What Teh Debil????

hm...SF King just gave me his old rims with his 16" BF Goodrich tires. They look sweet and I have tons of grip now...but I encountered a small problem. When we took it out for a test to see how she handeled, we got on the highway and once we hit about 70, my Spedometer crapped out and died. I ran fine and shifted ok, but it was really weird. plus once we got back home, My CEL came on, which freaked me out. I'm wondering if this is just bad timing and something died in my car that needs replaced, or if the larger tire size made my car crap out. Any help would be greatly appreciated.