I just installed 4.10's in my car, will my speedo be off? If so how much?

 

Yes, it will indicate that you're going faster than you really are.

Let's see: 4.10/3.42=~1.2

So, 20%? It's just an estimate, and it does sound a little high...

That means the speedo will indicate 72 mph when you are going 60 mph...

 

You can see how much it'll be off here:

www.f-body.org/gears/