position on your gauge.  
Good luck!