my el camino used to do that, it 'pegged' at 85 but i pulled out the little peg. and since i had 4.10's in the rear and a 700R4 instead of the stock TH250 and 2.73's the speedo was really messed up.
so with that compination (of the rear end gears, tranny, and missing peg) the speedo used to really be off, 30 mph was about 55 on the speedo.. so when i was going 75+ it would say i was in netural or reverse.. it was funny to me, confused a lot of people, and since im a loser like i am, when ever i would hit 'reverse' or what not i would be like, 'we're going back to the future!!!' and make all sorts of sound effects and such....
yeah i know i'm a loser... leave me be... i'm simple minded, and thats the way i like it...
-Neil