3.73 speedometer gear change?

[5.0PiMpin]

wats up guys , i just got my 3.73 gears installed today and i wasnt as impressed as i thought i would be. first thing im driving my car home and my speedometer was reading 70 mph and i only felt like i was going 55. i called my girlfriend who was following me and asked her wat hers said and she said 55. so i jumped on the phone with the guy who installed the gears and he said that i need to go to ford and get a new gear for the speedometer. i never heard of this before and i was wondering if he is BS'in me. wat didnt impress me about the gear change was that i thought there would a grand cloud of smoke and chirping into second with the swap. i didnt no this but the car came with 3.08's alot of people told me that the stock was 2. sumthin. i also added the aluminum drive shaft. it really works in cutting down on the interior noise that i used to get. is there sumthing that i left out that i should have added with the gear swap? ne answers would be greatly apreciated.

thanks, danny