Is my speedometer wrong?

Well I was driving to school this morning, and the radar detector started going off. I was doing about 50 (in a 45), so I played it safe and slowed to about 44 or 45. I come around the corner and it turns out it wasn't a cop but one of those stand-up speed things that displays your speed in big red letters. As I come up on it I look and it says I'm doin 51, even though the speedometer says I'm doin 44, 45 at the most. I double check both, and sure enough my speedometer was 5-6 mph off from that. I have stock gears (and everything else except shorties, o/r h and cai), so I don't know what the deal is. Even though it's only 5 or 6 off, regardless which is wrong, I usually cruise about 5 over, which they will let slide, but 11 is a different story. Just wondering whats going on.
 
buddyf87 said:
Yeah, stock tire size. I was in fifth so I was probably doing 1800-1900 rpm.

With stock gears you can do "45" in 5th? I have 3.27s and I only slide it into fifth around 50ish. Are you sure you weren't in 4th?

That you were able to be in 5th, with a stock geared car, and not notice the engine stall, and were reading 45, leads me to believe you were actually higher then 45, more around the 55ish mark, especially at 1800 rpms.

- Adam