Noticed when I had a GPS that my speed on the GPS and the speed on my speedometer don't match. Compared it with one of those MPH/radar things on the side of the road, it matched the GPS. Compared side by side with a friends car, still off.

My speedometer's 5 sometimes 10 mph off. The higher the speed the more off it is. Ie: When the speedometer says I'm doing 30, I'm really doing 27. When the speed. says I'm doing 65, I'm really doing 62. 80, more like 73/4ish.

Any ideas what can be causing this, and how to fix it?

 

You drive a vehicle that is over 14 years old - things are not going to be as perfect as they used to. There really is nothing you can do about it - to be like 5mph really is not that bad.

 

True, I was just hoping to find a fix for it. =/

Don't think it was ever fine though, bought it off my mom, she's had it since 98, but never took care of it. She always thought it ran a bit slow but wasn't sure.

 

Tires worn? Correct size? And this is a common thing. By federal Law manufactures are allowed to be off by a certain percentage. I would rather be going slower than what the speedometer reads than faster and not get a ticket!

 

tires are good.
I wouldn't have been so worried about it if it was always off by just 5 or 3, but it's a bigger gap as it goes higher.
I really just wanna be able to say I clocked 100+ and actually mean it. I could do it, but I'm only really doing 80.

 

IMOP it is the tires. If you go to Tirerack.com and compare the tire spec's, specifically the revolutions per mile of the same size tire, you will see a difference.The ones w/ the fewest revolutions per mile are larger.