Ok, so I bought a 00 WS6 and the guy had it dyno tuned then he had a Viper T56 tranny (same tranny just stronger) and put bigger tires on it. he only drove it a few miles because he was getting everything done while he was over seas and the car was here. everything was perfessionally installed and evverything runs great but the speedometer is off and I would like to get it fixed. the performace shop here on town says it needs a full dyno tune. im calling B.S. on that. know here are the questions I am asking.

1.can the speedo just be changed with like a preditor?
2. since the car was dyno tuned once does the diablo need to be changed from base line or will it read the changes?
3.is it worth getting the diablo? ( I plan on doing LT's h/c eventully and will get it dyno tuned after the h/c)
4. how much will them just inputing the stuff into my car cost?