help i think my speedo is cracked out
 

ok so i have a 87 turbo supra. and im sure my speedo is WAY off. i have 215/45/17's in the front and 235/45/17's n the back and wen im in 5th gear at 3K my speedo says im doing 80mph. wen i check with gps or anything of the sort i says im doing about 5mph slower than wat my speedo is reading. how can i fix this or should i just leave it be?