I don't know if you've seen the C-130 Hercules that they run around the air-show circuit?
6 JATO units, and it lifts off the runway and climbs at about 40* for several hundred feet.
250 with one strapped to the roof doesn't sound out of line.
The P-motor rocket that the Mythbusters used wasn't anywhere NEAR the size of a real JATO.
And the story originally was a mid-80s Impala, the guy hit a banked curve at around 250mph and went airborne, then hit the cliff. The way the story went, an old couple called the cops, and when the cops saw the rocket motor, they called the FAA, who called the Air Force, who hushed it up, because they didn't want to admit that someone just drove out of the base with a ten foot long rocket in their truck.
He was supposed to have burned off the brakes, the tires, the wheels, the suspension, and halfway through the floorpan when he hit the curve that threw him into the cliff, and the only reason the Air Force figured out it was a car was they found the Impala emblem that got blown off the car.