I have a 2007 that I got awhile ago and ever since I've had it the speedometer has been off. As I go faster the bigger the distance from my actual speed. When I'm going around 60 MPH it reads around 75 MPH. It has aftermarket tires on it the stock tires are 245 50 r18 now it has 275 40zr18. I have a tuner and I change the revolutions per mile and it only helped a little. The car is lowered not sure how much(not sure if this would effect the speedometer or not) Please help I never know how fast I'm going hahah

 

You could tweak the the tire revolutions/mile to get there, but it sounds like your rear gear ratio is wrong.
I'd check with your tuner to see what it says your gear ratio is and then adjust the ratio number up until you get it there. The normal ratios are 3.31, 3.55, 3.73, and 4.10, so get the tire revs correct for your tires and then step up the numerical ratio and test until it reads correct.

 

Your stock axle ratio would have been 3.31. It's possible that the previous owner did a 4.10 gear swap and didn't correct the speedo.

 

Yeah, what Dino says. I just ran the calcs, I'm willing to bet your tune says 3.31 when the actual gear ratio is 4.10...
And The calc says that when you are doing 60 but the speedo shows 75, your RPMs are probably reading at about 2140.