car companys have always low-balled their horsepower figures on musle cars. for example the 1970 chevy chevelle ls6 was rated at 450hp for the factory when they were actually closer to 500 hp. also in a issue of MM&FF they dyno tested a 2000 cobra R and they said that it was making around 370 rwhp, yet ford only rates the car at 385 at the engine. so it is really nothing new for the companies to lie, and as for the insurance company, they are not going to dyno test every new car that comes out, they just take the companys word.
