speedometer problem..

well i recently bought my mustang...and as far as i knew all the last owner did was put flowmaster exhaust and 17 inch cobra wheels on the pony. well, the speedometer is like 15 mph over what i'm really going, so i took it into ford for them to look at.

they looked around and it turns out the last owner put 4.10 gears in, and that they tried to recalibrate it but only got it 4mph closer..so now its 11mph over. any ideas on where to take it to get it recalibrated...if i even can?

a friend of mine also told me that puts extra miles on the odo...

you could try to get new speedo gear, I know for 3.73 gears it is 23 tooth that you should get, so maybe for 4.10 it is higher. The problem is that those wear down. If you wanted to get exact speed and all that I believe you can get like a Diablo chip or some other chip that will correct that for you.