faulty spedo

my spedo is off about 20mph , it says im going fater than i really am, and my gas gauge is empty when it reads half of a tank, any ideas?

Would help if you told us what car

Do you have aftermarket rims/wheels?  Are you saying that even while at idle, you speedo reads 20mph?
For the gas gauge, that's an easy fix, fill up the tank and move the needle to full.

it goes up to full and goes down to half a tank but doesnt go any further than that so when i hit half of a tank its really on empty, the guy said this stuff started happening when he put the new cluster in it before i bought it

Oh this is just great. What car, again?

its a eagle talon tsi awd

The gas gauge fix that Silver proposed should work... whereas the speedo issue... try removing the pin and remounting it at 0 while at idle...

My Gas Meter works somewhat but when i fill my tank allway up  it goes to one below full...so i think to three quarters

ok, i think that i know how i can fix my gas gauge ill run it till the light comes on then push the thing down to empty, but the spedo goes down to 0 when im not moving but as soon as i start moving it goes to 20

it wont go perfect but it will work alittle bit...some problem i had with mine did what you said...when i got it

Rapperbasher, you may have to remove the needle off the shaft, put the needle back on in the full spot, with the needle still in place..rotate it down to it's new "empty" position  (the half spot) and then carefully remove the needle again and place it back on the shaft with it pointing in the full spot.    It's a good chance that the fuel gauge shaft range may have been moved up and that's why it shows empty near the half spot instead of the needle going to "E".  If that doesn't work, you'll need a new gauge.

For the speedo, you may have to follow a buddy to set your speedo.  I had to do this (I used DSMLink and my turbo timer to help me set my speed).  I also had all the shrouds removed and the needle lightly attached before hand.  If that doesn't work, get a new gauge cluster, lol.  I don't see it being bad, just out of caliberation.
You still haven't answered my rim question though, is it stock or aftermarket?  If aftermarket, what size?

it has after market rims, 17" dc rims with lo pro yokahoma tires i think the tires are .40's

you know if the gas thing doesnt work out you could always go to a junkyard and get one out of a console and just replace yours with it i did that with a tacometer haha cause my 96 eclipse didnt come with one and i just put it in and it worked fine so i suggest a junkyard.

It sounds to me that the speedo needs re calibration due to the spring becoming weak.
 
As for the fuel gauge, due to the fact full defection is not possible have you considered that the fualt might be in the sender unit (float and pot)?