The speedometer is about 20 miles below how fast it is really going. So, if l am going 70 says 50. The tires are the right size and they have the right pressure. What could be wrong ad how would l go about fixing this?

Thanks in advance!!

l recommend you check the wire.

.which one?

the one holding ur car together

First try replacing the speedometer cable, if this does not do the trick you will have to replace the speedometer, in a lot of cases this means that the wind shield and dash board will have to be taken out, before you can extract the speedometer. The best place to go for another speedometer would be the junk yard. They can check it for you, to make sure it is in proper working order before you purchase it.

You may need to relace the spedometer cable, but that is probably not the case. Usually ur spedometer will read very random speeds & will bounce to different speeds if this is the case.
Who says ur tires r the right size? Talk to a different mechanic if u have just spoken to only one. My 89 Grand Am had the same problem. A ''professional'' put the right size tires on my car (at least he thought). l was going about 15 miles per hour slower than what my spedometer showed. l went to a different mechanic & found that the guy had put on the tires for the wrong year. After that, my speed was perfect.

edit: If u tell me the specific model & engine size for ur vehicle, l can tell u if u have the right size tires.
For example, if u have a Grand Am SE u should have the size 185/75R14
However, a Grand Am LE would have 205/70R15 for a tire.