This is a traffic infraction. Although traffic infractions are criminal, they are not afforded the same proceedures as misdemeanors and felonies.
Set the case for trial. After the officer testifies, you can question the officer. Present your evidence./tape. You may want to bring in a transcription of the tape to that the judge and officer can read along with it while you are playing it. Then you can argue that the officer is not credible and that the evidence of your speed has not be proven beyond a reasonable doubt.
An infraction is not a crime; it is a public offense. That's why you get to have a commissioner hear your case instead of a real judge and have no right to counsel or to a jury.
As far as the substance of your question goes, 80 or 85 either one is speeding. This will get you nowhere.
There is no statutory provision for defendant to file a motion to dismiss in a criminal case, however, there is a history of these motions honored in California based upon the common law which is the collective body of law established in cases throughout the legal history of our courts.