First--what is the posted speed limit? Unless that specific area is with the city limits of an incorporated city the speed limit is probably 55 mph. La 342 is Ridge Road in Lafayette. If in fact your tires are brand new ,your speedometer was calibrated by the manufacturer when the tires were placed on the car. If the tires are oversized (not the same size that were designed for the vehicle) then yes your speedometer will be off as little as 5 mph--sometimes more. Any dealership can calibrate your speedometer. And if you question how fast you really were going at the time you were captured on radar---you could have brought your vehicle into a dealership and gotten a written record of what your vehicle was calibrated at,but you would have to have done it immediately after receiving the ticket to present as evidence in court to contest the ticket. If in fact it was off, a judge could take that into consideration. But you should note there is a law on the books making you responsible for the operating condition of your vehicle--that means everything--including the windshield wiper squitter that sprays water on your windshield.