you want Analog inputs.
I got this board for controlling all my appliances in the car, and it comes with two analog inputs, you could hook these up to your speedo etc and calibrate them.
UK site, sorry if your elseware, i have seen a US supplier somewhere.
just search for USB Analog Input board or something of the like.
if your in the US this with this board would be great for you. 8 analog inputs!!!
(i'm also considering going the Speedo volemeter route for my Speed and Temperature etc... ODBII can only reach 1 reading per sec in my car (VW Polo year: 2000)