MPH wayyy offf...HELP!!

[cattrikk]

I've got both the sr20 engine and transmission in my coupe. i bought a gear for my speedometer(digital) that would supposedly fit into the side of the transmission and fix it...but no, the casing is too big to go in the hole in the transmission. So i tried to swap the little gears so that i could use the new gear on the old casing, didnt work. So how do i get my damn speedometer to read right? B/c i know when im cruisin down the interstate in 5th, im doin more that 27mph.