JellyBeanDriver's MPGuino has worked flawlessly for me. Shipping couldn't have been quicker if I drove to Cali myself! If he/she isn't in business in September I would suspect foul play.
Here is the wiki on calibrating the MPGuino. It's pretty straight forward.
Mpguino calibration - EcoModder
IIRC the VSS for a Jeep is 8000PPM... but calibrating your MPGuino using the wiki is best.