We've had a few of those little "dashboard dynos" in the drag car over the past years. They're a lot of fun and can get pretty close, 'though never dead on. You really have to be accurate when entering the weight of the car, including driver and fuel and such in order to get close to correct. We use four scales (out at the wheels) and still get different readings at the track and salvage yard.
BUT, they are a ton of fun, and gives the passenger something to look at while they laugh and scream ! ! !